Speaker
Dr
Romain Holzmann
(GSI Helmholtzzentrum fuer Schwerionenforschung)
Description
The existence of a photon-like massive particle, the gamma' or dark photon, is postulated in several extensions of the Standard Model. Such a particle could indeed
help to understand the puzzling behavior of the observed cosmic positron fraction
as well as to solve the sofar unexplained deviation between the measured and
calculated values of the muon g−2 anomaly. The dark photon, unlike its conventional counterpart, would have mass and would be detectable via its mixing with the latter. We present a search for the e+e− decay of such a hypothetical dark photon, also named U boson, in inclusive dielectron spectra measured with HADES in the p(3.5 GeV)+p, Nb reactions, as well as in the Ar(1.756 GeV/u) +KCl reaction. A new upper limit on the kinetic mixing parameter squared (epsilon^2) at 90% CL has been obtained in the mass range M(U) = 0.02 – 0.55 GeV and is compared here with the present world data set. For masses 0.03 – 0.1 GeV, the limit has been lowered with respect to previous results, allowing to exclude a large part of the parameter space favored by the muon g−2 anomaly. From our data also an improved upper limit of 2.3 × 10^-6 (90% CL) could be set on the branching ratio of the helicity-suppressed direct decay of the eta meson, i.e. eta -> e+e−.
Primary authors
Dr
Malgorzata Gumberidze
(Technical University Darmstadt)
Dr
Romain Holzmann
(GSI Helmholtzzentrum fuer Schwerionenforschung)
