Search for Dark Sector Physics with NA64
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earch for Dark Sector Physics with NA64 S. N. Gninenkoa, N. V. Krasnikova, b, *, and V. A. Matveeva, b aINR
RAS, Moscow, 117312 Russia Joint Institute for Nuclear Research, Dubna, 141980 Russia *e-mail: [email protected]
b
Received January 12, 2020; revised February 12, 2020; accepted March 13, 2020
Abstract—The NA64 experiment consists of two detectors which are planned to be located at the electron (NA64e) and muon (NA64μ) beams of the CERN SPS and start operation after the LHC long-stop 2 in 2021. Its main goals include searches for dark sector physics—particularly light dark matter (LDM), visible and invisible decays of dark photons ( A ' ), and new light particles that could explain the 8Be and gμ − 2 anomalies. Here we review these physics goals, the current status of NA64 including recent results and perspectives of further searches, as well as other ongoing or planned experiments in this field. The main theoretical results on LDM, the problem of the origin of the γ-A ' mixing term and its connection to loop corrections, possible existence of a new light Z ' coupled to Lμ − Lτ current are also discussed. DOI: 10.1134/S1063779620050044
1. INTRODUCTION At present the most striking evidence in favour of new physics beyond the Standard model (SM) is the observation of Dark Matter (DM) [1, 2]. The nature of DM is one of challenging questions in physics. If DM is a thermal relic from the hot early Universe then its existence motivates to look for models with nongravitational interactions between dark and ordinary matter. There is a lot of candidates for the role of dark matter [1, 2]. In particular, there are LDM(light dark matter) models [3–7] with the mass of DM particles ≤ 0(1) GeV . LDM particles with masses below 0(1) GeV were generally expected to be ruled out because they overclose the Universe [8]. However there are models [3–7] with additional light vector boson and LDM particles that avoid the arguments [8] excluding the LDM. The standard assumption that in the hot early Universe the DM particles are in equilibrium with ordinary matter is often used. During the Universe expansion the temperature decreases and at some point the thermal decoupling of the DM starts to work. Namely, at some freeze-out temperature the annihilation cross-section of DM paricles
DM particles → SM particles becomes too small to obey the equilibrium of DM particles with the SM particles and the DM decouples. The experimental data are in favour of scenario with cold relic for which the freeze-out temperature is much lower than the mass of the DM particle. In other words the DM particles decouple in non-relativistic regime. The value of the DM annihilation cross-sec-
tion at the decoupling epoch determines the value of the current DM density in the Universe. Too big annihilation cross-section leads to small DM density and vise versa too small annihilation cross section leads to DM overproduction. The observed value of the DM ρ density fraction d ≈ 0.23 [9] allows to estimate the ρc DM annihilation cross-section into the
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