Searching a Dark Photon with HADES

Abstract

We present a search for the e_s16e_s13_s178722_s19 decay of a hypothetical dark photon, also named U vector boson, in inclusive dielectron spectra measured by HADES in the p(3.5 GeV) _s16 p, Nb reactions, as well as the Ar (1.756 GeV/u) _s16 KCl reaction. An upper limit on the kinetic mixing parameter squared _s13_s171013_s192 at 90% CL has been obtained for the mass range MU=0.02–0.55 GeV/c2 and is compared with the present world data set. For masses 0.03–0.1 GeV/c2, the limit has been lowered with respect to previous results, allowing now to exclude a large part of the parameter region favored by the muon g_s13_s178722_s192 anomaly. Furthermore, an improved upper limit on the branching ratio of 2.3×10_s13_s178722_s196 has been set on the helicity-suppressed direct decay of the eta meson, _s13_s17951_s19_s13_s178594_s19e_s16e_s13_s178722_s19, at 90% CL.