Vassili Kachanov

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 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.

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 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.

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^{'}}.

Measurements were made of the cross section of the reactions π−p → ν′(958)n, η′ → 2γ at momenta at 15, 20, 25, 30 and 40 GeV/c. The experiment was carried out on the IHEP 70 GeV accelerator using the 648 channel hodoscope spectrometer NICE for γ-ray detection. A total of 6000 η′ mesons were recorded. A sharp drop is seen in the differential cross section for t → 0. The dependences of the differential cross sections for the π−p → η′n and π−p → η n on t are identical. On the basis of the ratio of the cross sections for these reactions at t = 0, i.e. R(η′n)t=0 = 0.55 ± 0.06, the singlet-octet mixing angle for pseudoscalar mesons was determined to be β = −(18.2 ± 1.4)°.

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 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.

Measurements have been performed on production of particles with mass 1.5 GeV/c2 and charge ⩾ 23 for θlab = 62.5° and s = 53 GeV. At pT = 0.7 GeV/c the relative rate of production of antideuterons to π− is (5 ± 1) × 10−5. The deuteron to antideuteron ratio is 3.7 ± 1.2. No new stable particle has been amongst 0.7 × 108 charged particles entering our detector.

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 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.

A high statistics measurement of the reaction π−p → π0n has been performed at the Serpukhov accelerator for 15, 20, 25, 30 and 40 GeV/c incident pion momentum using the NICE set-up with its associated 648-channel hodoscope spectrometer for γ-ray detection. More than 3 million charge-exchange events have been recorded in total. It is found that the spin-flip and non-spin-flip amplitudes can be parametrized, for small |t|, as exponentials with the same slopes to within a few percent. Also the behaviour of the differential cross section for small and medium |t| agrees with the prediction of a geometrical s-channel model which describes binary reactions in terms of a complex pole b0(s). The imaginary part of this universal pole, Im b0(s), has been determined and found to be growing logarithmically with s.

The invariant mass spectrum of neutral final states produced in π−p charge-exchange scattering at 40 GeV/c has been studied, searching for heavy particles decaying in 2γ. A peak is observed around 2.85 GeV/c2. The cross section of the reaction π−p→X(2.85)+n, times the branching ratio of the X→2γ decay, is measured to be σ × BR ⋍ 2 × 10−34cm2.

The rare radiative decay η → π0γγ has been effectively observed and studied at the 70 GeV IHEP accelerator with the hodoscope spectrometer GAMS-2000. The branching ratio for this decay is found to be Br(η → π0γγ) = (9.5 ± 2.3)-10-4.

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.

The production of χ particles in the reaction π−p → χ + … has been studied near threshold. The measurements have been performed at the 70 GeV IHEP proton synchrotron using the hodoscope spectrometer GAMS-2000. The χ particles have been identified through their decay χ → J/ψ + γ, J/ψ → e+e−. In the region XF ≳0.4 the fraction of obsered J/ψ particles which is obtained via the decay of χ states is 0.44 ± 0.16. The χ longitudinal and transverse momentum distributions are similar to those obtained for J/ψ. The inclusive production cross section of χ particles (3P1 and 3P2 states) at 38 GeV/c is σXF>0 (π−p → χ + …) = (28±11) nb.

π− p→π0 n differential cross-sections have been measured in the region of small 4-momentum transfer at 40 GeV/c incident momentum. The experiment performed at the IHEP 70 GeV accelerator makes use of a hodoscope γ-spectrometer. Thet- dependence of the cross-section points to a dominance of the spin-flip amplitude.

In this study, 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 momentum. The analysis was performed using data collected by the NA64-$e$ experiment in 2022. Our study is based on calorimetric measurements, exploiting the different interaction mechanisms of electrons and hadrons in the NA64-ECAL and NA64-HCAL detectors. We determined the intrinsic hadronic contamination by comparing the results obtained using the nominal electron/positron beamline configuration with those obtained in a dedicated setup, in which only hadrons impinged on the detector. The significant differences in the experimental signatures of electrons and hadrons motivated our approach, resulting in a small and well-controlled systematic uncertainty for the measurement. Our study allowed us to precisely determine the intrinsic hadronic contamination, which represents a crucial parameter for the NA64 experiment in which the hadron contaminants may result in non-trivial backgrounds. Moreover, we performed dedicated Monte Carlo simulations for the hadron production induced by the primary T2 target. We found a good agreement between measurements and simulation results, confirming the validity of the applied methodology and our evaluation of the intrinsic hadronic contamination.

