Search for vector mediator of dark matter production in invisible decay mode

A search is performed for a new sub-GeV vector boson (A') mediated production of dark matter (chi) in the fixed-target experiment, NA64, at the CERN SPS. The A', called dark photon, can be generated in the reaction e(-)Z -> e(-)ZA' of 100 GeV electrons dumped against an active target followed by its prompt invisible decay A' -> chi(chi) over bar. 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 x 10(10) electrons on target no evidence of such a process has been found. New stringent constraints on the A' mixing strength with photons, 10(-5) less than or similar to epsilon less than or similar to 10(-2), for the A' mass range m(A') less than or similar to 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) less than or similar to y less than or similar to 10(-6) on the dark-matter parameter y = epsilon(2)alpha(D)(m(chi)/m(A'))(4) are obtained for the dark coupling constant alpha(D) = 0.5 and dark-matter masses 0.001 less than or similar to m(chi) less than or similar to 0.5 GeV. The lower limits alpha(D) greater than or similar to 10(-3) for pseudo-Dirac dark matter in the mass region m(chi) less than or similar to 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 A' production cross sections, which turn out to be significantly smaller compared to the one obtained in the Weizsacker-Williams approximation for the mass region m(A') greater than or similar to 0.1 GeV.