Nikolai Krasnikov

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.

We propose a mechanism for generating the baryon asymmetry of the Universe in the framework of spontaneously broken gauge theories with CP-nonconversation and baryon number violation. An SU(2) ⊗ U(1) model is worked out in detail, in which it proves to be possible to construct universal superweak baryon and CP-nonconserving interaction mediated by new scalar boson with mass not exceeding that of W-boson.

We show that non-zero gaugino condensates of several non-abelian gauge groups G1⊗…⊗Gk∃E8 in low-energy d=4 superstring E8⊗E6 gauge theory can lead to the exponentially small (compared to the Planck scale) supersymmetry breaking scale. The Hosotani mechanism can provide the E8→G1⊗…⊗Gk breaking.

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.

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.

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.

We calculate two-loop corrections to the propagators of gluonic currentsjPG=αGμvaG¯μv and jSG = (β(α)/α)Gμν2. The results of the calculations are used for finding the connection between the characteristic scales of quark and gluon worlds in perturbative QCD and for an estimate of the mass of the first radial excitation of the η′ meson [we obtain (mη′)1 = 1.55GeV].

We use finite energy sum rules, taking into account nonperturbative corrections, for the calculation of hadron masses within QCD. As a rule, we obtain reasonable agreement with experimental data. We also estimate the proton lifetime in theSU(5) model to beτ p =1028 (M x /1014GeV)4 years.

Finite energy sum rules (FESR) for the cross section of e+e−-annihilation into hadrons are obtained within the framework of QCD. The FESR for the standard 4-quark model are in satisfactory agreement with experiment. The best description of the data is given by the 5-quark model, with the electric charge of the 5th quark Q5 = −1/3.

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 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.

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.\

We investigate the influence of the analytical continuation effects on the value of QCD scale parameter Λ extracted from the data on charmonium and upsilon hadron decays. The account of the analytical continuation leads to the increasing of the value ΛMS of 1.5 times approximately. We obtained the following values: ΛMS = 143 +73−55MeV (Ψ-meson), ΛMS = 145 +107−72MeV (ϒ-meson).

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.

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.

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.

We present the package for the simulation of DM (Dark Matter) particles in fixed target experiments. The most convenient way of this simulation (and the only possible way in the case of beam-dump) is to simulate it in the framework of the Monte-Carlo program performing the particle tracing in the experimental setup. The Geant4 toolkit framework was chosen as the most popular and versatile solution nowadays. Specifically, the package includes the codes for the simulation of the processes of DM particles production via electron and muon bremsstrahlung off nuclei, resonant in-flight positron annihilation on atomic electrons and gamma to ALP (axion-like particles) conversion on nuclei. Four types of DM mediator particles are considered: vector, scalar, pseudoscalar and axial vector. The total cross sections of bremsstrahlung processes are calculated numerically at exact tree level (ETL). The code handles both the case of invisible DM decay and of visible decay into e+e− (μ+μ− for Z′, γγ for ALP). The proposed extension implements native Geant4 application programming interfaces (API) designed for these needs and can be unobtrusively embedded into the existing applications. As an example of its usage, we discuss the results obtained from the simulation of a typical active beam-dump experiment. We consider 5×1012 100 GeV electrons impinging on a lead/plastic heterogeneous calorimeter playing a role of an active thick target. The expected sensitivity of the experiment to the four types of DM mediator particles mentioned above is then derived. Program Title: DMG4 CPC Library link to program files: https://doi.org/10.17632/6m3yhx4ssw.1 Licensing provisions: GNU General Public License 3 Programming language: c++ Nature of problem: The optimal way to simulate Dark Matter production processes in fixed target experiments in most cases is to do it inside the program for the full simulation of the experimental setup and not separately, in event generators. The code that can be easily embedded in such programs is needed. The code should be able to simulate various DM production processes that happen in a thick target, in particular on nuclei, with maximal accuracy. Solution method: We created a Geant4 compatible DM simulation package for this purpose. The choice of this simulation framework is suggested by its popularity and versatility. The code includes the cross sections precalculated at exact tree level for a wide variety of DM particles.

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^{'}}.

The five-dimensional Yang-Mills theory is shown to have an ultraviolet fixed point. We point out that the models with a compactification radius of the order of (TeV)−1 can solve the gauge hierarchy problem, as a consequence we predict the existence of an infinite tower of radial excitations of the quarks, leptons and the vector bosons with the masses of the first excitations at the TeV scale.

