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Register a new userSearch for a Hypothetical 16.7 MeV Gauge Boson and Dark Photons in the NA64 Experiment at CERN
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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 Be-8 nucleus decays. Due to its coupling to electrons, the X could be produced in the bremsstrahlung reaction e- Z -> e- Z X by a 100 GeV e- beam incident on an active target in the NA64 experiment at the CERN SPS and observed through the subsequent decay into an e+e- pair. With 5.4times 10^{10} electrons on target, no evidence for such decays was found, allowing to set first limits on the X-e^- coupling in the range 1.3times 10^{-4} < epsilon_e < 4.2times 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 non-observation of the decay A->e+e- of the bremsstrahlung A with a mass <~ 23 MeV.
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We present a search at Jefferson Laboratory for new forces mediated by sub-GeV vector bosons with weak coupling $alpha$ to electrons. Such a particle $A$ can be produced in electron-nucleus fixed-target scattering and then decay to an $e^+e^-$ pair, producing a narrow resonance in the QED trident spectrum. Using APEX test run data, we searched in the mass range 175--250 MeV, found no evidence for an $Ato e^+e^-$ reaction, and set an upper limit of $alpha/alpha simeq 10^{-6}$. Our findings demonstrate that fixed-target searches can explore a new, wide, and important range of masses and couplings for sub-GeV forces.
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 $a(s)togamma gamma$ 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 $2.84times10^{11}$ electrons on target allowing to set new limits on the $a(s)gammagamma$-coupling strength for a(s) masses below 55 MeV.
The improved results on a direct search for a new X(16.7 MeV) boson which could explain the anomalous excess of $e^+e^-$ pairs observed in the excited 8Be nucleus decays (Berillium anomaly) are reported. Due to its coupling to electrons, the X boson could be produced in the bremsstrahlung reaction e-Z -> e-ZX by a high-energy beam of electrons incident on active target in the NA64 experiment at the CERN SPS and observed through its subsequent decay into $e^+e^-$ pair. No evidence for such decays was found from the combined analysis of the data samples with total statistics corresponding to 8.4times 10^{10} electrons on target collected in 2017 and 2018. This allows to set the new limits on the $X$--$e^-$ coupling in the range 1.2 times 10^{-4} < epsilon_e < 6.8 times 10^{-4}, excluding part of the parameter space favored by the Berillium anomaly. We also set new bounds on the mixing strength of photons with dark photons (A) from non-observation of the decay $A to e^+e^-$ of the bremsstrahlung A with a mass below 24 MeV.
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 As would decay invisibly into dark matter particles resulting in events with large missing energy. No evidence for such decays was found with 2.75cdot 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.
A search for sub-GeV dark matter production mediated by a new vector boson $A$, called 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 $2.84times10^{11}$ 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 $lesssim 0.2$ GeV are derived, thus demonstrating the power of the active beam dump approach for the dark matter search.
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