Thermal dark matter models with particle $\chi$ 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 $\chi$ 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 $9.37\times10^{11}$ 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 $m_{A'} \lesssim 0.35$ GeV, and to exclude scalar and Majorana dark matter with the $\chi-A'$ coupling $\alpha_D \leq 0.1$ for masses $0.001 \lesssim m_\chi \lesssim 0.1$ GeV and $3m_\chi \leq m_{A'}$.

We present the results of a missing-energy search for Light Dark Matter which has a new interaction with ordinary matter transmitted by a vector boson, called dark photon $A^\prime$. For the first time, this search is performed with a positron beam by using the significantly enhanced production of $A^\prime$ in the resonant annihilation of positrons with atomic electrons of the target nuclei, followed by the invisible decay of $A^\prime$ into dark matter. No events were found in the signal region with $(10.1 \pm 0.1)~\times~10^{9}$ positrons on target with 100 GeV energy. This allowed us to set new exclusion limits that, relative to the collected statistics, prove the power of this experimental technique. This measurement is a crucial first step toward a future exploration program with positron beams, whose estimated sensitivity is here presented.

The $$K^ - p \to \bar K^0 n$$ charge exchange differential cross-section has been measured at 30 GeV/c with high statistical accuracy and high angular resolution. The experiment was made at the IHEP 70 GeV accelerator, by using a hodoscope hadron calorimeter to detect K L 0 . The cross-section shows a dip at small |t|, which indicates a dominance of spin-flip amplitudes in ρ and A2 exchanges in thet-channel.

A mathematical model for  calculation of a jet power impact on a barrier consisting of a corrosive deposit layer  has been proposed on the basis of an upper-bound method and accepted assumptions. A variational problem associated with determination of a minimum destruction pressure has been solved and a theoretical dependence for pressure calculation at the point of jet-barrier collision with due account of mechanical properties of the destroyed material and kinematic jet parameters has been obtained in the paper. Comparison of experimental data obtained by means of the EDP-30 sensor with the theoretical ones has shown a divergence of 15–19 % within velocity range of the jet impact from 150 to 300 m/s.

GAMMA-400 is a future high-energy gamma-ray telescope, designed to measure the fluxes of gamma-rays and cosmic-ray electrons + positrons, which can be produced by annihilation or decay of dark matter particles, and to survey the celestial sphere in order to study point and extended sources of gamma-rays, measure energy spectra of Galactic and extragalactic diffuse gamma-ray emission, gamma-ray bursts, and gamma-ray emission from the Sun. GAMMA-400 covers the energy range from 100 MeV to ~3000 GeV. Its angular resolution is ~0.01 deg(Eg &gt; 100 GeV), and the energy resolution ~1% (Eg &gt; 10 GeV). GAMMA-400 is planned to be launched on the Russian space platform Navigator in 2019. The GAMMA-400 perspectives in the search for dark matter in various scenarios are presented in this paper

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 shielding, and would be observed either through their $a(s)\to\gamma \gamma$ decay in the rest of the HCAL detector or as events with large missing energy if the a(s) decays downstream of the HCAL. This method allows for the probing 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 $2.84\times10^{11}$ electrons on target allowing to set new limits on the $a(s)\gamma\gamma$-coupling strength for a(s) masses below 55 MeV.

Recently, the ATOMKI experiment has reported new evidence for the excess of $e^+ e^-$ events with a mass $\sim$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 $X17 \rightarrow e^+ e^-$ 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 [Phys. Rev. D101, 071101 (2020)], validating the tracking procedure. The detailed Monte Carlo study of the proposed setup and the background estimate shows that the goal of the proposed search is feasible.

