Search for Axionlike and Scalar Particles with the NA64 Experiment

Indexación: Scopus.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 2.84×1011 electrons on target, allowing us to set new limits on the a(s)γγ-coupling strength for a(s) masses below 55 MeV. © 2020 authors. Published by the American Physical Society. Funded by SCOAP3.https://journals-aps-org.recursosbiblioteca.unab.cl/prl/abstract/10.1103/PhysRevLett.125.08180

Hunting down the X17 boson at the CERN SPS

Indexación ScopusRecently, the ATOMKI experiment has reported new evidence for the excess of e+e- events with a mass ∼ 17 MeV in the nuclear transitions of 4He, that they previously observed in measurements with 8Be. 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. © 2020, The Author(s).https://link-springer-com.recursosbiblioteca.unab.cl/article/10.1140%2Fepjc%2Fs10052-020-08725-

Photoproduction of axionlike particles in the NA64 experiment

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→γγ 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.Axion-like particles $a$ (ALPs) that couple to the Standard Model (SM) gauge fields could be observed in the high-energy photon scattering $\gamma N\to N a$ off nuclei followed by the $a\to \gamma\gamma$ 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 SPS. We study the prospects of the NA64 experiment to search for ALP in the 10\, \mbox{MeV} \lesssim m_a\lesssim 100 MeV mass range for the statistics corresponding to up to $5\times 10^{12}$ electrons on target (EOT)

Search for Light Dark Matter with NA64 at CERN

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 9.37×10¹¹ 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 mA′ ≲ 0.35 GeV, and to exclude scalar and Majorana dark matter with the χ-A′ coupling αD ≤ 0.1 for masses 0.001 ≲ mχ ≲ 0.1 GeV and 3 mχ ≤ mA′.ISSN:0031-9007ISSN:1079-711

Performance of Multiplexed XY Resistive Micromegas detectors in a high intensity beam

We present the performance of multiplexed XY resistive Micromegas detectors tested in the CERN SPS 100 GeV/c electron beam at intensities up to 3.3×105e−∕(s⋅cm2) . So far, all studies with multiplexed Micromegas have only been reported for tests with radioactive sources and cosmic rays. The use of multiplexed modules in high intensity environments was not explored due to the effect of ambiguities in the reconstruction of the hit point caused by the multiplexing feature. For the specific mapping and beam intensities analyzed in this work with a multiplexing factor of five, more than 50% level of ambiguity is introduced due to particle pile-up as well as fake clusters due to the mapping feature. Our results prove that by using the additional information of cluster size and integrated charge from the signal clusters induced on the XY strips, the ambiguities can be reduced to a level below 2%. The tested detectors are used in the CERN NA64 experiment for tracking the incoming particles bending in a magnetic field in order to reconstruct their momentum. The average hit detection efficiency of each module was found to be ∼ 96% at the highest beam intensities. By using four modules a tracking resolution of 1.1% was obtained with ∼ 85% combined tracking efficiency.We present the performance of multiplexed XY resistive Micromegas detectors tested in the CERN SPS 100 GeV/c electron beam at intensities up to 3.3 $\times$ 10$^5$ e$^-$/(s$\cdot$cm$^2$). So far, all studies with multiplexed Micromegas have only been reported for tests with radioactive sources and cosmic rays. The use of multiplexed modules in high intensity environments was not explored due to the effect of ambiguities in the reconstruction of the hit point caused by the multiplexing feature. At the beam intensities analysed in this work and with a multiplexing factor of 5, more than 50% level of ambiguity is introduced. Our results prove that by using the additional information of cluster size and integrated charge from the signal clusters induced on the XY strips, the ambiguities can be reduced to a level below 2%. The tested detectors are used in the CERN NA64 experiment for tracking the incoming particles bending in a magnetic field in order to reconstruct their momentum. The average hit detection efficiency of each module was found to be $\sim$ 96% at the highest beam intensities. By using four modules a tracking resolution of 1.1% was obtained with $\sim$ 85% combined tracking efficiency

Performance of Multiplexed XY Resistive Micromegas detectors in a high intensity beam

We present the performance of multiplexed XY resistive Micromegas detectors tested in the CERN SPS 100 GeV/c electron beam at intensities up to 3.3 × 10 5e−/(s-cm2). So far, all studies with multiplexed Micromegas have only been reported for tests with radioactive sources and cosmic rays. The use of multiplexed modules in high intensity environments was not explored due to the effect of ambiguities in the reconstruction of the hit point caused by the multiplexing feature. For the specific mapping and beam intensities analyzed in this work with a multiplexing factor of five, more than 50% level of ambiguity is introduced due to particle pile-up as well as fake clusters due to the mapping feature. Our results prove that by using the additional information of cluster size and integrated charge from the signal clusters induced on the XY strips, the ambiguities can be reduced to a level below 2%. The tested detectors are used in the CERN NA64 experiment for tracking the incoming particles bending in a magnetic field in order to reconstruct their momentum. The average hit detection efficiency of each module was found to be 96% at the highest beam intensities. By using four modules a tracking resolution of 1.1% was obtained with 85% combined tracking efficiency.ISSN:0168-9002ISSN:1872-957

Dark matter search in missing energy events with NA64

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.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 $2.84\times10^{11}$ 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

Improved limits on a hypothetical X(16.7) boson and a dark photon decaying into e+e−e^+e^- pairs

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 Be*8 nuclei (“Berillium or X17 anomaly”) are reported. Interestingly, new recent results in the nuclear transitions of another nucleus, He4, 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-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- 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≲εe≲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.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 $e^-Z\rightarrow e^-ZX$ 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 $8.4 \times 10^{10}$ electrons on target collected in 2017 and 2018. This allows to set the new limits on the $X-e^-$ coupling in the range $1.2 \times 10^{-4}\underset{\sim}{<}\epsilon_e \underset{\sim}{<}6.8\times 10^{-4}$, 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

Improved exclusion limit for light dark matter from e+e−e^+e^- annihilation in NA64

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.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