Sergei Gninenko

Hypothetical low-mass particles, such as axions, provide a compelling explanation for the dark matter in the universe. Such particles are expected to emerge abundantly from the hot interior of stars. To test this prediction, the CERN Axion Solar Telescope (CAST) uses a 9 T refurbished Large Hadron Collider test magnet directed towards the Sun. In the strong magnetic field, solar axions can be converted to X-ray photons which can be recorded by X-ray detectors. In the 2013–2015 run, thanks to low-background detectors and a new X-ray telescope, the signal-to-noise ratio was increased by about a factor of three. Here, we report the best limit on the axion–photon coupling strength (0.66 × 10−10 GeV−1 at 95% confidence level) set by CAST, which now reaches similar levels to the most restrictive astrophysical bounds. Axions are hypothetical light particles that could explain the dark matter. They could be produced in the interior of the Sun and the CERN Axion Solar Telescope sets the best limit on how strongly axions can interact with light.

Hypothetical axionlike particles with a two-photon interaction would be produced in the sun by the Primakoff process. In a laboratory magnetic field ("axion helioscope"), they would be transformed into x-rays with energies of a few keV. Using a decommissioned Large Hadron Collider test magnet, the CERN Axion Solar Telescope ran for about 6 months during 2003. The first results from the analysis of these data are presented here. No signal above background was observed, implying an upper limit to the axion-photon coupling g(agamma)<1.16x10(-10) GeV-1 at 95% C.L. for m(a) less, similar 0.02 eV. This limit, assumption-free, is comparable to the limit from stellar energy-loss arguments and considerably more restrictive than any previous experiment over a broad range of axion masses.

Abstract The Physics Beyond Colliders initiative is an exploratory study aimed at exploiting the full scientific potential of the CERN’s accelerator complex and scientific infrastructures through projects complementary to the LHC and other possible future colliders. These projects will target fundamental physics questions in modern particle physics. This document presents the status of the proposals presented in the framework of the Beyond Standard Model physics working group, and explore their physics reach and the impact that CERN could have in the next 10–20 years on the international landscape.

The International Axion Observatory (IAXO) will be a forth generation axion helioscope. As its primary physics goal, IAXO will look for axions or axion-like particles (ALPs) originating in the Sun via the Primakoff conversion of the solar plasma photons. In terms of signal-to-noise ratio, IAXO will be about 4–5 orders of magnitude more sensitive than CAST, currently the most powerful axion helioscope, reaching sensitivity to axion-photon couplings down to a few × 10−12 GeV−1 and thus probing a large fraction of the currently unexplored axion and ALP parameter space. IAXO will also be sensitive to solar axions produced by mechanisms mediated by the axion-electron coupling gae with sensitivity — for the first time — to values of gae not previously excluded by astrophysics. With several other possible physics cases, IAXO has the potential to serve as a multi-purpose facility for generic axion and ALP research in the next decade. In this paper we present the conceptual design of IAXO, which follows the layout of an enhanced axion helioscope, based on a purpose-built 20 m-long 8-coils toroidal superconducting magnet. All the eight 60cm-diameter magnet bores are equipped with focusing x-ray optics, able to focus the signal photons into ∼ 0.2 cm2 spots that are imaged by ultra-low-background Micromegas x-ray detectors. The magnet is built into a structure with elevation and azimuth drives that will allow for solar tracking for ∼ 12 h each day.

We review the physics potential of a next generation search for solar axions: the International Axion Observatory (IAXO) . Endowed with a sensitivity to discover axion-like particles (ALPs) with a coupling to photons as small as gaγ∼ 10−12 GeV−1, or to electrons gae∼10−13, IAXO has the potential to find the QCD axion in the 1 meV∼1 eV mass range where it solves the strong CP problem, can account for the cold dark matter of the Universe and be responsible for the anomalous cooling observed in a number of stellar systems. At the same time, IAXO will have enough sensitivity to detect lower mass axions invoked to explain: 1) the origin of the anomalous "transparency" of the Universe to gamma-rays, 2) the observed soft X-ray excess from galaxy clusters or 3) some inflationary models. In addition, we review string theory axions with parameters accessible by IAXO and discuss their potential role in cosmology as Dark Matter and Dark Radiation as well as their connections to the above mentioned conundrums.

A search for sub-GeV dark matter production mediated by a new vector boson A^{'}, called a dark photon, is performed by the NA64 experiment in missing energy events from 100 GeV electron interactions in an active beam dump at the CERN SPS. From the analysis of the data collected in the years 2016, 2017, and 2018 with 2.84×10^{11} electrons on target no evidence of such a process has been found. The most stringent constraints on the A^{'} mixing strength with photons and the parameter space for the scalar and fermionic dark matter in the mass range ≲0.2 GeV are derived, thus demonstrating the power of the active beam dump approach for the dark matter search.

Abstract With the establishment and maturation of the experimental programs searching for new physics with sizeable couplings at the LHC, there is an increasing interest in the broader particle and astrophysics community for exploring the physics of light and feebly-interacting particles as a paradigm complementary to a New Physics sector at the TeV scale and beyond. FIPs 2020 has been the first workshop fully dedicated to the physics of feebly-interacting particles and was held virtually from 31 August to 4 September 2020. The workshop has gathered together experts from collider, beam dump, fixed target experiments, as well as from astrophysics, axions/ALPs searches, current/future neutrino experiments, and dark matter direct detection communities to discuss progress in experimental searches and underlying theory models for FIPs physics, and to enhance the cross-fertilisation across different fields. FIPs 2020 has been complemented by the topical workshop “Physics Beyond Colliders meets theory”, held at CERN from 7 June to 9 June 2020. This document presents the summary of the talks presented at the workshops and the outcome of the subsequent discussions held immediately after. It aims to provide a clear picture of this blooming field and proposes a few recommendations for the next round of experimental results.

We have searched for solar axions or similar particles that couple to two photons by using the CERN Axion Solar Telescope (CAST) setup with improved conditions in all detectors. From the absence of excess X-rays when the magnet was pointing to the Sun, we set an upper limit on the axion-photon coupling of 8.8 x 10^{-11} GeV^{-1} at 95% CL for m_a <~ 0.02 eV. This result is the best experimental limit over a broad range of axion masses and for m_a <~ 0.02 eV also supersedes the previous limit derived from energy-loss arguments on globular-cluster stars.

The principle of the equivalence of gravitational and inertial mass is one of the cornerstones of general relativity. Considerable efforts have been made and are still being made to verify its validity. A quantum-mechanical formulation of gravity allows for non-Newtonian contributions to the force which might lead to a difference in the gravitational force on matter and antimatter. While it is widely expected that the gravitational interaction of matter and of antimatter should be identical, this assertion has never been tested experimentally. With the production of large amounts of cold antihydrogen at the CERN Antiproton Decelerator, such a test with neutral antimatter atoms has now become feasible. For this purpose, we have proposed to set up the AEGIS experiment at CERN/AD, whose primary goal will be the direct measurement of the Earth's gravitational acceleration on antihydrogen with a classical Moiré deflectometer.

We have studied the muon neutrino and antineutrino quasi-elastic (QEL) scattering reactions (ν μ n→μ − p and $\bar{\nu }_{\mu}p\to\mu^{+}n$ ) using a set of experimental data collected by the NOMAD Collaboration. We have performed measurements of the cross-section of these processes on a nuclear target (mainly carbon) normalizing it to the total ν μ ( $\bar{\nu}_{\mu}$ ) charged-current cross section. The results for the flux-averaged QEL cross sections in the (anti)neutrino energy interval 3–100 GeV are $\langle \sigma_{\mathrm{qel}}\rangle_{\nu_{\mu}}=(0.92\pm0.02(\mathrm{stat})\pm0.06(\mathrm{syst}))\times10^{-38}~\mathrm{cm}^{2}$ and $\langle\sigma_{\mathrm{qel}}\rangle_{\bar{\nu}_{\mu}}=(0.81\pm0.05(\mathrm{stat})\pm0.09(\mathrm{syst}))\times10^{-38}~\mathrm{cm}^{2}$ for neutrino and antineutrino, respectively. The axial mass parameter M A was extracted from the measured quasi-elastic neutrino cross section. The corresponding result is M A =1.05±0.02(stat)±0.06(syst) GeV. It is consistent with the axial mass values recalculated from the antineutrino cross section and extracted from the pure Q 2 shape analysis of the high purity sample of ν μ quasi-elastic 2-track events, but has smaller systematic error and should be quoted as the main result of this work. Our measured M A is found to be in good agreement with the world average value obtained in previous deuterium filled bubble chamber experiments. The NOMAD measurement of M A is lower than those recently published by K2K and MiniBooNE Collaborations. However, within the large errors quoted by these experiments on M A , these results are compatible with the more precise NOMAD value.

We present the results of a search for νμ→νe oscillations in the NOMAD experiment at CERN. The experiment looked for the appearance of νe in a predominantly νμ wide-band neutrino beam at the CERN SPS. No evidence for oscillations was found. The 90% confidence limits obtained are Δm2<0.4 eV2 for maximal mixing and sin2(2θ)<1.4×10−3 for large Δm2. This result excludes the LSND allowed region of oscillation parameters with Δm2≳10 eV2.

We have searched for solar axions or other pseudoscalar particles that couple to two photons by using the CERN Axion Solar Telescope (CAST) setup. Whereas we previously have reported results from CAST with evacuated magnet bores (Phase I), setting limits on lower mass axions, here we report results from CAST where the magnet bores were filled with 4He gas (Phase II) of variable pressure. The introduction of gas generates a refractive photon mass mγ, thereby achieving the maximum possible conversion rate for those axion masses ma that match mγ. With 160 different pressure settings we have scanned ma up to about 0.4 eV, taking approximately 2 h of data for each setting. From the absence of excess x-rays when the magnet was pointing to the Sun, we set a typical upper limit on the axion-photon coupling of gaγ≲2.2 × 10−10 GeV−1 at 95% CL for ma≲0.4 eV, the exact result depending on the pressure setting. The excluded parameter range covers realistic axion models with a Peccei-Quinn scale in the neighborhood of fa ∼ 107 GeV. Currently in the second part of CAST Phase II, we are searching for axions with masses up to about 1.2 eV using 3He as a buffer gas.

This white paper addresses the hypothesis of light sterile neutrinos based on recent anomalies observed in neutrino experiments and the latest astrophysical data.

Several models of dark matter motivate the concept of hidden sectors consisting of SU(3)_C x SU(2)_L x U(1)_Y singlet fields. The interaction between our and hidden matter could be transmitted by new abelian U'(1) gauge bosons A' mixing with ordinary photons. If such A's with the mass in the sub-GeV range exist, they would be produced through mixing with photons emitted in two photon decays of \eta,\eta' neutral mesons generated by the high energy proton beam in a neutrino target. The A's would then penetrate the downstream shielding and be observed in a neutrino detector via their A'-> e+e- decays. Using bounds from the CHARM neutrino experiment at CERN that searched for an excess of e+e- pairs from heavy neutrino decays, the area excluding the \gamma - A' mixing range 10^{-7} < \epsilon < 10^{-4} for the A' mass region 1 < M_A' <500 MeV is derived. The obtained results are also used to constrain models, where a new gauge boson X interacts with quarks and leptons. New upper limits on the branching ratio as small as Br(\eta -> \gamma X) < 10^{-14} and Br(\eta' -> \gamma X) < 10^{-12} are obtained, which are several orders of magnitude more restrictive than the previous bounds from the Crystal Barrel experiment.

