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.
In this paper we estimate the sensitivity of the NA64 experiment to millicharged particles (χ). 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 ≃100 GeV electron and muon beams and show that the later one allows to obtain more stringent bounds on the millicharge Qχ, which for the χ masses 100 MeV≤mχ≤500 MeV at the level Qχ/e≲O(10−3)−O(10−2).
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.
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.
We carried out a model-independent search for light scalar (s) and pseudoscalar axionlike (a) particles that couple to two photons by using the high-energy CERN SPS H4 electron beam. The new ...particles, if they exist, could be produced through the Primakoff effect in interactions of hard bremsstrahlung photons generated by 100 GeV electrons in the NA64 active dump with virtual photons provided by the nuclei of the dump. The a (s) would penetrate the downstream HCAL module, serving as a shield, and would be observed either through their a (s) → γγ decay in the rest of the HCAL detector, or as events with a large missing energy if the a (s) decays downstream of the HCAL. This method allows for the probing of the a (s) parameter space, including those from generic axion models, inaccessible to previous experiments. No evidence of such processes has been found from the analysis of the data corresponding to 2.84 × 10 11 electrons on target, allowing us to set new limits on the a(s)γγ -coupling strength for a (s) masses below 55 MeV.
A 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.
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.
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.
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 → e− ZX, followed by its invisible decay in the NA64 experiment at CERN. No evidence for such a process was found in the full NA64 dataset of 2.84 × 1011 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 ae,|ΔaX|≲ 10−15 –10−13 for the X mass region 1 MeV ≲ mX ≲ 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.
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