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 report the results of a search for a new vector boson ($A'$) decaying into two dark matter particles $\chi_1 \chi_2$ of different mass. The heavier $\chi_2$ particle subsequently decays to $\chi_1$ and $A' \to e^- e^+$. 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 $A'\to \chi \overline{\chi}$ and axion-like or pseudo-scalar particles $a \to \gamma \gamma$. 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$m_e$ up to 390 MeV and mixing parameter $\epsilon$ between $3\times10^{-5}$ and $2\times10^{-2}$.

The current most stringent constraints for the existence of sub-GeV dark matter coupling to Standard Model via a massive vector boson $A^\prime$ were set by the NA64 experiment for the mass region $m_{A^\prime}\lesssim 250$ MeV, by analyzing data from the interaction of $2.84\cdot10^{11}$ 100-GeV electrons with an active thick target and searching for missing-energy events. In this work, by including $A^\prime$ 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.

Cross sections for the reaction ..pi../sup -/p..--> omega..n, ..omega --> pi../sup 0/..gamma.. have been measured with good statistics at incident-pion momenta of 15, 20, 25, 30, and 40 GeV/c. The experiments were performed at the 70-GeV IHEP accelerator, using the NICE 648-channel hodoscopic spectrometer. The density matrix elements and the differential and integrated cross sections for the reaction have been determined. The unnatural-parity exchange contribution to the production of ..omega.. mesons in the state with helicity 1 is already insignificant at 15 GeV/c. With increasing Vertical BartVertical Bar the differential cross sections pass through a maximum at Vertical BartVertical Barapprox. =0.12 (GeV/c)/sup 2/ and then fall exponentially until Vertical BartVertical Barapprox. =0.7 (GeV/c)/sup 2/; at that point the curve bends sharply, the cross section falling off much less rapidly for larger values of Vertical BartVertical Bar.

Cross sections for the reaction ..pi../sup -/p..-->..etan are measured at the momenta 15 and 40 GeV/c. About 30,000 events are recorded. A hodoscope spectrometer on line with a computer was used to detect ..gamma.. pairs from eta-meson decays. The differential cross section decreases exponentially with increasing t, the 4-momentum transfer squared, and for -t>2 (GeV/c)/sup 2/ it is less than 1 nb/(GeV/c)/sup 2/. The A/sub 2/ trajectory is determined; it has a linear form ..cap alpha../sub A/2(t) =0.45+0.7t in the region 0<-t9 or approx. =1 (GeV/c)/sup 2/.

The mass spectrum of muon pairs in the reaction ..pi../sup -/p..--> mu../sup +/..mu../sup -/n has been investigated at momenta 25 and 33 GeV/c. A peak corresponding to the rare decay eta..--> mu../sup +/..mu../sup -/ is distinguished in the mass spectrum. The measured relative probability of this decay (6.5 +- 2.1) x 10/sup -6/ is in good agreement with theoretical estimates.

The cross sections of the reactions ..pi../sup -/p..-->..X/sup 0/n with the decay X/sup 0/..-->..2..gamma.. were measured at momenta 15 and 40 GeV/c. About 500 events were registered. The ..gamma../sup -/ quantum pairs were detected with a hodoscopic spectrometer on line with a computer. The t-dependence of the differential cross section was found to be similar to that of the reaction ..pi../sup -/p..-->..etan. The angle of the singlet-octet mixing of the pseudoscalar mesons was determined from the ratio of the differential cross sections for the production of X/sup 0/ and eta mesons at t=0 and found to be ..beta..=-(16 +- 2) /sup 0/.

Differential cross sections for the charge-exchange scattering of K/sup -/ mesons by protons have been measured at 25 and 40 GeV/c by a method that does not involve the use of a magnetic field. The reaction cross section is proportional to P/sup -1.52/ in the incident-momentum interval 5

The 70-GeV accelerator at the Institute of High Energy Physics has been used to carry out a search for associated production of particles with the new hadronic quantum number charm in the reaction ..pi../sup -/p..-->..D/sup 0/anti D/sup 0/n at a pion momentum of 40 GeV/c. The decays D/sup 0/..-->..anti K/sup 0/..pi../sup 0/ were identified by spark chambers, proportional chambers, and hodoscopes recording the decay K/sup 0//sub S/..--> pi../sup +/..pi../sup -/, and a 648-channel hodoscopic spectrometer on-line with a computer, recording ..gamma.. rays from decay of ..pi../sup 0/ mesons.