The light quark masses are calculated on the basis of finite energy sum rules. The obtained result [(mu+md(GeV)∼20MeV] allows us to estimate the ratio mb/ms∼20 which should be compared with the prediction of the SU(5) model, mb/ms≈ 17. We estimate a value of the π′ meson residue and propose a model for the mass spectrum of the radial excitations of the π meson. In particular, we obtain a mass of the second π meson excitation mπ2(2) = 2mπ′2 in agreement with experimental data.

The mass spectrum and the leptonic decay widths of the meson resonances in QCD are calculated on the basis of the use of finite-energy sum rules and the 1/N-expansion. The obtained results are in good agreement with experiment, in particular we derive the mass formulas for the ϱ-meson A1-meson excitations m2ϱ(n)=m2ϱ(1+2n), m2A1 (n)=m2A1 (n+1)(n = 0, 1, 2, ...).

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.

We point out that the notion of an unparticle, recently introduced by Georgi, can be interpreted as a particular case of a field with continuously distributed mass considered in Phys. Lett. B325, 430 (1994). We also point out that the simplest renormalizable extension of the SU c (3) ⊗ SU L (2) ⊗ U (1) Standard Model is the extension with the replacement of the U (1) gauge propagator [Formula: see text] with [Formula: see text].

A new approach is proposed to the problem of the relationship between the e+e− hadron annihilation cross section and the behaviour of the hadronic vacuum polarization tensor Πμν(q) in the space-like region. By using this approach based on the principles of functional analysis, one can uniquely transform the information about the behaviour of Πμν(q) at q2 → −∞, which is available in asymptotically free theories and in theories with anomalous dimensions, into constraints on the e+e− total cross section.

We present the computation of the αs3 correction to the two-point function of the pseudoscalar gluon currents. The obtained information is used to determine the relation between the scales of the gluon and quark channels. We apply the finite energy sum rules and estimate the value of the η'-meson decay constant and the mass of the η'-excitation.

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 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.

We propose a method to estimate the probability of new physics discovery in future high energy physics experiments. Physics simulation gives both the average numbers &lt;N b &gt; of background and &lt;N s &gt; of signal events. We find that the proper definition of the significance for &lt;N b &gt;, &lt;N s &gt; ≫ 1 is [Formula: see text] in comparison with often used significances: [Formula: see text] and [Formula: see text]. We propose a method of taking into account the systematical errors related to nonexact knowledge of background and signal cross-sections. An account of such systematics is essential in the search for supersymmetry at LHC. We also propose a method for estimating exclusion limits on new physics in future experiments.

We prove the inequalities ψ(y, α) ⩽α, |αs(d/dαs)(β(αs)/αs| ⩽ 1 (for the Paterman-Stueckelberg-Gell-Mann-Low functions in QED and QCD) and γ0(αs ⩽ 1 (for the anomalous dimension of the gauge-invariant operator O(x)). The consequences of the inequalities are discussed: for modern energies, comparison of theoretical and experimental moments of deep inelastic structure functions has a meaning only for N ⩾ 7 (singlet case) and N ⩾ 50 (non-singlet case); perturbation theory in QCD has a meaning only for αrms ⩽ 0.45.

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.

A review is provided of the concept confidence distributions. Material covered include: fundamentals, extensions, applications of confidence distributions and available computer software. We expect that this review could serve as a source of reference and encourage further research with respect to confidence distributions.

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.

Several criteria used by physicists to quantify the ratio of signal to background in planned experiments are compared. An equal probability test is proposed for the evaluation of the uncertainty in planned search experiments.

We review the search for new physics to be performed at the Large Hadron Collider(LHC). Namely, we review the expectations for the Higgs boson, supersymmetry and exotica detection at LHC. We also describe the main parameters of the CMS and ATLAS detectors.

We present measurements of the reduction of light output by plastic scintillators irradiated in the CMS detector during the 8 TeV run of the Large Hadron Collider and show that they indicate a strong dose rate effect. The damage for a given dose is larger for lower dose rate exposures. The results agree with previous measurements of dose rate effects, but are stronger due to the very low dose rates probed. We show that the scaling with dose rate is consistent with that expected from diffusion effects.

Recently, the ATOMKI experiment has reported new evidence for the excess of e+e- events with a mass ∼ 17 MeV in the nuclear transitions of 4 He, that they previously observed in measurements with 8 Be. These observations could be explained by the existence of a new vector X17 boson. So far, the search for the decay X17→e+e- with the NA64 experiment at the CERN SPS gave negative results. Here, we present a new technique that could be implemented in NA64 aiming to improve the sensitivity and to cover the remaining X17 parameter space. If a signal-like event is detected, an unambiguous observation is achieved by reconstructing the invariant mass of the X17 decay with the proposed method. To reach this goal an optimization of the X17 production target, as well as an efficient and accurate reconstruction of two close decay tracks, is required. A dedicated analysis of the available experimental data making use of the trackers information is presented. This method provides independent confirmation of the NA64 published results [1], validating the tracking procedure. The detailed Monte Carlo study of the proposed setup and the background estimate show that the goal of the proposed search is feasible.