The CERN Axion Solar Telescope (CAST) has extended its search for solar axions by using 3He as a buffer gas. At T=1.8 K this allows for larger pressure settings and hence sensitivity to higher axion masses than our previous measurements with 4He. With about 1 h of data taking at each of 252 different pressure settings we have scanned the axion mass range 0.39 eV < m_a < 0.64 eV. From the absence of excess X-rays when the magnet was pointing to the Sun we set a typical upper limit on the axion-photon coupling of g_ag < 2.3 x 10^{-10} GeV^{-1} at 95% CL, the exact value depending on the pressure setting. KSVZ axions are excluded at the upper end of our mass range, the first time ever for any solar axion search. In future we will extend our search to m_a < 1.15 eV, comfortably overlapping with cosmological hot dark matter bounds.

We report on a direct search for sub-GeV dark photons (A') which might be produced in the reaction e^- Z \to e^- Z A' via kinetic mixing with photons by 100 GeV electrons incident on an active target in the NA64 experiment at the CERN SPS. The A's would decay invisibly into dark matter particles resulting in events with large missing energy. No evidence for such decays was found with 2.75\cdot 10^{9} electrons on target. We set new limits on the \gamma-A' mixing strength and exclude the invisible A' with a mass < 100 MeV as an explanation of the muon g_\mu-2 anomaly.

The CERN Axion Solar Telescope has finished its search for solar axions with (3)He buffer gas, covering the search range 0.64 eV ≲ ma ≲ 1.17 eV. This closes the gap to the cosmological hot dark matter limit and actually overlaps with it. From the absence of excess x rays when the magnet was pointing to the Sun we set a typical upper limit on the axion-photon coupling of gaγ ≲ 3.3 × 10(-10) GeV(-1) at 95% C.L., with the exact value depending on the pressure setting. Future direct solar axion searches will focus on increasing the sensitivity to smaller values of gaγ, for example by the currently discussed next generation helioscope International AXion Observatory.

Important open questions are still present in fundamental Physics and Cosmology, like the nature of Dark Matter, the matter-antimatter asymmetry and the validity of the Standard Model of particle interactions. Addressing these questions requires a new generation of massive particle detectors to explore the subatomic and astrophysical worlds. ICARUS T600 is the first large mass (760 tons) example of a new generation of detectors able to combine the imaging capabilities of the old famous ``bubble chamber'' with the excellent energy measurement of huge electronic detectors. ICARUS T600 now operates at the Gran Sasso underground laboratory and is used to study cosmic rays, neutrino oscillations and the proton decay. The potential for doing physics of this novel telescope is presented through a few examples of neutrino interactions reconstructed with unprecedented detail. Detector design and early operation are also reported.

We report the first results on a direct search for a new 16.7 MeV boson (X) which could explain the anomalous excess of e^{+}e^{-} pairs observed in the excited ^{8}Be^{*} nucleus decays. Because of its coupling to electrons, the X could be produced in the bremsstrahlung reaction e^{-}Z→e^{-}ZX by a 100 GeV e^{-} beam incident on an active target in the NA64 experiment at the CERN Super Proton Synchrotron and observed through the subsequent decay into a e^{+}e^{-} pair. With 5.4×10^{10} electrons on target, no evidence for such decays was found, allowing us to set first limits on the X-e^{-} coupling in the range 1.3×10^{-4}≲ε_{e}≲4.2×10^{-4} excluding part of the allowed parameter space. We also set new bounds on the mixing strength of photons with dark photons (A^{'}) from nonobservation of the decay A^{'}→e^{+}e^{-} of the bremsstrahlung A^{'} with a mass ≲23 MeV.

A search is performed for a new sub-GeV vector boson ($A'$) mediated production of Dark Matter ($\chi$) in the fixed-target experiment, NA64, at the CERN SPS. The $A'$, called dark photon, could be generated in the reaction $ e^- Z \to e^- Z A'$ of 100 GeV electrons dumped against an active target which is followed by the prompt invisible decay $A' \to \chi \overline{\chi}$. The experimental signature of this process would be an event with an isolated electron and large missing energy in the detector. From the analysis of the data sample collected in 2016 corresponding to $4.3\times10^{10}$ electrons on target no evidence of such a process has been found. New stringent constraints on the $A'$ mixing strength with photons, $10^{-5}\lesssim \epsilon \lesssim 10^{-2}$, for the $A'$ mass range $m_{A'} \lesssim 1$ GeV are derived. For models considering scalar and fermionic thermal Dark Matter interacting with the visible sector through the vector portal the 90% C.L. limits $10^{-11}\lesssim y \lesssim 10^{-6}$ on the dark-matter parameter $y = \epsilon^2 \alpha_D (\frac{m_\chi}{m_{A'}})^4 $ are obtained for the dark coupling constant $\alpha_D = 0.5$ and dark-matter masses $0.001 \lesssim m_\chi \lesssim 0.5 $ GeV. The lower limits $\alpha_D \gtrsim 10^{-3} $ for pseudo-Dirac Dark Matter in the mass region $m_\chi \lesssim 0.05 $ GeV are more stringent than the corresponding bounds from beam dump experiments. The results are obtained by using tree level, exact calculations of the $A'$ production cross-sections, which turn out to be significantly smaller compared to the one obtained in the Weizs\"{a}cker-Williams approximation for the mass region $m_{A'} \gtrsim 0.1$ GeV.

The $3.6\ensuremath{\sigma}$ discrepancy between the predicted and measured values of the anomalous magnetic moment of positive muons can be explained by the existence of a new dark boson ${Z}_{\ensuremath{\mu}}$ with a mass in the sub-GeV range, which is coupled predominantly to the second and third lepton generations through the ${L}_{\ensuremath{\mu}}\ensuremath{-}{L}_{\ensuremath{\tau}}$ current. After a discussion of the present phenomenological bounds on the ${Z}_{\ensuremath{\mu}}$ coupling, we show that if the ${Z}_{\ensuremath{\mu}}$ exists, it could be observed in the reaction $\ensuremath{\mu}+Z\ensuremath{\rightarrow}\ensuremath{\mu}+Z+{Z}_{\ensuremath{\mu}}$ of a muon scattering off nuclei by looking for an excess of events with large missing muon beam energy in a detector due to the prompt bremsstrahlung ${Z}_{\ensuremath{\mu}}$ decay ${Z}_{\ensuremath{\mu}}\ensuremath{\rightarrow}\ensuremath{\nu}\ensuremath{\nu}$ into a couple of neutrinos. We describe the experimental technique and the preliminary study of the feasibility for the proposed search. We show that this specific signal allows for a search for the ${Z}_{\ensuremath{\mu}}$ with a sensitivity in the coupling constant ${\ensuremath{\alpha}}_{\ensuremath{\mu}}\ensuremath{\gtrsim}1{0}^{\ensuremath{-}11}$, which is 3 orders of magnitude higher than the value required to explain the discrepancy. We point out that the availability of high-energy and -intensity muon beams at CERN SPS provides a unique opportunity to either discover or rule out the ${Z}_{\ensuremath{\mu}}$ in the proposed search in the near future. The experiment is based on the missing-energy approach developed for the searches for invisible decays of dark photons and (pseudo)scalar mesons at CERN and is complementary to these experiments.

The improved results on a direct search for a new $X(16.7\text{ }\text{ }\mathrm{MeV})$ boson that could explain the anomalous excess of ${e}^{+}{e}^{\ensuremath{-}}$ pairs observed in the decays of the excited $^{8}{\mathrm{Be}}^{*}$ nuclei (``Berillium or X17 anomaly'') are reported. Interestingly, new recent results in the nuclear transitions of another nucleus, $^{4}\mathrm{He}$, seems to support this anomaly spurring the need for an independent measurement. If the $X$ boson exists, it could be produced in the bremsstrahlung reaction ${e}^{\ensuremath{-}}Z\ensuremath{\rightarrow}{e}^{\ensuremath{-}}ZX$ by a high energy beam of electrons incident on the active target in the NA64 experiment at the CERN Super Proton Synchrotron and observed through its subsequent decay into ${e}^{+}{e}^{\ensuremath{-}}$ pairs. No evidence for such decays was found from the combined analysis of the data samples with total statistics corresponding to $8.4\ifmmode\times\else\texttimes\fi{}{10}^{10}$ electrons on target collected in 2017 and 2018. This allows one to set new limits on the $X\ensuremath{-}{e}^{\ensuremath{-}}$ coupling in the range $1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\ensuremath{\lesssim}{\ensuremath{\epsilon}}_{e}\ensuremath{\lesssim}6.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, excluding part of the parameter space favored by the X17 anomaly, and setting new bounds on the mixing strength of photons with dark photons (${A}^{\ensuremath{'}}$) with a mass $\ensuremath{\lesssim}24\text{ }\text{ }\mathrm{MeV}$. For the 2018 run, the setup was optimized to probe the region of parameter space characterized by a large coupling $\ensuremath{\epsilon}$. This allowed a significant improvement in sensitivity despite a relatively modest increase in statistics.

We carried out a model-independent search for light scalar (s) and pseudoscalar axionlike (a) particles that couple to two photons by using the high-energy CERN SPS H4 electron beam. The new particles, if they exist, could be produced through the Primakoff effect in interactions of hard bremsstrahlung photons generated by 100 GeV electrons in the NA64 active dump with virtual photons provided by the nuclei of the dump. The a(s) would penetrate the downstream HCAL module, serving as a shield, and would be observed either through their a(s)→γγ decay in the rest of the HCAL detector, or as events with a large missing energy if the a(s) decays downstream of the HCAL. This method allows for the probing of the a(s) parameter space, including those from generic axion models, inaccessible to previous experiments. No evidence of such processes has been found from the analysis of the data corresponding to 2.84×10^{11} electrons on target, allowing us to set new limits on the a(s)γγ-coupling strength for a(s) masses below 55 MeV.

A bstract We present results of the Relic Axion Dark-Matter Exploratory Setup (RADES), a detector which is part of the CERN Axion Solar Telescope (CAST), searching for axion dark matter in the 34.67 μ eV mass range. A radio frequency cavity consisting of 5 sub-cavities coupled by inductive irises took physics data inside the CAST dipole magnet for the first time using this filter-like haloscope geometry. An exclusion limit with a 95% credibility level on the axion-photon coupling constant of g aγ ≳ 4 × 10 − 13 GeV − 1 over a mass range of 34 . 6738 μ eV &lt; m a &lt; 34 . 6771 μ eV is set. This constitutes a significant improvement over the current strongest limit set by CAST at this mass and is at the same time one of the most sensitive direct searches for an axion dark matter candidate above the mass of 25 μ eV. The results also demonstrate the feasibility of exploring a wider mass range around the value probed by CAST-RADES in this work using similar coherent resonant cavities.

