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 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 \(2\times10^{-2}\).
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.
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.
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 5.4\times 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 1.3\times 10^{-4} < \epsilon_e < 4.2\times 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.
A search is performed for a new sub-GeV vector boson (\(A'\)) mediated production of Dark Matter (\(\chi\)) in the fixed-target experiment, NA64, at the CERN SPS. The \(A'\), called dark photon, ...could be generated in the reaction \( e^- Z \to e^- Z A'\) of 100 GeV electrons dumped against an active target which is followed by the prompt invisible decay \(A' \to \chi \overline{\chi}\). 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 \(4.3\times10^{10}\) electrons on target no evidence of such a process has been found. New stringent constraints on the \(A'\) mixing strength with photons, \(10^{-5}\lesssim \epsilon \lesssim 10^{-2}\), for the \(A'\) mass range \(m_{A'} \lesssim 1\) 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 \(10^{-11}\lesssim y \lesssim 10^{-6}\) on the dark-matter parameter \(y = \epsilon^2 \alpha_D (\frac{m_\chi}{m_{A'}})^4 \) are obtained for the dark coupling constant \(\alpha_D = 0.5\) and dark-matter masses \(0.001 \lesssim m_\chi \lesssim 0.5 \) GeV. The lower limits \(\alpha_D \gtrsim 10^{-3} \) for pseudo-Dirac Dark Matter in the mass region \(m_\chi \lesssim 0.05 \) GeV are more stringent than the corresponding bounds from beam dump experiments. The results are obtained by using tree level, exact calculations of the \(A'\) production cross-sections, which turn out to be significantly smaller compared to the one obtained in the Weizs\"{a}cker-Williams approximation for the mass region \(m_{A'} \gtrsim 0.1\) GeV.
In high energy experiments such as active beam dump searches for rare decays and missing energy events, the beam purity is a crucial parameter. In this paper we present a technique to reject heavy ...charged particle contamination in the 100 GeV electron beam of the H4 beam line at CERN SPS. The method is based on the detection with BGO scintillators of the synchrotron radiation emitted by the electrons passing through a bending dipole magnet. A 100 GeV \(\pi^-\) beam is used to test the method in the NA64 experiment resulting in a suppression factor of \(10^{-5}\) while the efficiency for electron detection is \(\sim\)95%. The spectra and the rejection factors are in very good agreement with the Monte Carlo simulation. The reported suppression factors are significantly better than previously achieved.
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.
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.
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.
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