We report on a search for a new $Z'$ ($L_\mu-L_\tau$) vector boson performed at the NA64 experiment employing a high energy muon beam and a missing energy-momentum technique. Muons from the M2 beamline at the CERN Super Proton Synchrotron with a momentum of 160 GeV/c are directed to an active target. A signal event is a single scattered muon with momentum $<$ 80 GeV/c in the final state, accompanied by missing energy, i.e. no detectable activity in the downstream calorimeters. For a total statistic of $(1.98\pm0.02)\times10^{10}$ muons on target, no event is observed in the expected signal region. This allows us to set new limits on part of the remaining $(m_{Z'},\ g_{Z'})$ parameter space which could provide an explanation for the muon $(g-2)_\mu$ anomaly. Additionally, our study excludes part of the parameter space suggested by the thermal Dark Matter relic abundance. Our results pave the way to explore Dark Sectors and light Dark Matter with muon beams in a unique and complementary way to other experiments.

The search for new comparably light (well below the electroweak scale) feebly interacting particles is an exciting possibility to explain some mysterious phenomena in physics, among them the origin of Dark Matter. The sensitivity study through detailed simulation of projected experiments is a key point in estimating their potential for discovery. Several years ago we created the DMG4 package for the simulation of DM (Dark Matter) particles in fixed target experiments. The natural approach is to integrate this simulation into the same program that performs the full simulation of particles in the experiment setup. The Geant4 toolkit framework was chosen as the most popular and versatile solution nowadays. The simulation of DM particles production by this package accommodates several possible scenarios, employing electron, muon or photon beams and involving various mediators, such as vector, axial vector, scalar, pseudoscalar, or spin 2 particles. The bremsstrahlung, annihilation or Primakoff processes can be simulated. The package DMG4 contains a subpackage DarkMatter with cross section methods weakly connected to Geant4. It can be used in different frameworks. In this paper, we present the latest developments of the package, such as extending the list of possible mediator particle types, refining formulas for the simulation and extending the mediator mass range. The user interface is also made more flexible and convenient. In this work, we also demonstrate the usage of the package, the improvements in the simulation accuracy and some cross check validations.

The inclusion of an additional U(1) gauge $L_{\mu} - L_{\tau}$ symmetry would release the tension between the measured and the predicted value of the anomalous muon magnetic moment: this paradigm assumes the existence of a new, light $Z'$ vector boson, with dominant coupling to ${\mu}$ and ${\tau}$ and interacting with electrons via a loop mechanism. The $L_{\mu} - L_{\tau}$ model can also explain the Dark Matter relic abundance, by assuming that the $Z'$ boson acts as a "portal" to a new Dark Sector of particles in Nature, not charged under known interactions. In this work we present the results of the $Z'$ search performed by the NA64-$e$ experiment at CERN SPS, that collected $ \sim 9 \times 10^{11}$ 100-GeV electrons impinging on an active thick target. Despite the suppressed $Z'$ production yield with an electron beam, the limits sets by NA64-$e$ are competitive with other experimental searches, and partially exclude the $g-2$ preferred model parameter values for $Z'$ masses lighter than 2 MeV. This result proves the complementarity of this search with NA64-${\mu}$, the parallel effort of the NA64 collaboration with a muon beam.

The search for new comparably light (well below the electroweak scale) feebly interacting particles is an exciting possibility to explain some mysterious phenomena in physics, among them the origin of Dark Matter. The sensitivity study through detailed simulation of projected experiments is a key point in estimating their potential for discovery. Several years ago we created the DMG4 package for the simulation of DM (Dark Matter) particles in fixed target experiments. The natural approach is to integrate this simulation into the same program that performs the full simulation of particles in the experiment setup. The Geant4 toolkit framework was chosen as the most popular and versatile solution nowadays. The simulation of DM particles production by this package accommodates several possible scenarios, employing electron, muon or photon beams and involving various mediators, such as vector, axial vector, scalar, pseudoscalar, or spin 2 particles. The bremsstrahlung, annihilation or Primakoff processes can be simulated. The package DMG4 contains a subpackage DarkMatter with cross section methods weakly connected to Geant4. It can be used in different frameworks. In this paper, we present the latest developments of the package, such as extending the list of possible mediator particle types, refining formulas for the simulation and extending the mediator mass range. The user interface is also made more flexible and convenient. In this work, we also demonstrate the usage of the package, the improvements in the simulation accuracy and some cross check validations. Program title: DMG4 CPC Library link to program files: https://doi.org/10.17632/cmr4bcrj6j.1 Licensing provisions: GNU General Public License 3 Programming language: c++ Journal reference of previous version: Comput. Phys. Commun. 269 (2021) 108129 Does the new version supersede the previous version?: Yes Reasons for the new version: Numerous developments, addition of new features Summary of revisions: WW approximation cross sections for the muon beam are implemented and cross-checked, models with semivisible A′ (inelastic Dark Matter) and spin 2 mediators are added. The range of possible mediator masses is extended. Several important improvements for the annihilation processes are made, the number of possible annihilation processes is extended. User interface is improved. Several bugs are fixed. Nature of problem: For the simulation of Dark Matter production processes in fixed target experiments a code that can be easily integrated in programs for the full simulation of experimental setup is needed. Solution method: A fully Geant4 compatible DM simulation package DMG4 was presented in 2020. We present numerous further developments of this package.