The CAST-CAPP axion haloscope, operating at CERN inside the CAST dipole magnet, has searched for axions in the 19.74 μeV to 22.47 μeV mass range. The detection concept follows the Sikivie haloscope principle, where Dark Matter axions convert into photons within a resonator immersed in a magnetic field. The CAST-CAPP resonator is an array of four individual rectangular cavities inserted in a strong dipole magnet, phase-matched to maximize the detection sensitivity. Here we report on the data acquired for 4124 h from 2019 to 2021. Each cavity is equipped with a fast frequency tuning mechanism of 10 MHz/ min between 4.774 GHz and 5.434 GHz. In the present work, we exclude axion-photon couplings for virialized galactic axions down to gaγγ = 8 × 10-14 GeV-1 at the 90% confidence level. The here implemented phase-matching technique also allows for future large-scale upgrades.

A light ${Z}^{\ensuremath{'}}$ vector boson coupled to the second and third lepton generations through the ${L}_{\ensuremath{\mu}}\ensuremath{-}{L}_{\ensuremath{\tau}}$ current with mass below 200 MeV provides a very viable explanation in terms of new physics to the recently confirmed $(g\ensuremath{-}2{)}_{\ensuremath{\mu}}$ anomaly. This boson can be produced in the bremsstrahlung reaction $\ensuremath{\mu}N\ensuremath{\rightarrow}\ensuremath{\mu}N{Z}^{\ensuremath{'}}$ after a high energy muon beam collides with a target. $\mathrm{NA}64\ensuremath{\mu}$ is a fixed-target experiment using a 160 GeV muon beam from the CERN Super Proton Synchrotron accelerator looking for ${Z}^{\ensuremath{'}}$ production and its subsequent decays, ${Z}^{\ensuremath{'}}\ensuremath{\rightarrow}\mathrm{invisible}$. In this paper, we present the study of the $\mathrm{NA}64\ensuremath{\mu}$ sensitivity to search for such a boson. This includes a realistic beam simulation, a detailed description of the detectors and a discussion about the main potential background sources. A pilot run is scheduled in order to validate the simulation results. If those are confirmed, $\mathrm{NA}64\ensuremath{\mu}$ will be able to explore all the remaining parameter space which could provide an explanation for the $g\ensuremath{-}2$ muon anomaly in the ${L}_{\ensuremath{\mu}}\ensuremath{-}{L}_{\ensuremath{\tau}}$ model.

The NOMAD experiment is a short base-line search for νμ − ντ oscillations in the CERN neutrino beam. The ντ's are searched for through their charged current interactions followed by the observation of the resulting τ− through its electronic, muonic or hadronic decays. These decays are recognized using kinematical criteria necessitating the use of a light target which enables the reconstruction of individual particles produced in the neutrino interactions. This paper describes the various components of the NOMAD detector: the target and muon drift chambers, the electromagnetic and hadronic calorimeters, the preshower and transition radiation detectors and the veto and trigger scintillation counters. The beam and data acquisition system are also described. The quality of the reconstruction and individual particles is demonstrated through the ability of NOMAD to observe Ks0's, Λ0's and π0's. Finally, the observation of τ− through its electronic decay being one of the most promising channels in the search, the identification of electrons in NOMAD is discussed.

It is shown that the 2.6 $\sigma$ discrepancy between the predicted and recently measured value of the anomalous magnetic moment of positive muons could be explained by the existence of a new light gauge boson X with a mass $M_X \leq O(5) GeV$. Phenomenological bounds on the X coupling are discussed.

The results of a new search for positronium decays into invisible final states are reported. Convincing detection of this decay mode would be a strong evidence for new physics beyond the standard model (SM): for example, the existence of extra--dimensions, of milli-charged particles, of new light gauge bosons or of mirror particles. Mirror matter could be a relevant dark matter candidate. In this paper the setup and the results of a new experiment are presented. In a collected sample of about $(6.31\ifmmode\pm\else\textpm\fi{}0.28)\ifmmode\times\else\texttimes\fi{}{10}^{6}$ orthopositronium decays, no evidence for invisible decays in an energy [0,80] keV was found and an upper limit on the branching ratio of orthopositronium $\mathrm{o}\mathrm{\text{\ensuremath{-}}}\mathrm{Ps}\ensuremath{\rightarrow}\mathrm{\text{invisible}}$ could be set: $\mathrm{Br}(\mathrm{o}\mathrm{\text{\ensuremath{-}}}\mathrm{Ps}\ensuremath{\rightarrow}\mathrm{\text{invisible}})&lt;4.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}(90%\mathrm{C}.\mathrm{L}.)$ Our results provide a limit on the photon mirror-photon mixing strength $ϵ\ensuremath{\le}1.55\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}(90%\mathrm{C}.\mathrm{L}.)$ and rule out particles lighter than the electron mass with a fraction ${Q}_{x}\ensuremath{\le}3.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ of the electron charge. Furthermore, upper limits on the branching ratios for the decay of parapositronium $\mathrm{Br}(\mathrm{p}\mathrm{\text{\ensuremath{-}}}\mathrm{Ps}\ensuremath{\rightarrow}\mathrm{\text{invisible}})\ensuremath{\le}4.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}(90%\mathrm{C}.\mathrm{L}.)$ and the direct annihilation $\mathrm{Br}({e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\mathrm{\text{invisible}})\ensuremath{\le}2.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}(90%\mathrm{C}.\mathrm{L}.)$ could be set.

The anomaly in the low-energy distribution of quasielastic neutrino events reported by the MiniBooNE Collaboration is discussed. We show that the observed excess of electronlike events could originate from the production and decay of a heavy neutrino (${\ensuremath{\nu}}_{h}$) in the MiniBooNE detector. The ${\ensuremath{\nu}}_{h}$ with the mass around 500 MeV is created by mixing in ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ neutral-current interactions and decays radiatively into $\ensuremath{\nu}\ensuremath{\gamma}$ with the lifetime ${\ensuremath{\tau}}_{{\ensuremath{\nu}}_{h}}\ensuremath{\lesssim}{10}^{\ensuremath{-}9}$ s due to a transition magnetic moment between the ${\ensuremath{\nu}}_{h}$ and a light neutrino $\ensuremath{\nu}$. Existing experimental data are found to be consistent with a mixing strength between the ${\ensuremath{\nu}}_{h}$ and the ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ of $|{U}_{\ensuremath{\mu}h}{|}^{2}\ensuremath{\simeq}(1\ensuremath{-}4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ and a ${\ensuremath{\nu}}_{h}$ transition magnetic moment of ${\ensuremath{\mu}}_{\mathrm{tr}}\ensuremath{\simeq}(1\ensuremath{-}6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{\ensuremath{\mu}}_{B}$. Finally, we discuss the reason why no significant excess of low-energy events has been observed in the recent antineutrino data.

This document reports on a series of experimental and theoretical studies conducted to assess the astro-particle physics potential of three future large-scale particle detectors proposed in Europe as next generation underground observatories. The proposed apparatus employ three different and, to some extent, complementary detection techniques: GLACIER (liquid Argon TPC), LENA (liquid scintillator) and MEMPHYS (\WC), based on the use of large mass of liquids as active detection media. The results of these studies are presented along with a critical discussion of the performance attainable by the three proposed approaches coupled to existing or planned underground laboratories, in relation to open and outstanding physics issues such as the search for matter instability, the detection of astrophysical- and geo-neutrinos and to the possible use of these detectors in future high-intensity neutrino beams.

The AEGIS experiment, currently being set up at the Antiproton Decelerator at CERN, has the objective of studying the free fall of antimatter in the Earth's gravitational field by means of a pulsed cold atomic beam of antihydrogen atoms. Both duration of free fall and vertical displacement of the horizontally emitted atoms will be measured, allowing a first test of the WEP with antimatter.

This work has attempted to reconcile puzzling neutrino oscillation results from the LSND, KARMEN, and MiniBooNE experiments. We show that the LSND evidence for ${\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}\ensuremath{\rightarrow}{\overline{\ensuremath{\nu}}}_{e}$ oscillations, its long-standing disagreement with the results from KARMEN, and the anomalous event excess observed by MiniBooNE in ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ and ${\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}$ data could all be explained by the existence of a heavy sterile neutrino (${\ensuremath{\nu}}_{h}$). All these results are found to be consistent with each other, assuming that the ${\ensuremath{\nu}}_{h}$ is created in ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ neutral-current interactions and decays radiatively into a photon and a light neutrino. Assuming the ${\ensuremath{\nu}}_{h}$ is produced through mixing with ${\ensuremath{\nu}}_{\ensuremath{\mu}}$, the combined analysis of the LSND and MiniBooNe excess events suggests that the ${\ensuremath{\nu}}_{h}$ mass is in the range from 40 to 80 MeV, the mixing strength is $|{U}_{\ensuremath{\mu}h}{|}^{2}\ensuremath{\simeq}{10}^{\ensuremath{-}3}--{10}^{\ensuremath{-}2}$, and the lifetime is ${\ensuremath{\tau}}_{{\ensuremath{\nu}}_{h}}\ensuremath{\lesssim}{10}^{\ensuremath{-}9}\text{ }\text{ }\mathrm{s}$. Surprisingly, this LSND-MiniBooNE parameter window is found to be unconstrained by the results from the most sensitive experiments. We set new limits on $|{U}_{\ensuremath{\mu}h}{|}^{2}$ for the favorable mass region from the precision measurements of the Michel spectrum by the TWIST experiment. The results obtained provide a strong motivation for a sensitive search for the ${\ensuremath{\nu}}_{h}$ in a near future $K$ decay or neutrino experiments, which fit well in the existing and planned experimental programs at CERN or FNAL. The question of whether the heavy neutrino is a Dirac or Majorana particle is briefly discussed.

A Short-Baseline Neutrino (SBN) physics program of three LAr-TPC detectors located along the Booster Neutrino Beam (BNB) at Fermilab is presented. This new SBN Program will deliver a rich and compelling physics opportunity, including the ability to resolve a class of experimental anomalies in neutrino physics and to perform the most sensitive search to date for sterile neutrinos at the eV mass-scale through both appearance and disappearance oscillation channels. Using data sets of 6.6e20 protons on target (P.O.T.) in the LAr1-ND and ICARUS T600 detectors plus 13.2e20 P.O.T. in the MicroBooNE detector, we estimate that a search for muon neutrino to electron neutrino appearance can be performed with ~5 sigma sensitivity for the LSND allowed (99% C.L.) parameter region. In this proposal for the SBN Program, we describe the physics analysis, the conceptual design of the LAr1-ND detector, the design and refurbishment of the T600 detector, the necessary infrastructure required to execute the program, and a possible reconfiguration of the BNB target and horn system to improve its performance for oscillation searches.

