Searching a dark photon with HADES
We present a search for the 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/u) KCl reaction. An upper limit on the kinetic mixing parameter squared epsilon(2) at 90% CL has been obtained for the mass range M-U = 0.02-0.55 GeV/c(2) and is compared with the present world data set. For masses 0.03-0.1 GeV/c(2), 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 - 2 anomaly. Furthermore, an improved upper limit on the branching ratio of 2.3 x 10(-6) has been set on the helicity-suppressed direct decay of the eta meson, eta --> e(+)e(-), at 90% CL. (C) 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license