Data are presented on the charge-exchange ..pi../sup -/p scattering at a momentum 40 GeV/c at large momentum transfers. The experiment was performed with the 70-GeV Serpukhov accelerator using a 648-channel hodoscopic ..gamma.. spectrometer. The total statistics amounted to more than a million ..pi../sup 0/ mesons, making it possible to advance into the momentum-transfer region up to -t=1.8 (GeV/c)/sup 2/. The experimental data are compared with the optical model of the impact parameter. The imaginary part of the trajectory of the b/sub 0/(s) pole is determined from the slopes of the maxima of the function ..sqrt..-t dsigma/dt. A linear increase of Imb/sub 0/(s) with log s is observed.

We have detected 600 events of the rare electromagnetic decay eta..--> mu../sup +/..mu../sup -/..gamma... The measured relative decay probability is BR(eta..--> mu../sup +/..mu../sup -/..gamma..) = (3.1 +- 0.4) x 10/sup -4/. The electromagnetic form factor of the eta meson was found to be F(m/sup 2//sub mumu/;0) = (1-m/sup 2//sub mumu//..lambda../sup 2/)/sup -1/, where ..lambda.. = 0.72 +- 0.09 GeV/c/sup 2/, which is in good agreement with the vector-dominance model.

The mass spectrum of ..mu../sup +/..mu../sup -/..gamma.. systems produced in ..pi../sup -/p collisions at momentum 25 GeV/c is investigated. The muon pairs and ..gamma.. rays were recorded with a wide-aperture magnetic spectrometer with a hodoscopic ..gamma.. detector. A distinct peak, corresponding to a hitherto unobserved eta..--> mu../sup +/..mu../sup -/..gamma.. decay, is separated in the mass spectrum. The relative probability of the observed decay was found to be 1.5 x 10/sup -4/.

The cross sections of the reaction ..pi../sup -/p..--> pi../sup 0/n were measured with high statistical accuracy at momenta 15, 20, 25, 30, and 40 GeV/c. The experiments were performed with the 70-GeV IHEP accelerator using the NICE 648-channel hodoscopic spectrometer. About 3 million ..pi../sup 0/ mesons were registered. A pulse-height analysis of the reaction at small t has shown that the amplitudes of scattering with and without spin flip are parametrized by exponentials whose arguments are equal within several per cent. The t-dependence of the differential cross section at medium and large t agrees with the model of the geometrical s-channel approximation. The imaginary part Imb/sub 0/(s) of the universal pole is determined. A logarithmic growth of Imb/sub 0/(s) with increasing energy is observed.

The mass spectrum of ..mu../sup +/..mu../sup -/..gamma.. systems produced in the ..pi../sup -/ rho interactions at momenta of 25 and 33 GeV/c was investigated. A wide-aperture magnetic spectrometer and a hodoscopic ..gamma..-ray detector were used for the recording of muon pairs and ..gamma.. quanta. A sharp peak corresponding to the eta' ..-->.. ..mu../sup +/..mu../sup -/..gamma.. decay not observed earlier was detected in the mass spectrum. The relative probability of this decay (8 x 10/sup -5/) and the spectrum of muon pairs are in agreement with the model of the form factor of the eta' meson, which is based on vector dominance.

The invariant-mass spectrum of neutral states produced in charge-exchange scattering of 40-GeV/c ..pi../sup -/ mesons on protons is studied to search for heavy particles which decay into two ..gamma.. rays. The ..gamma.. rays were detected by a hodoscope spectrometer on-line with a computer. The mass spectrum shows a peak which is identified with chi (2.85). The reaction cross section times the branching ratio of the decay chi..-->..2..gamma.. is found to be 2 x 10/sup -34/ cm/sup 2/.