The precise measurement of forces is one way to obtain deep insight into the fundamental interactions present in nature. In the context of neutral antimatter, the gravitational interaction is of high interest, potentially revealing new forces that violate the weak equivalence principle. Here we report on a successful extension of a tool from atom optics--the moiré deflectometer--for a measurement of the acceleration of slow antiprotons. The setup consists of two identical transmission gratings and a spatially resolving emulsion detector for antiproton annihilations. Absolute referencing of the observed antimatter pattern with a photon pattern experiencing no deflection allows the direct inference of forces present. The concept is also straightforwardly applicable to antihydrogen measurements as pursued by the AEgIS collaboration. The combination of these very different techniques from high energy and atomic physics opens a very promising route to the direct detection of the gravitational acceleration of neutral antimatter.

In non-hadronic axion models, which have a tree-level axion-electron interaction, the Sun produces a strong axion flux by bremsstrahlung, Compton scattering, and axio-recombination, the ``BCA processes.'' Based on a new calculation of this flux, including for the first time axio-recombination, we derive limits on the axion-electron Yukawa coupling gae and axion-photon interaction strength gaγ using the CAST phase-I data (vacuum phase). For ma≲10 meV/c2 we find gaγ gae < 8.1 × 10−23 GeV−1 at 95% CL. We stress that a next-generation axion helioscope such as the proposed IAXO could push this sensitivity into a range beyond stellar energy-loss limits and test the hypothesis that white-dwarf cooling is dominated by axion emission.

In addition to gravity, there might be another very weak interaction between the ordinary and dark matter transmitted by U'(1) gauge bosons A' (dark photons) mixing with our photons. If such A's exist, they could be searched for in a light-shining-through-a-wall experiment with a high energy electron beam. The electron energy absorption in a calorimeter (CAL1) is accompanied by the emission of bremsstrahlung A's in the reaction eZ -> eZA' of electrons scattering on nuclei due to the \gamma - A' mixing. A part of the primary beam energy is deposited in the CAL1, while the rest of the energy is transmitted by the A' through the "CAL1 wall" and deposited in another downstream calorimeter CAL2 by the e+e- pair from the A'->e+e- decay in flight. Thus, the A's could be observed by looking for an excess of events with the two-shower signature generated by a single high energy electron in the CAL1 and CAL2. A proposal to perform such an experiment to probe the still unexplored area of the mixing strength 10^{-5} < \epsilon < 10^{-3} and masses M_{A'} < 100 MeV by using 10-300 GeV electron beams from the CERN SPS is presented. The experiment can provide complementary coverage of the parameter space, which is intended to be probed by other searches. It has also a capability for a sensitive search for A's decaying invisibly to dark-sector particles, such as dark matter, which could cover a significant part of the still allowed parameter space.

We report an early result from the ICARUS experiment on the search for a ν μ →ν e signal due to the LSND anomaly. The search was performed with the ICARUS T600 detector located at the Gran Sasso Laboratory, receiving CNGS neutrinos from CERN at an average energy of about 20 GeV, after a flight path of ∼730 km. The LSND anomaly would manifest as an excess of ν e events, characterized by a fast energy oscillation averaging approximately to $\sin^{2}(1.27\Delta m^{2}_{\mathrm{new}}L/E_{\nu})\approx 1/2$ with probability $P_{\nu_{\mu}\rightarrow \nu_{e}} = 1/2 \sin^{2}(2\theta_{\mathrm{new}})$ . The present analysis is based on 1091 neutrino events, which are about 50 % of the ICARUS data collected in 2010–2011. Two clear ν e events have been found, compared with the expectation of 3.7±0.6 events from conventional sources. Within the range of our observations, this result is compatible with the absence of a LSND anomaly. At 90 % and 99 % confidence levels the limits of 3.4 and 7.3 events corresponding to oscillation probabilities $\langle P_{\nu_{\mu}\rightarrow \nu_{e}}\rangle \le 5.4 \times 10^{-3}$ and $\langle P_{\nu_{\mu}\rightarrow \nu_{e}}\rangle \le 1.1 \times 10^{-2} $ are set respectively. The result strongly limits the window of open options for the LSND anomaly to a narrow region around (Δm 2,sin2(2θ))new=(0.5 eV2,0.005), where there is an overall agreement (90 % CL) between the present ICARUS limit, the published limits of KARMEN and the published positive signals of LSND and MiniBooNE Collaborations.

We report an updated result from the ICARUS experiment on the search for ν μ →ν e anomalies with the CNGS beam, produced at CERN with an average energy of 20 GeV and traveling 730 km to the Gran Sasso Laboratory. The present analysis is based on a total sample of 1995 events of CNGS neutrino interactions, which corresponds to an almost doubled sample with respect to the previously published result. Four clear ν e events have been visually identified over the full sample, compared with an expectation of 6.4±0.9 events from conventional sources. The result is compatible with the absence of additional anomalous contributions. At 90 % and 99 % confidence levels, the limits to possible oscillated events are 3.7 and 8.3 respectively. The corresponding limit to oscillation probability becomes consequently 3.4×10−3 and 7.6×10−3, respectively. The present result confirms, with an improved sensitivity, the early result already published by the ICARUS Collaboration.

In the Lμ−Lτ model the 3.6 σ discrepancy between the predicted and measured values of the anomalous magnetic moment of positive muons can be explained by the existence of a new dark boson Z′ with a mass in the sub-GeV range, which is coupled at tree level predominantly to the second and third lepton generations. However, at the one-loop level the Z′ coupling to electrons or quarks can be induced via the γ−Z′ kinetic mixing, which is generated through the loop involving the muon and tau lepton. This loophole has important experimental consequences since it opens up new possibilities for the complementary searches of the Z′ with high-energy electron beams, in particular in the ongoing NA64 and incoming dark photon experiments. An extension of the model able to explain relic Dark Matter density is also discussed.

We demonstrate the laser excitation of the n = 3 state of positronium (Ps) in vacuum.A combination of a specially designed pulsed slow positron beam and a high-efficiency converter target was used to produce Ps.Its annihilation was recorded by single-shot positronium annihilation lifetime spectroscopy.Pulsed laser excitation of the n = 3 level at a wavelength λ ≈ 205 nm was monitored via Ps photoionization induced by a second intense laser pulse at λ = 1064 nm.About 15% of the overall positronium emitted into vacuum was excited to n = 3 and photoionized.Saturation of both the n = 3 excitation and the following photoionization was observed and explained by a simple rate equation model.The positronium's transverse temperature was extracted by measuring 2469

We discuss prospects of searching for a dark photon (A′) which serves as mediator between Standard model (SM) particles and light dark matter (LDM) by using the combined results from the NA64 experiment at the CERN SPS running in high-energy electron (NA64e) and muon (NA64μ) modes. We discuss the most natural values and upper bounds on the A′ coupling constant to LDM and show they are lying in the range accessible at NA64. While for the projected 5×1012 electrons on target (EOT) NA64e is able to probe the scalar and Majorana LDM scenarios, the combined NA64e and NA64μ results with ≃1013 EOT and a few 1013 MOT, respectively, will allow covering significant region in the parameter space of the most interesting LDM models. This makes NA64e and NA64μ extremely complementary to each other and increases significantly the discovery potential of sub-GeV DM.

A bstract This article describes BabyIAXO, an intermediate experimental stage of the International Axion Observatory (IAXO), proposed to be sited at DESY. IAXO is a large-scale axion helioscope that will look for axions and axion-like particles (ALPs), produced in the Sun, with unprecedented sensitivity. BabyIAXO is conceived to test all IAXO subsystems (magnet, optics and detectors) at a relevant scale for the final system and thus serve as prototype for IAXO, but at the same time as a fully-fledged helioscope with relevant physics reach itself, and with potential for discovery. The BabyIAXO magnet will feature two 10 m long, 70 cm diameter bores, and will host two detection lines (optics and detector) of dimensions similar to the final ones foreseen for IAXO. BabyIAXO will detect or reject solar axions or ALPs with axion-photon couplings down to g aγ ∼ 1 . 5 × 10 − 11 GeV − 1 , and masses up to m a ∼ 0 . 25 eV. BabyIAXO will offer additional opportunities for axion research in view of IAXO, like the development of precision x-ray detectors to identify particular spectral features in the solar axion spectrum, and the implementation of radiofrequency-cavity-based axion dark matter setups.

The current most stringent constraints for the existence of sub-GeV dark matter coupling to Standard Model via a massive vector boson A′ were set by the NA64 experiment for the mass region mA′≲250 MeV, by analyzing data from the interaction of 2.84×1011 100-GeV electrons with an active thick target and searching for missing-energy events. In this work, by including A′ production via secondary positron annihilation with atomic electrons, we extend these limits in the 200–300 MeV region by almost an order of magnitude, touching for the first time the dark matter relic density constrained parameter combinations. Our new results demonstrate the power of the resonant annihilation process in missing energy dark-matter searches, paving the road to future dedicated e+ beam efforts.Received 16 August 2021Accepted 15 October 2021DOI:https://doi.org/10.1103/PhysRevD.104.L091701Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasDark matterExtensions of gauge sectorParticle dark matterPhysical SystemsPositronsTechniquesElectromagnetic calorimetersParticle productionParticles & Fields

We report on a search for heavy neutrinos ($\nus$) produced in the decay $D_s\to τ\nus$ at the SPS proton target followed by the decay $\nudecay$ in the NOMAD detector. Both decays are expected to occur if $\nus$ is a component of $ν_τ$.\ From the analysis of the data collected during the 1996-1998 runs with $4.1\times10^{19}$ protons on target, a single candidate event consistent with background expectations was found. This allows to derive an upper limit on the mixing strength between the heavy neutrino and the tau neutrino in the $\nus$ mass range from 10 to 190 $\rm MeV$. Windows between the SN1987a and Big Bang Nucleosynthesis lower limits and our result are still open for future experimental searches. The results obtained are used to constrain an interpretation of the time anomaly observed in the KARMEN1 detector.\

Results from the ντ appearance search in a neutrino beam using the full NOMAD data sample are reported. A new analysis unifies all the hadronic τ decays, significantly improving the overall sensitivity of the experiment to oscillations. The “blind analysis” of all topologies yields no evidence for an oscillation signal. In the two-family oscillation scenario, this sets a 90% CL allowed region in the sin22θμτ–Δm2 plane which includes sin22θμτ<3.3×10−4 at large Δm2 and Δm2< 0.7 eV2/c4 at sin22θμτ=1. The corresponding contour in the νe→ντ oscillation hypothesis results in sin22θeτ<1.5×10−2 at large Δm2 and Δm2<5.9 eV2/c4 at sin22θeτ=1. We also derive limits on effective couplings of the τ lepton to νμ or νe.

We present a measurement of the muon neutrino–nucleon inclusive charged current cross section, off an isoscalar target, in the neutrino energy range 2.5⩽Eν⩽40GeV. The significance of this measurement is its precision, ±4% in 2.5⩽Eν⩽10GeV, and ±2.6% in 10⩽Eν⩽40GeV regions, where significant uncertainties in previous experiments still exist, and its importance to the current and proposed long baseline neutrino oscillation experiments.