We propose two generalizations of the dark photon model which predict the suppressed elastic dark matter nuclei cross section in comparison with the corresponding prediction of the dark photon model. In the first model the main difference from dark photon model is that the mixing parameter $\epsilon$ is nonlocal formfactor $\epsilon(q^2)= \frac{q^2}{\Lambda^2}V(\frac{q^2}{\Lambda^2}) $ depending on the square of the momentum transfer $q^2$. Here $V(\frac{q^2}{\Lambda^2})$ is an entire function of the growth $\rho \geq \frac{1}{2}$ and $\Lambda$ is nonlocal scale. In this model our world and dark world are described by renormalizable field theories while the communication between them is performed by nonlocal interaction. The second model is renormalizable model where besides dark photon field $A'$ additional vector boson $Z'$ interacts with $B - L$ current. The communication between our world and dark world is performed due to nonzero kinetic mixing between $Z'$ and $A'$ fields. The predictions of the models for the search for dark matter at the accelerators don't contain additional suppression factors.

It is shown that the non-zero transition magnetic moment (μtran) between the sterile neutrino (νs) and the muon neutrino (νμ) could be effectively searched for via the Primakoff effect, in the process of νμZ→νsZ conversion in the external Coulomb field of a nucleus Z, with the subsequent νs→νμ+γ decay. From the recent results of the NOMAD neutrino detector at CERN a model-independent constraint of μtran<(10−6−10−9)μB is obtained depending on the value of νs mass. For the ms∼O(1) GeV region these bounds are comparable with the present experimental ones on νμ and νe diagonal magnetic moments and are more sensitive than those on ντ magnetic moment. Further improvement of the bounds might be interesting for understanding of the origin of the KARMEN anomaly. From the same analysis the constraint on ντ(νs)→νμ+γ+γ decay lifetime τ>2×1013 s/mν7 MeV) is obtained. The limit is valid for neutrino masses up to mν∼O(1) GeV.

We show that in supersymmetric models with explicit flavor lepton number violation due to soft supersymmetry breaking terms there must be flavor lepton number violation in slepton decays. We propose to look for flavor lepton number violation in righthanded selectron and smuon decays. For selectron and smuon lighter than 80 Gev flavor lepton number violation in slepton decays could be discovered at LEP2 provided the mixing betwen selectron and smuon is not small. We also estimate NLC discovery potential of the lepton flavor number violation in slepton decays.

Various ways on how higher order corrections can reveal themselves if integrated over the ir region are discussed. We show that in different renormalization group (RG) schemes and for some observables one has no factorial divergences. We argue that for treating things in the ir region it is preferable to start with a RG scheme without the ir Landau pole in the running coupling constant. The uncertainties for the τ lepton width resulting from accounting for higher order corrections are also discussed.

This is an erratum to the paper published in Nucl. Phys. B198(1982)508.

In this paper we estimate the LHC sgoldstino discovery potential for the signatures with gamma gamma and ZZ in a final state.

We study a possibility to detect sleptons at post-WMAP benchmark points at LHC(CMS). We find that at $L_{tot}=30 fb^{-1}$ it would be possible to detect sleptons at points A, B, C, D, G. We also investigate the production and decays of right and left sleptons separately. We find that at $L_{tot}=30 fb^{-1}$ it would be possible to detect right sleptons with a mass up to 200 GeV and left ones with a mass up to 300 GeV.