Experiments have been carried out to search for the decay eta..--> pi../sup 0/..gamma gamma.. at a level of accuracy ten times better than that in earlier studies in which this decay was observed. The measurements were carried out at the 70 GeV IHEP accelerator using the hodoscopic ..gamma.. spectrometer GAMS-2000. It is shown that the decay eta..--> pi../sup 0/..gamma gamma.. was not observed in the earlier studies. An upper limit is obtained for the partial width of this decay at the 90% confidence level: BR (eta..--> pi../sup 0/..gamma gamma..)<3 x 10/sup -3/.

We report the results of an experimental study of the reaction ..pi../sup -/+p..-->..m..gamma..+n at the 70-GeV accelerator of the Institute of High Energy Physics. A hodoscopic spectrometer on-line with a computer was used to record ..gamma.. rays in the final state of the reaction. The reaction ..pi../sup -/p..-->..etaetan (eta..-->..2..gamma..) was identified for the first time. In the region of invariant mass of the etaeta system 1600--2800 MeV we have separated a massif of ''clean'' events free of admixture of ..pi../sup 0/..pi../sup 0/ states. The ratio of the cross sections for formation of the final states etaeta and ..pi../sup 0/..pi../sup 0/ was found to be approx.1:5 in agreement with the quark model. A distinct anisotropy was found in the angular distribution of the etaeta system in the Gottfried-Jackson center-of-mass system for masses above 1600 MeV.

The cross sections of the reaction ..pi../sup -/p..-->..eta'(958)n, eta'--2..gamma.. were measured at the momenta 15, 20, 25, 30, and 40 GeV/c. The experiments were performed on the 70-GeV IHEP accelerator using the NICE 648-channel hodoscopic spectrometer. A total of 6000 eta' mesons were registered. The observed dip of the differential cross sections of the reactions ..pi..'p..-->..eta'n and ..pi../sup -/p..-->..etan as functions of t are identical. The ratio R(eta'/eta)/sub t/=0=0.55 +- 0.06 of the cross sections of these reactions yields the angle ..beta..=-(18.2 +- 1.4)/sup 0/ of the singlet-octet mixing of the pseudoscalar mesons.

Meson states formed in ..pi../sup -/p collisions at 38 GeV/c and decaying to two ..pi../sup 0/ mesons are studied in the mass range up to 3 GeV/c/sup 2/. The experiment was performed with high statistics (200 events/nb) at the 70 GeV IHEP accelerator. The hodoscope spectrometer GAMS-2000 was used for detection of photons from the decay M /sup 0/..--> pi../sup 0/..pi../sup 0/..-->..4..gamma.. in the reaction ..pi../sup -/p..-->..M /sup 0/n. A new ..pi pi.. resonance was observed with mass 2510 +- 30 MeV/c/sup 2/ and width GAMMA = 240 +- 60 MeV/c/sup 2/. The quantum numbers of the observed r(2510) meson have been determined: J /sup P/C = 6/sup + +/. The cross section for production of f, h, and r mesons has an exponential dependence on spin approx.exp(-0.9J).

The reaction ..pi../sup -/p..--> pi../sup 0/..pi../sup 0/n has been studied at momentum 25 GeV/c. The 648-channel hodoscopic spectrometer NICE was used for detection of the four ..gamma.. rays from decay of the ..pi../sup 0/ mesons. The D wave of the ..pi../sup 0/..pi../sup 0/ system agrees completely with the contribution of the f meson determined in ..pi../sup +/..pi../sup -/ experiments. On the basis of fit to the angular distributions, the intensity of the S wave of the ..pi../sup 0/..pi../sup 0/ system and the cosine of its phase shift relative to the known D wave have been determined. Argand diagrams are given for the S-wave amplitude with isospin I = 0 in the mass region 1000--1500 MeV. Two solutions are obtained. One of them exceeds the unitary limit for masses of the ..pi../sup 0/..pi../sup 0/ system above 1200 MeV. The other solution remains within the limits of unitarity and is almost elastic up to 1450 MeV. It indicates the existence of a resonance in the S/sub 0/ wave in the region of 1300 MeV (the Epsilon meson).