We report new limits on the ${\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}\ensuremath{\gamma}X$ decay of the neutral pion into a photon and a new gauge boson $X$ followed by the decay $X\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}$. If this process exists, one would expect a flux of high energy $X$'s produced from ${\ensuremath{\pi}}^{0}$'s generated by the proton beam in a neutrino target. The $X$'s would then penetrate the downstream shielding and be observed in a neutrino detector via their decays. Using bounds from the NOMAD and PS191 neutrino experiments at CERN that searched for an excess of ${e}^{+}{e}^{\ensuremath{-}}$ pairs from heavy neutrino decays, stringent limits on the branching ratio as small as $\mathrm{Br}({\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}\ensuremath{\gamma}X)\ensuremath{\lesssim}{10}^{\ensuremath{-}15}$ are obtained. These limits are several orders of magnitude smaller than the previous experimental and cosmological bounds. The obtained results are used to constrain models, where the $X$ interacts with quarks and leptons, or it is a new vector boson mixing with photons that transmits interaction between our world and hidden sectors consisting of $SU(3{)}_{C}\ifmmode\times\else\texttimes\fi{}SU(2{)}_{L}\ifmmode\times\else\texttimes\fi{}U(1{)}_{Y}$ singlet fields.

We present our new measurement of the cross-section for charm dimuon production in neutrino–iron interactions based upon the full statistics collected by the NOMAD experiment. After background subtraction we observe 15 344 charm dimuon events, providing the largest sample currently available. The analysis exploits the large inclusive charged current sample – about 9×106 events after all analysis cuts – and the high resolution NOMAD detector to constrain the total systematic uncertainty on the ratio of charm dimuon to inclusive Charged Current (CC) cross-sections to ∼2%. We also perform a fit to the NOMAD data to extract the charm production parameters and the strange quark sea content of the nucleon within the NLO QCD approximation. We obtain a value of mc(mc)=1.159±0.075 GeV/c2 for the running mass of the charm quark in the MS¯ scheme and a strange quark sea suppression factor of κs=0.591±0.019 at Q2=20 GeV2/c2.

The CERN-SPS accelerator has been briefly operated in a new, lower intensity neutrino mode with ~10^12 p.o.t. /pulse and with a beam structure made of four LHC-like extractions, each with a narrow width of 3 ns, separated by 524 ns. This very tightly bunched beam structure represents a substantial progress with respect to the ordinary operation of the CNGS beam, since it allows a very accurate time-of-flight measurement of neutrinos from CERN to LNGS on an event-to-event basis. The ICARUS T600 detector has collected 7 beam-associated events, consistent with the CNGS delivered neutrino flux of 2.2 10^16 p.o.t. and in agreement with the well known characteristics of neutrino events in the LAr-TPC. The time of flight difference between the speed of light and the arriving neutrino LAr-TPC events has been analysed. The result is compatible with the simultaneous arrival of all events with equal speed, the one of light. This is in a striking difference with the reported result of OPERA that claimed that high energy neutrinos from CERN should arrive at LNGS about 60 ns earlier than expected from luminal speed.

detectors and shielding, we returned to 4 He in 2012 to investigate a narrow m a range around 0.2 eV ("candidate setting" of our earlier search) and 0.39-0.42eV, the upper axion mass range reachable with 4 He, to "cross the axion line" for the KSVZ model.We have improved the limit on the axion-photon coupling to g aγ < 1.47 × 10 -10 GeV -1 (95% C.L.), depending on the pressure settings.Since 2013, we have returned to the vacuum and aim for a significant increase in sensitivity.

Dark photon ($A'$) that couples to the standard model fermions via the kinetic mixing with photons and serves as a mediator of dark matter production could be observed in the high-energy electron scattering $e^- + Z ~\rightarrow e^- + Z + A'$ off nuclei followed by the $A' \to invisible $ decay. We have performed the exact, tree-level calculations of the $A'$ production cross sections and implemented them in the program for the full simulation of such events in the experiment NA64 at the CERN SPS. Using simulations results, we study the missing energy signature for the bremsstrahlung $A' \rightarrow $ invisible decay that permits the determination of the $\gamma-A'$ mixing strength in a wide, from sub-MeV to sub-GeV, $A'$ mass range. We refine and expand our earlier studies of this signature for discovering $A'$ by including corrections to the previously used calculations based on the improved Weizsaker-Williams approximation, which turn out to be significant. We compare our cross sections values with the results from other calculations and find a good agreement between them. The possibility of future measurements with high-energy electron beams and the sensitivity to $A'$ are briefly discussed.

The dark photon ($A'$) production through the mixing with the bremsstrahlung photon from the electron scattering off nuclei can be accompanied by the dominant invisible $A'$ decay into dark-sector particles. In this work we discuss the missing energy signature of this process in the experiment NA64 aiming at the search for $A'\to invisible$ decays with a high-energy electron beam at the CERN SPS. We show the distinctive distributions of variables that can be used to distinguish the $A'\to invisible$ signal from background. The results of the detailed simulation of the detector response for the events with and without $A'$ emission are presented. The efficiency of the signal event selection is estimated. It is used to evaluate the sensitivity of the experiment and show that it allows to probe the still unexplored area of the mixing strength $10^{-6}\lesssim \epsilon \lesssim 10^{-2}$ and masses up to $M_{A'} \lesssim 1$ GeV. The results obtained are compared with the results from other calculations. In the case of the signal observation, a possibility of extraction of the parameters $M_{A'}$ and $\epsilon$ by using the missing energy spectrum shape is discussed. We consider as an example the $A'$ with the mass 16.7 MeV and mixing $\epsilon \lesssim 10^{-3}$, which can explain an excess of events recently observed in nuclear transitions of an excited state of $^8$Be. We show that if such $A'$ exists its invisible decay can be observed in NA64 within a month of running, while data accumulated during a few months would allow also to determine the $\epsilon$ and $M_{A'}$ parameters.

We performed a search for a new generic $X$ boson, which could be a scalar ($S$), pseudoscalar ($P$), vector ($V$) or an axial vector ($A$) particle produced in the 100 GeV electron scattering off nuclei, $e^- Z \to e^- Z X$, followed by its invisible decay in the NA64 experiment at CERN. No evidence for such process was found in the full NA64 data set of $2.84\times 10^{11}$ electrons on target. We place new bounds on the $S, P, V, A$ coupling strengths to electrons, and set constraints on their contributions to the electron anomalous magnetic moment $a_e$, $|\Delta a_{X}| \lesssim 10^{-15} - 10^{-13}$ for the $X$ mass region $m_X\lesssim 1$ GeV. These results are an order of magnitude more sensitive compared to the current accuracy on $a_e$ from the electron $g-2$ experiments and recent high-precision determination of the fine structure constant.

A search for a new Z^{'} gauge boson associated with (un)broken B-L symmetry in the keV-GeV mass range is carried out for the first time using the missing-energy technique in the NA64 experiment at the CERN SPS. From the analysis of the data with 3.22×10^{11} electrons on target collected during 2016-2021 runs, no signal events were found. This allows us to derive new constraints on the Z^{'}-e coupling strength, which, for the mass range 0.3≲m_{Z^{'}}≲100 MeV, are more stringent compared to those obtained from the neutrino-electron scattering data.

In addition to vector ($V$) type new particles extensively discussed previously, both $CP$-even ($S$) and $CP$-odd ($P$) spin-0 dark matter (DM) mediators can couple to muons and be produced in the bremsstrahlung reaction ${\ensuremath{\mu}}^{\ensuremath{-}}+N\ensuremath{\rightarrow}{\ensuremath{\mu}}^{\ensuremath{-}}+N+S(P)$. Their possible subsequent invisible decay into a pair of Dirac DM particles, $S(P)\ensuremath{\rightarrow}\ensuremath{\chi}\overline{\ensuremath{\chi}}$, can be detected in fixed target experiments through missing energy signature. In this paper, we focus on the case of experiments using high-energy muon beams. For this reason, we derive the differential cross sections involved using the phase space Weisz\"acker-Williams approximation and compare them to the exact-tree-level calculations. The formalism derived can be applied in various experiments that could observe muon-spin-0 DM interactions. This can happen in present and future proton beam-dump experiments such as NA62, SHIP, HIKE, and SHADOWS; in muon fixed target experiments as $\mathrm{NA}64\ensuremath{\mu}$, MUonE and M3; in neutrino experiments using powerful proton beams such as DUNE. In particular, we focus on the $\mathrm{NA}64\ensuremath{\mu}$ experiment case, which uses a 160 GeV muon beam at the CERN Super Proton Synchrotron accelerator. We compute the derived cross sections, the resulting signal yields and we discuss the experiment projected sensitivity to probe the relic DM parameter space and the $(g\ensuremath{-}2{)}_{\ensuremath{\mu}}$ anomaly favored region considering ${10}^{11}$ and ${10}^{13}$ muons on target.

The Λ polarization in νμ charged current interactions has been measured in the NOMAD experiment. The event sample (8087 reconstructed Λ 's) is more than an order of magnitude larger than that of previous bubble chamber experiments, while the quality of event reconstruction is comparable. We observe negative polarization along the W -boson direction which is enhanced in the target fragmentation region: Px(xF<0)=−0.21±0.04(stat)±0.02(sys) . In the current fragmentation region we find Px(xF>0)=−0.09±0.06(stat)±0.03(sys) . These results provide a test of different models describing the nucleon spin composition and the spin transfer mechanisms. A significant transverse polarization (in the direction orthogonal to the Λ production plane) has been observed for the first time in a neutrino experiment: Py=−0.22±0.03(stat)±0.01(sys) . The dependence of the absolute value of Py on the Λ transverse momentum with respect to the hadronic jet direction is in qualitative agreement with the results from unpolarized hadron–hadron experiments.

A study has been made of the characteristics of electromagnetic calorimeter modules with the structure (1.4 mm Pb + 4 mm Sc)×60 layers using 72 wavelength shifting fibers for readout. These modules were made according to a specially developed technology and were tested with a beam of electrons, pions and muons with energies between 0.5 and 5 GeV. It was found that their energy resolution is σE/E = 0.014+0.067/E[GeV], response nonuniformity <2%, and π/e rejection at the level 10−2 to 10−3.

It has been recently suggested that the vacuum magnetic dichroism observed by the PVLAS experiment could be explained by pair production of a new light, ${m}_{\ensuremath{\epsilon}}\ensuremath{\simeq}0.1\text{ }\text{ }\mathrm{eV}$, millicharged, ${q}_{\ensuremath{\epsilon}}\ensuremath{\simeq}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}e$, fermions ($\ensuremath{\epsilon}$). In addition, it has been pointed out that millicharged particles with ${q}_{\ensuremath{\epsilon}}\ensuremath{\gtrsim}{10}^{\ensuremath{-}9}e$ appear naturally in models based on the string theory. We show that low energy reactor neutrino experiments provide a sensitive probe of millicharged particles. Considering, as an example, recent results of the TEXONO experiment searching for the neutrino magnetic moment, a new upper bound ${q}_{\ensuremath{\epsilon}}\ensuremath{\lesssim}{10}^{\ensuremath{-}5}e$ for the mass region ${m}_{\ensuremath{\epsilon}}&lt;1\text{ }\text{ }\mathrm{keV}$ is derived. These results enhance motivations for a more sensitive search for such particles in near future experiments. Furthermore, a direct experimental limit on the electric charge of the electron antineutrino ${q}_{{\overline{\ensuremath{\nu}}}_{e}}&lt;3.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}e$ is obtained.