In the Standard Model there are several canonical examples of pure leptonic processes involving the muon, the electron and the corresponding neutrinos which are connected by the crossing symmetry: i) the decay of muon, ii) the inverse muon decay, and iii) the annihilation of a muon and an electron into two neutrinos. Although the first two reactions have been observed and measured since long ago, the third process, resulting in the invisible final state, has never been experimentally tested. It may go either directly, or, at low energies, via the annihilation of a muon and an electron from an atomic bound state, called muonium (M=\mu^+e^-). The M\to \nu_\mu \nu_e decay is expected to be a very rare process, with the branching fraction predicted to be Br(M\to \nu_\mu\nu_e) = 6.6 10^{-12} with respect to the ordinary muon decay rate. Using the reported experimental results on precision measurements of the positive muon lifetime by the MuLan Collaboration, we set the first limit Br(M \to invisible) < 5.7 10^{-6}, while still leaving a big gap of about six orders of magnitude between this bound and the predictions. To improve substantially the limit, we proposed to perform an experiment dedicated to the sensitive search for the M\to invisible decay. A feasibility study of the experimental setup shows that the sensitivity of the search for this decay mode in branching fraction Br(M\to invisible) at the level of 10^{-12} could be achieved. If the proposed search results in a substantially higher branching fraction than predicted, say Br(M \to invisible) < 10^{-10}, this would unambiguously indicate the presence of new physics. We point out that such a possibility may occur due the muonium-mirror muonium conversion in the mirror matter model. A result in agreement with the Standard Model prediction would be a clean check of the pure leptonic bound state annihilation.

We study lepton flavour-violating interactions which could result in the τ-lepton production in the ν μ N scattering or in μ → τ conversion on nucleons at high energies. Phenomenological bounds on the strength of [Formula: see text] interactions are extracted from the combined result of the NOMAD and CHORUS experiments on searching for ν μ - ν τ oscillations. Some of these bounds supersede limits from rare decays. We also propose a "missing energy" type experiment searching for μ → τ conversion on nucleons. The experiment can be performed at a present accelerator or at a future neutrino factory.

Possible manifestations of new physics in rare (exotic) decays of orthopositronium (o - Ps) are briefly reviewed. It is pointed out that models with infinite additional dimension(s) of Randall–Sundrum type predict disappearance of orthopositronium into additional dimension(s). The experimental signature of this effect is the invisible decay of orthopositronium. We point out that this process may occur at a rate within two or three orders of magnitude of the present experimental upper limit. We also propose a model with a light weakly interacting boson leading to o - Ps → invisible decays at the experimentally interesting rate. We discuss this in details and stress that the existence of invisible decay of orthopositronium in vacuum could explain the o - Ps decay rate puzzle. Thus, our result enhances the existing motivation and justifies efforts for a more sensitive search for o - Ps → invisible decay in a near future experiment.

We point out that some models with infinite additional dimension(s) of Randall-Sundrum type predict the disappearance of orthopositronium $(o\ensuremath{-}\mathrm{Ps})$ into additional dimension(s). The experimental signature of this effect is the $o\ensuremath{-}\mathrm{Ps}\ensuremath{\rightarrow}\mathrm{invisible}$ decay of orthopositronium which may occur at a rate within three orders of magnitude of the present experimental upper limit. This result enhances existing motivations for a more sensitive search for this decay mode and suggests additional directions for testing extra dimensions in nonaccelerator experiments.

We describe a method of calculation of the functional integral over euclidean Fermi fields in left-right asymmetric gauge models. The method is compatible with perturbation theory and leads to fermion number violation in the presence of instantons.

Several models of dark matter suggest the existence of dark sectors consisting of SU(3)_C x SU(2)_L x U(1)_Y singlet fields. These sectors of particles do not interact with the ordinary matter directly but could couple to it via gravity. In addition to gravity, there might be another very weak interaction between the ordinary and dark matter mediated by U'(1) gauge bosons A' (dark photons) mixing with our photons. In a class of models the corresponding dark gauge bosons could be light and have the $γ$-A' coupling strength laying in the experimentally accessible and theoretically interesting region. If such A' mediators exist, their di-electron decays A' -&gt; e+e- could be searched for in a light-shining-through-a-wall experiment looking for an excess of events with the two-shower signature generated by a single high energy electron in the detector. A proposal to perform such an experiment aiming to probe the still unexplored area of the mixing strength 10^-5 &lt; $ε$ &lt; 10^-3 and masses M_A' &lt; 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. The full running time of the proposed measurements is requested to be up to several months, and it could be taken at different SPS secondary beams.

We point out that last NA64 bound on coupling constant of hypothetical X[Formula: see text](16.7 MeV) vector boson with electrons plus the recent value of the anomalous electron magnetic moment exclude at 90% C.L. purely vector or axial–vector couplings of X[Formula: see text](16.7) boson with electrons. Models with nonzero [Formula: see text] coupling constant with electron survive and they can explain both the electron and muon [Formula: see text] anomalies.

