Searching a dark photon with HADES

We present a search for the e+e−e+e− decay of a hypothetical dark photon, also named U   vector boson, in inclusive dielectron spectra measured by HADES in the p(3.5 GeV) + p, Nb reactions, as well as the Ar (1.756 GeV/u1.756 GeV/u) + KCl reaction. An upper limit on the kinetic mixing parameter squared ϵ2ϵ2 at 90% CL has been obtained for the mass range MU=0.02–0.55 GeV/c2MU=0.02–0.55 GeV/c2 and is compared with the present world data set. For masses 0.03–0.1 GeV/c20.03–0.1 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−2g−2 anomaly. Furthermore, an improved upper limit on the branching ratio of 2.3×10−62.3×10−6 has been set on the helicity-suppressed direct decay of the eta meson, η→e+e−η→e+e−, at 90% CL.