Recent results of the NA64 experiment at the CERN SPS

Abstract

We report on the results of the search for a new sub-GeV vector boson (A’) mediated production of Dark Matter (χ) in the fixed-target experiment, NA64, at the CERN SPS. The A’, called dark photon, could be generated in the reaction e-Z → e-ZA’ of 100 GeV electrons dumped against an active target which is followed by the prompt invisible decay A’ $ \to \chi \bar{\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 × 1010 electrons on target no evidence of such a process has been found. The constraints on the A’ mixing strength with photons, 10−5 ≤ ɛ ≤ 10−2, for the A’ mass range mA’ ≤ 1GeV are derived. Significantly more data were taken in 2017 and 2018. The analysis of these data is not yet finished, although the sensitivity is already estimated as about 3 times better than in the previously published result on the 2016 data. For models considering scalar and fermionic thermal Dark Matter interacting with the visible sector through the vector portal the experiment starts to be sensitive to the regions of the dark-matter parameter $ y = \in^{2} \alpha_{D} (\frac{{m_{\chi } }}{{m_{{A^{\prime}}} }})^{4} $ predicted from the cosmological observations of Dark Matter. We also report the improved 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 8Be* nucleus decays. Due to its coupling to electrons, the X could be produced in the bremsstrahlung reaction e-Z → e-ZX by a beam of electrons incident on an active target in the NA64 experiment at the CERN SPS and observed through the subsequent decay into a e+e- pair. With 8.4 × 1010 electrons on target no evidence for such decays was found, allowing to set limits on the X - e- coupling in the range 1 × 10−4 ≤ ɛe ≤ 6 × 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 ≤ 23MeV. Future plans of the NA64 experiment are presented.