Abstract We review the most important models of light dark matter and discuss the NA64 experiment aimed at searching for hypothetical particles, including dark matter, in the mass range ⩽ O (1) GeV with the use of electron and muon beams at the Super Proton Synchrotron (SPS) accelerator at CERN. We consider the methods and results of searches in the NA64 and other accelerator experiments and also discuss their further prospects.

We present a model explaining both the $4.2\ensuremath{\sigma}$ muon $g\ensuremath{-}2$ anomaly and the relic density of dark matter (DM) in which DM interacts with the Standard Model (SM) via a scalar portal boson $\ensuremath{\varphi}$ carrying both dark and SM leptonic numbers, and mediating a nondiagonal interaction between the electron and muon that allows $e\ensuremath{\leftrightarrow}\ensuremath{\mu}$ transitions. The $\ensuremath{\varphi}$ could be produced in high-energy electron scattering off a target nuclei in the reaction $eZ\ensuremath{\rightarrow}\ensuremath{\mu}Z\ensuremath{\varphi}$ followed by the prompt invisible decay $\ensuremath{\varphi}\ensuremath{\rightarrow}\mathrm{DM}$ particles and searched for in events with large missing energy accompanied by a single outgoing muon in the final state. Interestingly, several events with a similar signature have been observed in a data sample of $\ensuremath{\simeq}3\ifmmode\times\else\texttimes\fi{}{10}^{11}$ electrons on target collected during 2016-2018 for the search for light dark matter in the NA64 experiment at the CERN SPS [D. Banerjee et al. (NA64 Collaboration), Phys. Rev. Lett. 123, 121801 (2019).]. Attributing so far these events to background allows us to set first constraints on the $\ensuremath{\varphi}$ mass and couplings while leaving at the same time decisively probing the origin of these events and a large fraction of the remaining parameter space to a near exiting future with the upgraded NA64 detector or other planned experiments.

We discuss the most sensitive constraints on Light Dark Matter (LDM) from accelerator experiments NA64 and BaBar and compare it with recent results from direct searches at XENON1T, DAMIC-M, SuperCDMS, and DarkSide-50. We show that for the dark photon ($A'$) model with scalar LDM, NA64 gives more stringent bounds for $A'$ masses $m_{A'} \leq 0.15~GeV$ than direct searches. Moreover, for the case of Majorana LDM the damping DM velocity $v$ factor, $v^2 \sim O(10^{-6})$, for the elastic LDM electron(nucleon) cross section makes direct observation of Majorana LDM extremely challenging, while the absence of this suppression in the NA64 case gives an advantage to the experiment. The similar situation takes place for pseudo-Dirac LDM. The BaBar provides the most stringent bounds for $A'$ masses $m_{A'} \geq 0.35~GeV$. For scalar LDM the direct detection experiments give more stringent bounds at $m_{A'} \geq 0.35~GeV$ while for Majorana and pseudo-Dirac LDM case, the BaBar bounds are more stringent. The complementarity of the two approaches in searching for LDM is underlined.

We present the measurement of the intrinsic hadronic contamination at the CERN SPS H4 beamline configured to transport electrons and positrons at 100 GeV/c. The analysis, performed using data collected by the NA64-e experiment in 2022, is based on calorimetric measurements, exploiting the different interaction mechanisms of electrons and hadrons in the NA64 detector. We determined the contamination by comparing the results obtained using the nominal electron/positron beamline configuration with those from a dedicated setup, in which only hadrons impinged on the detector. We also obtained an estimate of the relative protons, anti-protons and pions yield by exploiting the different absorption probabilities of these particles in matter. We cross-checked our results with a dedicated Monte Carlo simulation for the hadron production at the primary T2 target, finding a good agreement with the experimental measurements.

We show that in supersymmetric models with explicit flavor lepton number violation due to soft supersymmetry breaking mass terms there could be detectable flavor lepton number violation in slepton decays. We estimate the potential for discovery of lepton flavor number violation in slepton decays at LHC.

ŽWe carried out a model-independent search for light scalar or pseudoscalar particles a's an example of which is the .axion that couple to two photons by using a photon-regeneration method at high energies allowing a substantial increase in the sensitivity to eV masses.The experimental set-up is based on elements of the CERN West Area Neutrino Facility Ž .WANF beam line and the NOMAD neutrino detector.The new particles, if they exist, could be produced through the Primakoff effect in interactions of high energy photons, generated by the 450 GeV protons in the CERN SPS neutrino target, with virtual photons from the WANF horn magnetic field.The particles would penetrate the downstream shielding and would be observed in the NOMAD neutrino detector through their re-conversion into real high energy photons by interacting with the virtual photons from the magnetic field of the NOMAD dipole magnet.From the analysis of the data collected during the 1996 run with 1.08 = 10 19 protons on target, 312 candidate events with energy between 5 and 140 GeV were found.This number is in general agreement with the expectation of 272 " 18 background events from standard neutrino processes.A 90 % CL upper limit on the agg-coupling g -1.5 = 10 y4 GeV y1 agg for a masses up to 40 eV is obtained.

