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NA64 Collaboration(Andreev, Y. M. et al), & Molina Bueno, L. (2022). Search for a light Z' in the L-mu – L-tau scenario with the NA64-e experiment at CERN. Phys. Rev. D, 106(3), 032015–12pp.
Abstract: The extension of Standard Model made by inclusion of additional U(1) gauge L-mu – L-tau symmetry can explain the difference between the measured and the predicted value of the muon magnetic moment and solve the tension in B meson decays. This model predicts the existence of a new, light Z' vector boson, predominantly coupled to second and third generation leptons, whose interaction with electrons is due to a loop mechanism involving muons and taus. In this work, we present a rigorous evaluation of the upper limits in the Z' parameter space, obtained from the analysis of the data collected by the NA64-e experiment at CERN SPS, that performed a search for light dark matter with 2.84 x 10(11) electrons impinging with 100 GeV on an active thick target. The resulting limits touch the muon g – 2 preferred band for values of the Z' mass of order of 1 MeV, while the sensitivity projections for the future high-statistics NA64-e runs demonstrate the power of the electrons/positron beam approach in this theoretical scenario.
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NA64 Collaboration(Andreev, Y. M. et al), & Molina Bueno, L. (2022). Search for a New B-L Z' Gauge Boson with the NA64 Experiment at CERN. Phys. Rev. Lett., 129, 161801–6pp.
Abstract: A search for a new Z′ gauge boson associated with (un)broken B−L symmetry in the keV–GeV mass range is carried out for the first time using the missing-energy technique in the NA64 experiment at the CERN SPS. From the analysis of the data with 3.22×10^11 electrons on target collected during 2016–2021 runs, no signal events were found. This allows us to derive new constraints on the Z′−e coupling strength, which, for the mass range 0.3≲ mZ′≲ 100 MeV, are more stringent compared to those obtained from the neutrino-electron scattering data.
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NA64 Collaboration(Andreev, Y. M. et al), Molina Bueno, L., & Tuzi, M. (2023). Search for Light Dark Matter with NA64 at CERN. Phys. Rev. Lett., 131(16), 161801–7pp.
Abstract: Thermal dark matter models with particle chi 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 chi 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 x 10(11) 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 m(A ') less than or similar to 0.35 GeV, and to exclude scalar and Majorana dark matter with the chi – A ' coupling alpha(D) <= 0.1 for masses 0.001 less than or similar to m(chi) less than or similar to 0.1 GeV and 3m(chi) <= m(A ').
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NA64 Collaboration(Andreev, Y. M. et al), & Molina Bueno, L. (2021). Search for pseudoscalar bosons decaying into e(+)e(-) pairs in the NA64 experiment at the CERN SPS. Phys. Rev. D, 104(11), L111102–5pp.
Abstract: We report the results of a search for a light pseudoscalar particle a that couples to electrons and decays to e(+) e(-) perfbnned using the high-energy CERN SPS H4 electron beam. If such light pseudoscalar exists, it could explain the ATOMKI anomaly (an excess of e(+) e(-) pairs in the nuclear transitions of Be-8 and 4 He nuclei at the invariant mass similar or equal to 17 MeV observed by the experiment at the 5 MV Van de Graaff accelerator at ATOMKI, Hungary). We used the NA64 data collected in the “visible mode” configuration with a total statistics corresponding to 8.4 x 10(10) electrons on target (EOT) in 2017 and 2018. In order to increase sensitivity to small coupling parameter epsilon we also used the data collected in 2016-2018 in the “invisible mode” configuration of NA64 with a total statistics corresponding to 2.84 x 10(11) EOT. The background and efficiency estimates for these two configurations were retained from our previous analyses searching for light vector bosons and axionlike particles (ALP) (the latter were assumed to couple predominantly to gamma). In this work we recalculate the signal yields, which are different due to different cross section and lifetime of a pseudoscalar particle a, and perform a new statistical analysis. As a result, the region of the two dimensional parameter space m(a) – epsilon in the mass range from 1 to 17.1 MeV is excluded. At the mass of the central value of the ATOMKI anomaly (the first result obtained on the beryllium nucleus, 16.7 MeV) the values of epsilon in the range 2.1 x 10(-4) < epsilon < 3.2 x 10(-4) are excluded.
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