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NA64 Collaboration(Andreev, Y. M. et al), & Molina Bueno, L. (2021). Improved exclusion limit for light dark matter from e(+) e(-) annihilation in NA64. Phys. Rev. D, 104(9), L091701–7pp.
Abstract: 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 m(A') less than or similar to 250 MeV, by analyzing data from the interaction of 2.84 x 10(11) 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.
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NA64 Collaboration(Andreev, Y. M. et al), Molina Bueno, L., & Tuzi, M. (2024). Probing light dark matter with positron beams at NA64. Phys. Rev. D, 109(3), L031103–6pp.
Abstract: We present the results of a missing-energy search for light dark matter which has a new interaction with ordinary matter transmitted by a vector boson, called dark photon A'. For the first time, this search is performed with a positron beam by using the significantly enhanced production of A' in the resonant annihilation of positrons with atomic electrons of the target nuclei, followed by the invisible decay of A' into dark matter. No events were found in the signal region with (10.1 +/- 0.1) x 109 positrons on target with 100 GeV energy. This allowed us to set new exclusion limits that, relative to the collected statistics, prove the power of this experimental technique. This measurement is a crucial first step toward a future exploration program with positron beams, whose estimated sensitivity is here presented.
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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(Cazzaniga, C. et al), & Molina Bueno, L. (2021). Probing the explanation of the muon (g-2) anomaly and thermal light dark matter with the semi-visible dark photon channel. Eur. Phys. J. C, 81(10), 959–6pp.
Abstract: 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 an off-shell Dark Photon A'* -> e(+)e(-). For a sufficiently largemass splitting, this model can explain in terms of new physics the recently confirmed discrepancy observed in themuon anomalous magnetic moment at Fermilab. Remark- ably, 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 reanalyzing the previous NA64 searches for an invisible decay A' -> chi(chi) over bar and axion-like or pseudo-scalar particles -> 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 2m(e) up to 390 MeV and mixing parameter e between 3 x 10(-5) and 2 x 10(-2).
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Sieber, H., Banerjee, D., Crivelli, P., Depero, E., Gninenko, S. N., Kirpichnikov, D. V., et al. (2022). Prospects in the search for a new light Z0 boson with the NA64 μexperiment at the CERN SPS. Phys. Rev. D, 105(5), 052006–9pp.
Abstract: A light Z0 vector boson coupled to the second and third lepton generations through the L μ- L tau current with mass below 200 MeV provides a very viable explanation in terms of new physics to the recently confirmed og – 2 thorn μanomaly. This boson can be produced in the bremsstrahlung reaction μN – μNZ0 after a high energy muon beam collides with a target. NA64 μis a fixed-target experiment using a 160 GeV muon beam from the CERN Super Proton Synchrotron accelerator looking for Z0 production and its subsequent decays, Z0 – invisible. In this paper, we present the study of the NA64 μsensitivity to search for such a boson. This includes a realistic beam simulation, a detailed description of the detectors and a discussion about the main potential background sources. A pilot run is scheduled in order to validate the simulation results. If those are confirmed, NA64 μwill be able to explore all the remaining parameter space which could provide an explanation for the g – 2 muon anomaly in the L μ- L tau model.
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