It is shown that a new light gauge boson X which might be produced in the decays of pseudoscalar mesons π0(η)→γ+X could be effectively searched for in neutrino experiments via the Primakoff effect, in the process of X+Z→π0(η)+Z conversion in the external Coulomb field of a nucleus. An estimate of the X→π0 conversion rate for the NOMAD neutrino detector at CERN is given.

We investigate squark and gluino pair production at LHC (CMS) with subsequent decays into quarks, leptons and LSP in models with effective supersymmetry where third generation of squarks is relatively light while the first two generations of squarks are heavy. We consider the general case of nonuniversal gaugino masses. Visibility of signal by an excess over SM background in (n \geq 2)jets + (m \geq 0)leptons + E^{miss}_T events depends rather strongly on the relation between LSP, second neutralino, gluino and squark masses and it decreases with the increase of LSP mass. We find that for relatively heavy gluino it is very difficult to detect SUSY signal even for light 3^{rd} generation squarks (m_{\tilde q_3}\le 1 TeV) if the LSP mass is closed to the 3^{rd} generation squark mass.

We investigate squark and gluino pair production at LHC (CMS) with subsequent decays into quarks and LSP for the case of nonuniversal gaugino masses. Visibility of signal by an excess over SM background in (n≥2) jets+EmissT events depends rather strongly on the relation between LSP, gluino and squark masses and it decreases with the increase of LSP mass. For relatively heavy LSP mass closed to squark or gluino masses and for (mq̃,mg̃)≥1.5 TeV signal is too small to be observable. For the case when only third generation squarks and LSP are relatively light signal is not visible without b-tagging.

This paper reviews the new methodology for statistical inferences. Point estimators, confidence intervals and p—values are fundamental tools for frequentist statisticians. Confidence distributions, which can be viewed as “distribution estimators”, are often convenient for constructing all of the above statistical procedures and more.

We consider the SM extension with additional light real singlet scalar, right-handed neutrino and nonrenormalizable Yukawa interaction for the first two generations. We show that the proposed model can explain the observed (g – 2) muon anomaly. Phenomenological consequenses as flavour violating decays τ → μμμ, μμe, μee are briefly discussed. We also propose the UR(1) gauge generalization of the SM with complex scalar singlet and nonzero right-handed charges for the first two generations.

An anomaly in time distribution of neutrinos from the ISIS pulsed beam stop source observed by the KARMEN collaboration is discussed. We show that the anomaly can be interpreted as a superposition of two exponentials, both having time constants consistent with the μ+ lifetime of 2.2 μs. It is assumed that they both originate from muon decays at rest. One of them describes the time distribution of the prompt neutrino events, while the other describes the time distribution of events from delayed decays of slowly moving (β≃0.02) particles in the KARMEN calorimeter. We propose here that these particles are produced in exotic decays of positive muons μ+→e++X, resulting in the second exponential time distribution shifted by the time of flight with respect to the time distribution of neutrino events. This model gives an acceptable fit to the KARMEN data if X has a mass of 103.9 MeV. The possible decay modes of this new massive neutral particle are discussed. This hypothesis can be experimentally tested in the near future by studying the low energy part of the e+ spectrum in the μ+ decays.

Abstract We point out that if the top quark is heavy (mt ⩾150 GeV then the predictions for the Higgs boson mass in the standard Glashow-Weinberg-Salam model and in its supersymmetric extension are in different regions and the measurement of the Higgs boson mass allows to distinguish between the models.

We show that the introduction of the infinite component local fields with higher-order derivatives in the interaction makes the theory completely ultraviolet finite. For the γ5-anomalous theories the introduction of the infinite component field makes the theory renormalizable or even superrenormalizable.

The LHC (CMS) discovery potential in the search for supersymmetry and lepton-flavor violation in neutralino decays using the e ± µ∓ + E T miss signature is studied. A detailed study is done for the CMS test points LM1-LM9. It is shown that, for the point LM1, it is possible to detect lepton-flavor violation in neutralino decays with lepton-flavor-violating branching Br( $$\tilde \chi _2^0 $$ → µ± e ∓ $$\tilde \chi _1^0 $$ ) ≥ 0.04 Br( $$\tilde \chi _2^0 $$ → e + e − $$\tilde \chi _1^0 $$ , µ+ µ− $$\tilde \chi _1^0 $$ ) for an integral luminosity of 10 fb−1. The discovery potential in the mSUGRA-SUSY scenario with tan β = 10, sgn(µ) = + in the (m 0, m 1/2) plane using the e ± µ∓ + E T miss signature is determined.

