| Περίληψη: | 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 → χ ¯χ. 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 ϵ2αDðmχ 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 theWeizsäcker-Williams approximation for the mass region mA0 ≳ 0.1 GeV.
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