We have analyzed the reaction ..pi../sup -/p ..-->.. ..pi../sup 0/etan(eta ..-->.. 2..gamma..) on the basis of the data from an experiment carried out in the 70-GeV accelerator at Serpukhov with use of the NICE 648-channel hodoscopic spectrometer for detection of the ..gamma.. rays. We used a method which permits determination of the number of ..pi../sup 0/eta events for each interval in mass, costheta/sub GJ/, and t. For the ..pi../sup 0/eta system we obtained mass spectra, t distributions, and distributions in the decay angle. The cross section for production of A/sup 0//sub 2/ mesons at momentum 40 GeV/c was found to be 2.7 +- 1.0 ..mu..b. No indications were obtained of a 1/sup -/ resonance in the ..pi../sup 0/eta system, although such a state is allowed for the ..pi../sup 0/eta system on the same basis as the observed 0/sup +/ and 2/sup +/ resonances.

The exclusive reactions for {pi}{sup 0}-, {eta}-, and {eta}{prime}-meson production in central {pi}{sup {minus}}{ital N} interaction are investigated with {pi}{sup {minus}} mesons with energy 300 GeV. The experiment is carried out at the CERN accelerator. To register {gamma} quanta from the decay of mesons we used the hodoscopic multiphoton spectrometer GAMS-4000. The differential cross sections are determined and are found to be constant in the longitudinal-momentum range 0.1{approx lt}{ital x}{sub F}{approx lt}0.3.

Structure in the vicinity of 2220 MeV has been observed in the mass spectrum and angular distributions of eta'eta systems produced by ..pi../sup -/ mesons of energy 38 and 100 GeV. The structure is observed independently in two experiments carried out in the accelerators at the Institute of High Energy Physics and at CERN by means of GAMS hodoscopic multiphoton spectrometers. The sharply anisotropic angular distribution of decay of the eta'eta systems shows that the spin of this state is Jgreater than or equal to2.

D(1285)- and E(1420)-meson production in ..pi../sup -/- and K/sup -/-meson charge-exchange reactions at 32.5 GeV/c are studied. The D(1285) production cross section is found to be sigma(..pi../sup -/p..-->..D(1285)n) = 4.3 +- 1.3 ..mu..b and the ratio sigma(..pi../sup -/p..-->..E(1420)n)/sigma(..pi../sup -/p..-->..D(1285)n) ..K/sup +/K/sup -/..pi../sup 0/Y. The large value sigma(K/sup -/p..-->..E(1420)Y)/sigma(..pi../sup -/p..-->..E(1420)n)>10 confirms this conclusion about the quark content of the E meson.

The parameters of the linear matrix element have been measured with high accuracy for the decay eta'..-->..eta..pi../sup 0/..pi../sup 0/. It is shown that the matrix element is practically constant; it depends weakly on only one of the Dalitz variables. The experiment has been performed at the 70-GeV IHEP accelerator. For detection of the six photons from the eta' decay we used the GAMS--2000 hodoscopic spectrometer.

Partial widths have been measured for the decays of the eta' meson into 2..gamma.., ..omega gamma.., 3..pi../sup 0/, and eta..pi../sup 0/..pi../sup 0/. Upper-limit estimates have been obtained for the decays of the eta' meson into 3..gamma.., ..pi../sup 0/..gamma gamma.., 2..pi../sup 0/, and 4..pi../sup 0/ . The slope of the linear matrix element for violating G parity of the decay eta'..-->..3..pi../sup 0/ has been determined. The experiment was performed with high accuracy at the 70-GeV accelerator at the Institute of High Energy Physics. An automated hodoscopic multiphoton spectrometer GAMS-2000 was used for detection of the ..gamma.. rays from decay of the eta' meson.

The rare decay eta'(958)..-->..3..pi../sup 0/, which does not conserve isospin, has been observed in an experiment carried out in the 70-GeV accelerator at the Institute of High Energy Physics. For detection of the six ..gamma.. rays representing the final decay products we used the GAMS-2000 hodoscopic spectrometer. The partial width of the decay is found to be BR(eta'..-->..3..pi../sup 0/) = (1.6 +- 0.4)x10/sup -3/. Estimates of the masses of the u and d quarks are obtained.