An experiment studying the radiative pion decay, π− → e−vγ, has been performed with a secondary 17 GeV negative pion beam on the IHEP machine with the ISTRA facility of the Institute for Nuclear Research. The high energy beam has enabled us to investigate this decay in a wide range of kinematic variables: Eγ > 21 MeV, Ee > 70−0.8Eγ MeV, which included events with θeγ > 60°. The axial-to-vector form factor ratio has been determined unambiguously: γ=0.41±0.023. The vector form factor has been determined in a model independent way: |Fv|=0.014±0.009. The probability of the π− → e−v γ decay was found to be BR = (1.61±0.23) × 10−7 for the region under consideration. The contributions of the inner bremsstrahlung and of the structure dependent radiation were investigated. A possible interpretation is discussed.

The SU c (3) ⊗ SU L (2) ⊗ SU R (2) ⊗ U(1) left-right (LR) symmetric model explains the origin of the parity violation in weak interactions and predicts the existence of additional gauge bosons W R and Z′. In addition, heavy right-handed Majorana neutrino states N arise naturally within the LR symmetric model. The states N could be partners of light neutrino states, related to their nonzero masses through the seesaw mechanism. This makes the searches for W R , Z′, and N interesting and important. In the framework of the minimal LR model, we study the possibility to observe signals from N and W R production in pp collisions after three years of running at low LHC luminosity. We show that their decay signals can be identified with a small background, especially in the case of same-sign leptons in the final state. For the integral LHC luminosity of L t = 30 fb−1, the 5σ discovery of W R boson and heavy Majorana neutrinos N e with masses $$M_{W_R } $$ up to 4 TeV and $$M_{N_e } $$ up to 2.4 TeV, respectively, is found to be possible.

This chapter of the "Flavor in the era of LHC" workshop report discusses flavor-related issues in the production and decays of heavy states at the LHC at high momentum transfer Q, both from the experimental and the theoretical perspective. We review top quark physics, and discuss the flavor aspects of several extensions of the standard model, such as supersymmetry, little Higgs models or models with extra dimensions. This includes discovery aspects, as well as the measurement of several properties of these heavy states. We also present publicly available computational tools related to this topic.

The main goal of the AEgIS experiment at CERN is to test the weak equivalence principle for antimatter. We will measure the Earth's gravitational acceleration g¯ with antihydrogen atoms being launched in a horizontal vacuum tube and traversing a moiré deflectometer. We intend to use a position sensitive device made of nuclear emulsions (combined with a time-of-flight detector such as silicon μ-strips) to measure precisely their annihilation points at the end of the tube. The goal is to determine g¯ with a 1% relative accuracy. In 2012 we tested emulsion films in vacuum and at room temperature with low energy antiprotons from the CERN antiproton decelerator. First results on the expected performance for AEgIS are presented.

The ICARUS T600 detector, the largest liquid Argon Time Projection Chamber (LAr-TPC) realized after many years of RD activities, was installed and successfully operated for 3 years at the INFN Gran Sasso underground Laboratory. One of the most important issues was the need of an extremely low residual electronegative impurity content in the liquid Argon, in order to transport the free electrons created by the ionizing particles with a very small attenuation along the drift path. The solutions adopted for the Argon re-circulation and purification systems have permitted to reach impressive results in terms of Argon purity and a free electron lifetime exceeding 15 ms, corresponding to about 20 parts per trillion of equivalent O2 contamination, a milestone for any future project involving LAr-TPC's and the development of higher detector mass scales.

We describe a system designed to re-bunch positron pulses delivered by an accumulator supplied by a positron source and a Surko-trap. Positron pulses from the accumulator are magnetically guided in a 0.085 T field and are injected into a region free of magnetic fields through a μ-metal field terminator. Here positrons are temporally compressed, electrostatically guided and accelerated towards a porous silicon target for the production and emission of positronium into vacuum. Positrons are focused in a spot of less than 4 mm FWTM in bunches of ∼8 ns FWHM. Emission of positronium into the vacuum is shown by single shot positron annihilation lifetime spectroscopy.

This report, based on the Dark Sectors workshop at SLAC in April 2016, summarizes the scientific importance of searches for dark sector dark matter and forces at masses beneath the weak-scale, the status of this broad international field, the important milestones motivating future exploration, and promising experimental opportunities to reach these milestones over the next 5-10 years.

Thermal dark matter models with particle χ masses below the electroweak scale can provide an explanation for the observed relic dark matter density. This would imply the existence of a new feeble interaction between the dark and ordinary matter. We report on a new search for the sub-GeV χ production through the interaction mediated by a new vector boson, called the dark photon A^{'}, in collisions of 100 GeV electrons with the active target of the NA64 experiment at the CERN SPS. With 9.37×10^{11} electrons on target collected during 2016-2022 runs NA64 probes for the first time the well-motivated region of parameter space of benchmark thermal scalar and fermionic dark matter models. No evidence for dark matter production has been found. This allows us to set the most sensitive limits on the A^{'} couplings to photons for masses m_{A^{'}}≲0.35 GeV, and to exclude scalar and Majorana dark matter with the χ-A^{'} coupling α_{D}≤0.1 for masses 0.001≲m_{χ}≲0.1 GeV and 3m_{χ}≤m_{A^{'}}.

We suggest that the discrepant lifetime measurements of ortho-positronium can be explained by ortho-positronium oscillations into mirror ortho-positronium. This explanation can be tested in future vacuum experiments.

We have searched for 14.4 keV solar axions or more general axion-like particles (ALPs), that may be emitted in the M1 nuclear transition of 57Fe, by using the axion-to-photon conversion in the CERN Axion Solar Telescope (CAST) with evacuated magnet bores (Phase I). From the absence of excess of the monoenergetic X-rays when the magnet was pointing to the Sun, we set model-independent constraints on the coupling constants of pseudoscalar particles that couple to two photons and to a nucleon gaγ|−1.19gaN0+gaN3| < 1.36 × 10−16 GeV−1 for ma < 0.03 eV at the 95% confidence level.

A bstract During May 2012, the CERN-CNGS neutrino beam has been operated for two weeks for a total of ~1.8 × 10 17 p.o.t., with the proton beam made of bunches, few ns wide and separated by 100 ns. This beam structure allows a very accurate time of flight measurement of neutrinos from CERN to LNGS on an event-by-event basis. Both the ICARUS-T600 PMT-DAQ and the CERN-LNGS timing synchronization have been substantially improved for this campaign, taking advantage of additional independent GPS receivers, both at CERN and LNGS as well as of the deployment of the “White Rabbit” protocol both at CERN and LNGS. The ICARUS-T600 detector has collected 25 beam-associated events; the corresponding time of flight has been accurately evaluated, using all different time synchronization paths. The measured neutrino time of flight is compatible with the arrival of all events with speed equivalent to the one of light: the difference between the expected value based on the speed of light and the measured value is δt = tof c −tof ν = 0.10 ± 0.67 stat. ± 2.39 syst. ns. This result is in agreement with the value previously reported by the ICARUS Collaboration, δt = 0.3 ± 4.9 stat. ± 9.0 syst. ns, but with improved statistical and systematic accuracy.

The main goal of the AEgIS experiment at CERN is to test the weak equivalence principle for antimatter. AEgIS will measure the free-fall of an antihydrogen beam traversing a moir&apos;e deflectometer. The goal is to determine the gravitational acceleration with an initial relative accuracy of 1% by using an emulsion detector combined with a silicon μ-strip detector to measure the time of flight. Nuclear emulsions can measure the annihilation vertex of antihydrogen atoms with a precision of ∼ 1–2 μm r.m.s. We present here results for emulsion detectors operated in vacuum using low energy antiprotons from the CERN antiproton decelerator. We compare with Monte Carlo simulations, and discuss the impact on the AEgIS project.

Liquid Argon Time Projection Chamber (LAr TPC) detectors offer charged particle imaging capability with remarkable spatial resolution. Precise event reconstruction procedures are critical in order to fully exploit the potential of this technology. In this paper we present a new, general approach of three-dimensional reconstruction for the LAr TPC with a practical application to track reconstruction. The efficiency of the method is evaluated on a sample of simulated tracks. We present also the application of the method to the analysis of real data tracks collected during the ICARUS T600 detector operation with the CNGS neutrino beam.

In the standard model the rate of the $\pi^0, \eta, \eta', K_S, K_L\to \nu \overline{\nu}$ decays is predicted to be extremely small. Therefore, observation of any of these mesons ($M^0$) decaying into an invisible final state would signal the presence of new physics. The Bell-Steinberger relation connects CP and CPT violation in the mass matrix to CP and CPT violation in all decay channels of neutral kaons. It is a powerful tool for testing CPT invariance in the $K^0-\overline{K}^0$ system, assuming that there are no significant undiscovered decay modes of either $K_S$ or $K_L$ which could contribute to the precision of the results. The $K_S,K_L\to invisible$ decays have never been tested and the question of how much these decays can influence the Bell-Steinberger analysis of the kaon system still remains open. In the present work we propose a new experiment to search for the $M^0\to invisible$ decays which aims at probing new physics and answering this question. The experiment utilizes high energy hadronic beams from the CERN SPS and the charge exchange reactions of pion or kaon on nucleons of an active target, e.g. $\pi^- (K^-) + p\to M^0 + n $, as a source of the well-tagged $M^0$s emitted with the beam energy. If the decay $M^0\to invisible$ exists, it could be observed by looking for an excess of events with a specific signature: the complete disappearance of the beam energy in the detector. This unique signal of $M^0\to invisible$ decays allows for searches of the decays $K_S,K_L\to invisible$ with a sensitivity in branching ratio Br$(K_S (K_L)\to invisible) \lesssim 10^{-8} (10^{-6})$, and $\pi^0,\eta, \eta' \to invisible$ decays with a sensitivity a few orders of magnitude beyond the present experimental limits. This experiment is complementary to the one recently proposed for the search for invisible decays of dark photons and fits well with the present kaon program at CERN.

The OPERA collaboration has claimed evidence of superluminal {\nu}{_\mu} propagation between CERN and the LNGS. Cohen and Glashow argued that such neutrinos should lose energy by producing photons and e+e- pairs, through Z0 mediated processes analogous to Cherenkov radiation. In terms of the parameter delta=(v^2_nu-v^2_c)/v^2_c, the OPERA result implies delta = 5 x 10^-5. For this value of \delta a very significant deformation of the neutrino energy spectrum and an abundant production of photons and e+e- pairs should be observed at LNGS. We present an analysis based on the 2010 and part of the 2011 data sets from the ICARUS experiment, located at Gran Sasso National Laboratory and using the same neutrino beam from CERN. We find that the rates and deposited energy distributions of neutrino events in ICARUS agree with the expectations for an unperturbed spectrum of the CERN neutrino beam. Our results therefore refute a superluminal interpretation of the OPERA result according to the Cohen and Glashow prediction for a weak current analog to Cherenkov radiation. In particular no superluminal Cherenkov like e+e- pair or gamma emission event has been directly observed inside the fiducial volume of the "bubble chamber like" ICARUS TPC-LAr detector, setting the much stricter limit of delta < 2.5 10^-8 at the 90% confidence level, comparable with the one due to the observations from the SN1987A.

