Kovalenko, Sergey

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 2.84×1011 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 ≲0.2 GeV are derived, thus demonstrating the power of the active beam dump approach for the dark matter search.

A search is performed for a new sub-GeV vector boson (A0 ) mediated production of dark matter (χ) in the fixed-target experiment, NA64, at the CERN SPS. The A0 , called dark photon, can be generated in the reaction e−Z → e−ZA0 of 100 GeV electrons dumped against an active target followed by its prompt invisible decay A0 → χχ¯. 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. New stringent constraints on the A0 mixing strength with photons, 10−5 ≲ ϵ ≲ 10−2, for the A0 mass range mA0 ≲ 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 ≲ y ≲ 10−6 on the dark-matter parameter y ¼ ϵ2αDð mχ mA0 Þ4 are obtained for the dark coupling constant αD ¼ 0.5 and dark-matter masses 0.001 ≲ mχ ≲ 0.5 GeV. The lower limits αD ≳ 10−3 for pseudo-Dirac dark matter in the mass region mχ ≲ 0.05 GeV are more stringent than the corresponding bounds from beam dump experiments. The results are obtained by using exact tree level calculations of the A0 production cross sections, which turn out to be significantly smaller compared to the one obtained in the Weizsäcker-Williams approximation for the mass region mA0 ≳ 0.1 GeV.

We report the first results on a direct search for a new 16.7 MeV boson (X) which could explain the anomalous excess of eþe− pairs observed in the excited 8 Be nucleus decays. Because of its coupling to electrons, the X could be produced in the bremsstrahlung reaction e−Z → e−ZX by a 100 GeV e− beam incident on an active target in the NA64 experiment at the CERN Super Proton Synchrotron and observed through the subsequent decay into a eþe− pair. With 5.4 × 1010 electrons on target, no evidence for such decays was found, allowing us to set first limits on the X − e− coupling in the range 1.3 × 10−4 ≲ ϵe ≲ 4.2 × 10−4 excluding part of the allowed parameter space. We also set new bounds on the mixing strength of photons with dark photons (A0 ) from nonobservation of the decay A0 → eþe− of the bremsstrahlung A0 with a mass ≲23 MeV

AbstractWe report the results of a search for a new vector boson ($$ A'$$ A ′ ) decaying into two dark matter particles $$\chi _1 \chi _2$$ χ 1 χ 2 of different mass. The heavier $$\chi _2$$ χ 2 particle subsequently decays to $$\chi _1$$ χ 1 and an off-shell Dark Photon $$ A'^* \rightarrow e^+e^-$$ A ′ ∗ → 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 $$A'\rightarrow \chi \overline{\chi }$$ A ′ → χ χ ¯ and axion-like or pseudo-scalar particles $$a \rightarrow \gamma \gamma $$ a → γ γ . 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 $$A'$$ A ′ masses from 2$$m_e$$ m e up to 390 MeV and mixing parameter $$\varepsilon $$ ε between $$3\times 10^{-5}$$ 3 × 10 - 5 and $$2\times 10^{-2}$$ 2 × 10 - 2 .

We report on a direct search for sub-GeV dark photons (A0 ), which might be produced in the reaction e−Z → e−ZA0 via kinetic mixing with photons by 100 GeV electrons incident on an active target in the NA64 experiment at the CERN SPS. The dark photons would decay invisibly into dark matter particles resulting in events with large missing energy. No evidence for such decays was found with 2.75 × 109 electrons on target. We set new limits on the γ − A0 mixing strength and exclude the invisible A0 with a mass ≲100 MeV as an explanation of the muon gμ − 2 anomaly.