We show that in some supersymmetric models with explicit flavor number violation due to soft supersymmetry breaking terms there is strong flavor lepton number violation in slepton decays due to slepton mixing. If sleptons are relatively light (m sl ≤ M Z ) flavor lepton number violation could be discovered at LEP2.

Abstract We calculate the relation between the pole and running masses for gluino and squarks in supersymmetric QCD with soft supersymmetry breaking in one loop approximation. We find that the difference between running and pole masses could be up to 15%.

We consider local quantum field theories with continuously distributed masses and in particular the Weinberg-Salam model with continuously distributed Higgs boson mass. The phenomenology of the Higgs boson with continuously distributed mass differs in a drastic way from the standard Higgs boson phenomenology, namely, there are invisible decay modes for the Higgs boson.

We show that the four-fermion (vector) ⊗ (vector) model is renormalizable in the framework of the 1/N expansion and moreover it is ultraviolet finite.

The double-θ vacuum structure of the Schwinger model is established in the framework of the pseudo-Euclidean functional integral method. The fermion number “spurionization” is found to be connected with the existence of a zero fermion mode in a background gauge field with nontrivial asymptotics. The relation between the pseudo-Euclidean approach and Euclidean one is described.

We consider an approach for testing the hypothesis that two realizations of the random variables in the form of histograms are taken from the same statistical population (i.e. two histograms are drawn from the same distribution). The approach is based on the notion of “significance of deviation”. This approach allows to estimate the statistical difference between two histograms using multi-dimensional test statistics. The distinguishability of the histograms is estimated by constructing a number of clones (rehistograms) of the observed histograms.

We present evidence that the Wess-Zumino model cannot be defined as a mathematically consistent quantum field theory. Our arguments are based on the use of Schwinger's equations, the Källen-Lehmann inequality and the supersymmetry Ward indentities. Possible consequences of our results are briefly discussed.

We use Schwinger's equations for the investigation of the conformal-invariant electrodynamics. We argue that the conformal-invariant solution and hence finite renormalization of the electric charge is impossible in QED.

We report on a direct search in NOMAD for a new 33.9 MeV/c2 neutral particle (X) produced in pion decay in flight, π→μX followed by the decay X→νe+e−. Both decays are postulated to occur to explain the time anomaly observed by the KARMEN experiment. From the analysis of the data collected during the 1996–1998 runs with 4.1×1019 protons on target, a single candidate event consistent with background expectations was found. The search is sensitive to a pion branching ratio BR(π→μX)>3.7×10−15, significantly smaller than previous experimental limits.

It is shown that zero-fermion modes exist in the external field with non-zero topological charge in Weinberg-Salam type models. An explicit expression for the zero modes is found in a particular case. Some physical consequences of the existence of the zero modes are discussed.

We discuss phenomenological consequences of renormalizable Z' models with continuously distributed mass. We point out that one of the possible LHC signatures for such model is the existence of broad resonance in Drell–Yan reaction pp →Z' →l + l - .

In the Standard Model (SM), the branching ratio for the decay [Formula: see text] is helicity suppressed and predicted to be very small [Formula: see text]. We consider two natural extensions of the SM as the two-Higgs-doublet model (2HDM) and the neutrino minimal Standard Model ([Formula: see text]MSM) with additional singlet scalar, whose main feature is that they can lead to an enhanced [Formula: see text]. In the 2HDM, the smallness of the neutrino mass is explained due to the smallness of the second Higgs doublet vacuum expectation value. Moreover, the [Formula: see text]MSM extension with additional singlet field can explain the [Formula: see text] anomaly. The considered models demonstrate that the [Formula: see text] decay is a clean probe of new physics scale well above 100 TeV, that is complementary to rare [Formula: see text] decay, and provide a strong motivation for its sensitive search in a near future low-energy experiment.

We show that in nonlocal generalization of standard nonsupersymmetric SU(5) GUT, it is possible to solve the problems with the proton lifetime and the Weinberg angle without introduction of additional particles in the spectrum of the theory. Nonlocal scale [Formula: see text] responsible for ultraviolet cutoff coincides (up to some factor) with GUT scale [Formula: see text]. We find that in the simplest nonlocal modification of the SU(5) model, [Formula: see text]GeV. In the general case, the value of [Formula: see text] is an arbitrary and the most interesting option [Formula: see text] could be realized.