We discuss the physics case for and the concept of a medium-scale axion helioscope with sensitivities in the axion-photon coupling a few times better than CERN Axion Solar Telescope (CAST). Search for an axion-like particle with these couplings is motivated by several persistent astrophysical anomalies. We present early conceptual design, existing infrastructure, projected sensitivity and timeline of such a helioscope (Troitsk Axion Solar Telescope Experiment, TASTE) to be constructed in the Institute for Nuclear Research, Troitsk, Russia. The proposed instrument may be also used for the search of dark-matter halo axions.

The NA64 experiment consists of two detectors which are planned to be located at the electron (NA64e) and muon (NA64μ) beams of the CERN SPS and start operation after the LHC long-stop 2 in 2021. Its main goals include searches for dark sector physics—particularly light dark matter (LDM), visible and invisible decays of dark photons ( $$A{\kern 1pt} '$$ ), and new light particles that could explain the 8Be and $${{g}_{\mu }} - 2$$ anomalies. Here we review these physics goals, the current status of NA64 including recent results and perspectives of further searches, as well as other ongoing or planned experiments in this field. The main theoretical results on LDM, the problem of the origin of the $$\gamma {\text{ - }}A{\kern 1pt} '$$ mixing term and its connection to loop corrections, possible existence of a new light $$Z{\kern 1pt} '$$ coupled to $${{L}_{\mu }} - {{L}_{\tau }}$$ current are also discussed.

NOMAD is a neutrino oscillation experiment designed to search for ντ appearance in the CERN-SPS wide band νμ beam. Signal detection relies on the identification of ντ charged current interactions using kinematic criteria. The analysis of the 1995 data sample yields no oscillation signal. Combining all studied τ decay modes, a limit of sin22θμτ<4.2×10−3 is obtained for large Δm2 at the 90% confidence level.

The method developed for the calculation of the flux and composition of the West Area Neutrino Beam used by NOMAD in its search for neutrino oscillations is described. The calculation is based on particle production rates computed using a recent version of FLUKA and modified to take into account the cross-sections measured by the SPY and NA20 experiments. These particles are propagated through the beam line taking into account the material and magnetic fields they traverse. The neutrinos produced through their decays are tracked to the NOMAD detector. The fluxes of the four neutrino flavours at NOMAD are predicted with an uncertainty of about 8% for νμ and νe, 10% for ν̄μ, and 12% for ν̄e. The energy-dependent uncertainty achieved on the νe/νμ prediction needed for a νμ→νe oscillation search ranges from 4% to 7%, whereas the overall normalization uncertainty on this ratio is 4.2%.

If mixing between orthopositronium (o-Ps) and its mirror analogue o-Ps′ exists it could be the origin of the present discrepancy between theory and experiment in the o-Ps lifetime in vacuum. It is pointed out that a recent experiment in search of the invisible decay of o-Ps provides considerably less stringent limits both on branching ratio for decay of o-Ps as mirror matter in vacuum, BR(o-Ps→o-Ps′) < 10−2, and on photon-mirror photon mixing strength ϵ<10−6 than was claimed. The new experimental limits BR(o-Ps→o-Ps′)<10−3 and ϵ<2.8×10−7 were obtained from measurements of the o-Ps lifetime in vacuum and low pressure gases.

The amplitude of the signal collected from the PbWO4 crystals of the CMS electromagnetic calorimeter is reconstructed by a digital filtering technique. The amplitude reconstruction has been studied with test beam data recorded from a fully equipped barrel supermodule. Issues specific to data taken in the test beam are investigated, and the implementation of the method for CMS data taking is discussed.

It has been recently shown that excess events observed by the LSND and MiniBooNE neutrino experiments could be interpreted as a signal from the radiative decay of a heavy sterile neutrino νh produced in νμ neutral-current-like neutrino interactions. If the νh exist, it would be also produced by the νμ beam from the CERN SPS in the neutrino beam line shielding. The νh's would penetrate the shielding and be observed through the νh→γν decay followed by the photon conversion into e+e− pair in the active target of the NOMAD detector. The νh's could be also produced in the iron of the magnetic spectrometer of the CHORUS detector, located just in front of NOMAD. Considering these two sources of νh's we set new constraints on νh properties and exclude part of the LSND/MiniBooNE νh parameter space using bounds on single photons production in neutrino reactions recently reported by the NOMAD Collaboration. We find that broad bands in the parameter space are still open for more sensitive searches for the νh in future neutrino experiments.

Standard Model fails to explain neutrino oscillations, dark matter, and baryon asymmetry of the Universe. All these problems can be solved with three sterile neutrinos added to SM. Quite remarkably, if sterile neutrino masses are well below the electroweak scale, this modification—Neutrino Minimal Standard Model (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M1"><mml:mrow><mml:mi>ν</mml:mi></mml:mrow></mml:math>MSM)—can be tested experimentally. We discuss a new experiment on search for decays of GeV-scale sterile neutrinos, which are responsible for the matter-antimatter asymmetry generation and for the active neutrino masses. If lighter than 2 GeV, these particles can be produced in decays of charm mesons generated by high energy protons in a target, and subsequently decay into SM particles. To fully explore this sector of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M2"><mml:mrow><mml:mi>ν</mml:mi></mml:mrow></mml:math>MSM, the new experiment requires data obtained with at least<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M3"><mml:mrow><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mn>20</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>incident protons on target (achievable at CERN SPS in future) and a big volume detector constructed from a large amount of identical single modules, with a total sterile neutrino decay length of few kilometers. The preliminary feasibility study for the proposed experiment shows that it has sensitivity which may either lead to the discovery of new particles below the Fermi scale—right-handed partners of neutrinos—or rule out seesaw sterile neutrinos with masses below 2 GeV.

In high energy experiments such as active beam dump searches for rare decays and missing energy events, the beam purity is a crucial parameter. In this paper we present a technique to reject heavy charged particle contamination in the 100 GeV electron beam of the H4 beam line at CERN SPS. The method is based on the detection with BGO scintillators of the synchrotron radiation emitted by the electrons passing through a bending dipole magnet. A 100 GeV $\pi^-$ beam is used to test the method in the NA64 experiment resulting in a suppression factor of $10^{-5}$ while the efficiency for electron detection is $\sim$95%. The spectra and the rejection factors are in very good agreement with the Monte Carlo simulation. The reported suppression factors are significantly better than previously achieved.

In this paper we estimate the sensitivity of the NA64 experiment to millicharged particles ($\ensuremath{\chi}$). That experimental facility is dedicated to the searching for dark sector particles in missing energy events at the CERN SPS. We consider missing momentum signatures in the $\ensuremath{\simeq}100\text{ }\text{ }\mathrm{GeV}$ electron and muon beams and show that the later one allows to obtain more stringent bounds on the millicharge ${Q}_{\ensuremath{\chi}}$, which for the $\ensuremath{\chi}$ masses $100\text{ }\text{ }\mathrm{MeV}\ensuremath{\le}{m}_{\ensuremath{\chi}}\ensuremath{\le}500\text{ }\text{ }\mathrm{MeV}$ at the level ${Q}_{\ensuremath{\chi}}/e\ensuremath{\lesssim}O({10}^{\ensuremath{-}3})\ensuremath{-}O({10}^{\ensuremath{-}2})$.

The recent confirmation by FNAL of the (g−2)μ muon anomaly gives strong evidence for the possible existence of new physics beyond the Standard Model in the muon sector. Thus it is worthy to revisit the existing experimental constraints on models suggesting theoretically consistent explanations of the anomaly. In this work, we point out that accounting for the loss of coherence between the wave packets (mass states) of solar neutrinos is important for setting limits on any model with new flavor-sensitive couplings in the neutrino sector. By taking into account this effect and considering more accurately the experimental constraints from the BOREXINO measurement of the 7Be solar neutrino interaction rate we corrected the limits previously placed on the coupling of the light Z′ to Lμ-Lτ current in the parameter space relevant to the muon (g−2)μ.

The extension of Standard Model made by inclusion of additional $U(1)$ gauge ${L}_{\ensuremath{\mu}}\ensuremath{-}{L}_{\ensuremath{\tau}}$ symmetry can explain the difference between the measured and the predicted value of the muon magnetic moment and solve the tension in $B$ meson decays. This model predicts the existence of a new, light ${Z}^{\ensuremath{'}}$ vector boson, predominantly coupled to second and third generation leptons, whose interaction with electrons is due to a loop mechanism involving muons and taus. In this work, we present a rigorous evaluation of the upper limits in the ${Z}^{\ensuremath{'}}$ parameter space, obtained from the analysis of the data collected by the NA64-$e$ experiment at CERN SPS, that performed a search for light dark matter with $2.84\ifmmode\times\else\texttimes\fi{}{10}^{11}$ electrons impinging with 100 GeV on an active thick target. The resulting limits touch the muon $g\ensuremath{-}2$ preferred band for values of the ${Z}^{\ensuremath{'}}$ mass of order of 1 MeV, while the sensitivity projections for the future high-statistics NA64-$e$ runs demonstrate the power of the electrons/positron beam approach in this theoretical scenario.

The NOMAD Collaboration presents a study of opposite sign dimuon events in the framework of Leading Order QCD. A total of 2714 neutrino- and 115 antineutrino-induced opposite sign dimuon events with Eμ1,Eμ2>4.5 GeV, 15<Eν<300 GeV and Q2>1(GeV/c)2 are observed in the Front-Calorimeter of NOMAD during the 1995 and 1996 runs. The analysis yields a value for the charm quark mass of mc=1.3+0.3+0.3−0.3−0.3GeV/c2 and for the average semileptonic branching ratio of Bc=0.095+0.007+0.014−0.007−0.013. The ratio of the strange to non-strange sea in the nucleon is measured to be κ=0.48+0.09+0.17−0.07−0.12. The measured rate of charm-induced dimuon relative to single muon, as a function of neutrino energy, is consistent with the slow rescaling hypothesis of heavy quark production.

If light hidden sector photons (γ′s) exist, they could be produced through kinetic mixing with solar photons in the eV energy range. We propose to search for this hypothetical γ′-flux with the Super-Kamiokande and/or upgraded CAST detectors. The proposed experiments are sensitive to the γ–γ′ mixing strength as small as 10−5≳χ≳10−9 for the γ′ mass region 10−4≲mγ′≲10−1 eV and, in the case of non-observation, would improve limits recently obtained from photon regeneration laser experiments for this mass region.

