Axionlike particles a (ALPs) that couple to the Standard Model gauge fields could be observed in the high-energy photon scattering γN → Na off nuclei followed by the a → γ γ decay. In the present ...paper we describe the calculation of the ALP production cross section and the properties of this production. The cross section formulas are implemented in the program for the simulation of events in the NA64 experiment, the active electron beam dump facility at the CERN Super Proton Synchrotron. We study the prospects of the NA64 experiment to search for ALP in the 10 MeV ≲ ma ≲ 100 MeV mass range for the statistics corresponding to up to 5 × 1012 electrons on target.
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′→invisible decays with a high-energy electron beam at the CERN SPS (The Super Proton Synchrotron). We show the distinctive distributions of variables that can be used to distinguish the A′→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 us to probe the still unexplored area of the mixing strength 10−6≲ε≲10−2 and masses up to MA′≲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 MA′ and ε by using the shape of the missing energy spectrum is discussed.
Vector bosons, such as dark photon A′ or Z′, can couple to muons and be produced in the bremsstrahlung reaction μ− + N → μ− + N + A′ (Z′). Their possible subsequent invisible decay can be detected in ...fixed target experiments through missing energy/momentum signature. In such experiments, not only is the energy transfer to A ′(Z′) important but also the recoil muon angle ψμ′. In this paper, we derive the total and the double differential cross sections involved in this process using the phase space Weizsäcker-Williams and improved Weizsäcker-Williams approximations, as well as using exact-tree-level calculations. As an example, we compare the derived cross sections and resulting signal yields in the NA 64μ experiment that uses a 160 GeV muon beam at the CERN Super Proton Synchrotron accelerator. We also discuss its impact on the NA 64μ expected sensitivity to explore the (g−2)μ anomaly favored region with a Z′ boson considering 1012 muons accumulated on target.
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 2.84×1011 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.
The improved results on a direct search for a new X ( 16.7 MeV ) boson that could explain the anomalous excess of e+e− pairs observed in the decays of the excited 8Be ∗ nuclei ("Berillium or X17 ...anomaly") are reported. Interestingly, new recent results in the nuclear transitions of another nucleus, 4 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 − Z → e − Z X 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− pairs. No evidence for such decays was found from the combined analysis of the data samples with total statistics corresponding to 8.4 × 1010 electrons on target collected in 2017 and 2018. This allows one to set new limits on the X−e− coupling in the range 1.2 × 10−4 ≲ ε e ≲ 6.8 × 10−4 , 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 ′ ) with a mass ≲ 24 MeV . For the 2018 run, the setup was optimized to probe the region of parameter space characterized by a large coupling ε . 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.
We report on a direct search for sub-GeV dark photons (A^{'}), which might be produced in the reaction e^{-}Z→e^{-}ZA^{'} via kinetic mixing with photons by 100 GeV electrons incident on an active ...target in the NA64 experiment at the CERN SPS. The dark photons would decay invisibly into dark matter particles resulting in events with large missing energy. No evidence for such decays was found with 2.75×10^{9} electrons on target. We set new limits on the γ-A^{'} mixing strength and exclude the invisible A^{'} with a mass ≲100 MeV as an explanation of the muon g_{μ}-2 anomaly.
A search is performed for a new sub-GeV vector boson (A′) mediated production of dark matter (χ) in the fixed-target experiment, NA64, at the CERN SPS. The A′, called dark photon, can be generated in ...the reaction e−Z→e−ZA′ of 100 GeV electrons dumped against an active target followed by its prompt invisible decay A′→χχ¯. 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 4.3×1010 electrons on target no evidence of such a process has been found. New stringent constraints on the A′ mixing strength with photons, 10−5≲ε≲10−2, for the A′ mass range mA′≲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≲y≲10−6 on the dark-matter parameter y=ε2αD(mχmA′)4 are obtained for the dark coupling constant αD=0.5 and dark-matter masses 0.001≲mχ≲0.5 GeV. The lower limits αD≳10−3 for pseudo-Dirac dark matter in the mass region mχ≲0.05 GeV are more stringent than the corresponding bounds from beam dump experiments. The results are obtained by using exact tree level calculations of the A′ production cross sections, which turn out to be significantly smaller compared to the one obtained in the Weizsäcker-Williams approximation for the mass region mA′≳0.1 GeV.
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.
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