We present a study of exclusive neutral pion production in neutrino-nucleus Neutral Current interactions using data from the NOMAD experiment at the CERN SPS. The data correspond to $1.44 \times 10^6$ muon-neutrino Charged Current interactions in the energy range $2.5 \leq E_{\nu} \leq 300$ GeV. Neutrino events with only one visible $\pi^0$ in the final state are expected to result from two Neutral Current processes: coherent $\pi^0$ production, {\boldmath $\nu + {\cal A} \to \nu + {\cal A} + \pi^0$} and single $\pi^0$ production in neutrino-nucleon scattering. The signature of coherent $\pi^0$ production is an emergent $\pi^0$ almost collinear with the incident neutrino while $\pi^0$'s produced in neutrino-nucleon deep inelastic scattering have larger transverse momenta. In this analysis all relevant backgrounds to the coherent $\pi^0$ production signal are measured using data themselves. Having determined the backgrounds, and using the Rein-Sehgal model for the coherent $\pi^0$ production to compute the detection efficiency, we obtain {\boldmath $4630 \pm 522 (stat) \pm 426 (syst)$} corrected coherent-$\pi^0$ events with $E_{\pi^0} \geq 0.5$ GeV. We measure {\boldmath $\sigma (\nu {\cal A} \to \nu {\cal A} \pi^0) = [ 72.6 \pm 8.1(stat) \pm 6.9(syst) ] \times 10^{-40} cm^2/nucleus$}. This is the most precise measurement of the coherent $\pi^0$ production to date.

It has been recently suggested that the anomalous excess of low-energy electron-like events observed by the MiniBooNE experiment, could be explained by the radiative decay of a heavy sterile neutrino nu_h of the mass around 500 MeV with a muonic mixing strength in the range |U_{\mu h}|^2 \simeq (1-4)\times 10^{-3}. If such nu_h exists its admixtures in the decay Ds -> mu+nu would result in the decay Ds -> mu+nu_h with the branching fraction \simeq (1.2-5.5)\times 10^{-4}, which is in the experimentally accessible range. Interestingly, the existence of the Ds -> mu+nu_h decay at this level may also explain why the currently measured decay rate of Ds -> mu+nu is slightly higher than the predicted one. This enhances motivation for a sensitive search for this decay mode and makes it interesting and complementary to neutrino experiments probing sterile-active neutrino mixing. Considering, as an example the CLEO-c experiment, we suggest to perform a search for the decay Ds -> mu+nu_h with the analysis of existing data. The discrepancy between the measurements and theoretical description of the decay Ds -> tau+nu is also discussed in brief.

We present a search for neutrino induced events containing a single, exclusive photon using data from the NOMAD experiment at the CERN SPS where the average energy of the neutrino flux is ≃25GeV. The search is motivated by an excess of electron-like events in the 200–475 MeV energy region as reported by the MiniBooNE experiment. In NOMAD, photons are identified via their conversion to e+e− in an active target embedded in a magnetic field. The background to the single photon signal is dominated by the asymmetric decay of neutral pions produced either in a coherent neutrino–nucleus interaction, or in a neutrino–nucleon neutral current deep inelastic scattering, or in an interaction occurring outside the fiducial volume. All three backgrounds are determined in situ using control data samples prior to opening the 'signal-box'. In the signal region, we observe 155 events with a predicted background of 129.2±8.5±3.3. We interpret this as null evidence for excess of single photon events, and set a limit. Assuming that the hypothetical single photon has a momentum distribution similar to that of a photon from the coherent π0 decay, the measurement yields an upper limit on single photon events, <4.0×10−4 per νμ charged current event. Narrowing the search to events where the photon is approximately collinear with the incident neutrino, we observe 78 events with a predicted background of 76.6±4.9±1.9 yielding a more stringent upper limit, <1.6×10−4 per νμ charged current event.

The understanding of the origin of dark matter has great importance for cosmology and particle physics. Several interesting extensions of the standard model dealing with solution of this problem motivate the concept of hidden sectors consisting of SU(3)xSU(2)_LxU(1)_Y singlet fields. Among these models, the mirror matter model is certainly one of the most interesting. The model explains the origin of parity violation in weak interactions, it could also explain the baryon asymmetry of the Universe and provide a natural ground for the explanation of dark matter. The mirror matter could have a portal to our world through photon-mirror photon mixing (epsilon). This mixing would lead to orthopositronium (o-Ps) to mirror orthopositronium oscillations, the experimental signature of which is the apparently invisible decay of o-Ps. In this paper, we describe an experiment to search for the decay o-Ps -> invisible in vacuum by using a pulsed slow positron beam and a massive 4pi BGO crystal calorimeter. The developed high efficiency positron tagging system, the low calorimeter energy threshold and high hermiticity allow the expected sensitivity in mixing strength to be epsilon about 10^-9, which is more than one order of magnitude below the current Big Bang Nucleosynthesis limit and in a region of parameter space of great theoretical and phenomenological interest. The vacuum experiment with such sensitivity is particularly timely in light of the recent DAMA/LIBRA observations of the annual modulation signal consistent with a mirror type dark matter interpretation.

The decay ${K}_{L}\ensuremath{\rightarrow}\mathit{\text{invisible}}$ has never been experimentally tested. In the Standard Model (SM), its branching ratio for the decay into two neutrinos is helicity suppressed and predicted to be $\mathrm{Br}({K}_{L}\ensuremath{\rightarrow}\ensuremath{\nu}\overline{\ensuremath{\nu}})\ensuremath{\lesssim}{10}^{\ensuremath{-}10}$. We consider several natural extensions of the SM, such as two-Higgs-doublet (2HDM), 2HDM and light scalar, and mirror dark matter models, whose main feature is that they allow us to avoid the helicity suppression factor and lead to an enhanced $\mathrm{Br}({K}_{L}\ensuremath{\rightarrow}\mathit{\text{invisible}})$. For the decay ${K}_{L}\ensuremath{\rightarrow}\ensuremath{\nu}\overline{\ensuremath{\nu}}$, the smallness of the neutrino mass in the considered 2HDM model is explained by the smallness of the second Higgs doublet vacuum expectation value. The small nonzero value of the second Higgs isodoublet can arise as a consequence of nonzero quark condensate. We show that taking into account the most stringent constraints from the $K\ensuremath{\rightarrow}\ensuremath{\pi}+\mathit{\text{invisible}}$ decay, this process could be in the region of $\mathrm{Br}({K}_{L}\ensuremath{\rightarrow}\mathit{\text{invisible}})\ensuremath{\simeq}{10}^{\ensuremath{-}8}--{10}^{\ensuremath{-}6}$, which is experimentally accessible. In some scenarios, the ${K}_{L}\ensuremath{\rightarrow}\mathit{\text{invisible}}$ decay could still be allowed while the $K\ensuremath{\rightarrow}\ensuremath{\pi}+\mathit{\text{invisible}}$ decay is forbidden. The results obtained show that the ${K}_{L}\ensuremath{\rightarrow}\mathit{\text{invisible}}$ decay is a clean probe of new physics scales well above 100 TeV that is complementary to rare $K\ensuremath{\rightarrow}\ensuremath{\pi}+\mathit{\text{invisible}}$ decay, and they provide a strong motivation for its sensitive search in a near-future experiment.

The measurement of muon momentum by Multiple Coulomb Scattering is a crucial ingredient to the reconstruction of νμ CC events in the ICARUS-T600 liquid argon TPC in absence of magnetic field, as in the search for sterile neutrinos at Fermilab where ICARUS will be exposed to ∼ 1 GeV Booster neutrino beam. A sample of ∼ 1000 stopping muons produced by charged current interactions of CNGS νμ in the surrounding rock at the INFN Gran Sasso underground Laboratory provides an ideal benchmark in the few-GeV range since their momentum can be directly and independently obtained by the calorimetric measurement. Stopping muon momentum in the 0.5–4.5 GeV/c range has been reconstructed via Multiple Coulomb Scattering with resolution ranging from 10 to 25% depending on muon energy, track length and uniformity of the electric field in the drift volume.

We report the results of a search for a new vector boson ( A' ) decaying into two dark matter particles χ1χ2 of different mass. The heavier χ2 particle subsequently decays to χ1 and an off-shell Dark Photon A'∗→e+e- . For a sufficiently large mass splitting, this model can explain in terms of new physics the recently confirmed discrepancy observed in the muon anomalous magnetic moment at Fermilab. Remarkably, it also predicts the observed yield of thermal dark matter relic abundance. A detailed Monte-Carlo simulation was used to determine the signal yield and detection efficiency for this channel in the NA64 setup. The results were obtained re-analyzing the previous NA64 searches for an invisible decay A'→χχ¯ and axion-like or pseudo-scalar particles a→γγ . With this method, we exclude a significant portion of the parameter space justifying the muon g-2 anomaly and being compatible with the observed dark matter relic density for A' masses from 2 me up to 390 MeV and mixing parameter ε between 3×10-5 and 2×10-2 .

We report the results of a search for a light pseudoscalar particle $a$ that couples to electrons and decays to $e^+e^-$ performed using the high-energy CERN SPS H4 electron beam. If such pseudoscalar with a mass $\simeq 17$ MeV exists, it could explain the ATOMKI anomaly. We used the NA64 data samples collected in the "visible mode" configuration with total statistics corresponding to $8.4\times 10^{10}$ electrons on target (EOT) in 2017 and 2018. In order to increase sensitivity to small coupling parameter $\epsilon$ we used also the data collected in 2016-2018 in the "invisible mode" configuration of NA64 with a total statistics corresponding to $2.84\times 10^{11}$ EOT. A thorough analysis of both these data samples in the sense of background and efficiency estimations was already performed and reported in our previous papers devoted to the search for light vector particles and axion-like particles (ALP). In this work we recalculate the signal yields, which are different due to different cross section and life time of a pseudoscalar particle $a$, and perform a new statistical analysis. As a result, the region of the two dimensional parameter space $m_a - \epsilon$ in the mass range from 1 to 17.1 MeV is excluded. At the mass of the ATOMKI anomaly the values of $\epsilon$ in the range $2.1 \times 10^{-4} < \epsilon < 3.2 \times 10^{-4}$ are excluded.

We have searched for the photonless annihilation of orthopositronium, o-Ps → nothing (nothing denotes the weakly-interacting particles which are not detected). The upper limit on the ratio of this decay rate to the 3γ decay rate obtained is Γ(o-Ps → nothing)/Γ(o-Ps → 3γ) < 5.8 × 10−4 (90% CL). This result excludes the hypothesis that this decay is responsible for the discrepancy between the theoretical and experimental values of the orthopositronium lifetime in vacuum.

Performance tests of some aspects of the CMS ECAL were carried out on modules of the "barrel" sub-system in 2002 and 2003. A brief test with high energy electron beams was made in late 2003 to validate prototypes of the new Very Front End electronics. The final versions of the monitoring and cooling systems, and of the high and low voltage regulation were used in these tests. The results are consistent with the performance targets including those for noise and overall energy resolution, required to fulfil the physics programme of CMS at the LHC.

The AEgIS experiment is presently almost completely installed at CERN. It is currently taking data with antiprotons, electrons and positrons. The apparatus is designed to form a cold, pulsed beam of antihydrogen to measure the Earth's gravitational acceleration g on antimatter and to perform spectroscopy measurements. This paper describes the main features of the apparatus and shows a selected review of some achieved results.