HADES has put an improved upper limit on the mixing of the photon with a hypothetical massive Dark Photon, the latter being the gauge boson mediating the interaction between Dark Matter particles.
Dark Matter in the Universe
The interpretation of current astrophysical observations results in the striking mass-energy budget of matter in the universe: ~75% Dark Energy, ~20% Dark Matter, and ~5% baryonic matter . The latter number refers to stars and inter/intra-galactic gas, i.e. mainly free hydrogen. Dark Energy drives the presently observed accelerated expansion of the universe. It is homogeneously distributed and can be attributed to a cosmological constant or vacuum energy.
In extreme cases it may cause, in the future, such a sudden expansion that anything in the universe is disrupted – this would be the Big Rip. Dark Matter, in contrast, is bumpy and is needed to explain the formation of the ditribution of observed visible matter in the evolving universe, evidenced by the hierarchy of structures from (super)clusters of galaxies, galaxies, stars, planets and other compact objects such as meteorites, etc.
Many attempts have been made to pin down the nature of Dark Matter. Researchers believe that Dark Matter most likely comprised hitherto unknown particles which do not fit into the Standard Model of particle physics. The Standard Model is a theoretically sound quantum field theory with fundamental matter particles, such as quarks (bound in hadrons) and leptons (e.g. electrons and neutrinos), which interact via the exchange of force-carrier quanta, called gauge bosons (e.g., photons). Some of these species acquire their masses by the interaction with the Higgs boson. While evidence for the Higgs boson has been found recently at CERN (resulting in the 2013 Physics Nobel prize), the Standard Model looks now complete when supplemented by some neutrino masses. Indeed, nothing else seems to be needed to understand the wealth of atomic, subnuclear and particle physics phenomena – besides the remaining Dark Matter puzzle! This unsatisfactory state of affairs has therefore initiated worldwide efforts to search for Dark Matter candidates.
Among the list of candidates of Dark Matter is a hypothetical particle, often dubbed U boson or Dark Photon. These nicknames refer to the underlying theory construction: a second unitary (”U”) symmetry allows for quanta which are, in one respect, similar to photons – namely gauge bosons – but in another respect different from them: In fact, attributing to these quanta a mass makes them to Dark Photons having only a very weak interaction with normal matter. Through its mixing with the normal photon, the Dark Photon can also decay into detectable electron-positron pairs. One arrives hence at a scenario where any electromagnetic process involving ”ordinary” virtual photons might be affected by the Dark Photon visible as (small) deviations from the expected Standad Model behavior. In particular, the recently observed discrepancy of the muon magnetic moment anomaly gμ − 2 has been discussed as a possible sign of a Dark Photon.
The HADES Dark Photon search
Figure 1: The 90% CL upper limit on ε2 versus the U-boson mass obtained
from the combined analysis of HADES data (solid black curve).
We have searched for a narrow U -> e−e+ decay signal in dielectron spectra obtained with HADES in 3.5 GeV p+p and p+Nb reactions, as well as in the 1.756 GeV/u Ar+KCl reaction . In contrast to previous experiments focussing on a specific decay channel, our analysis is based on the inclusive measurement of all e+e− pairs produced in a given mass range, i.e. from Dalitz decays of the π0, η, and Δ mostly. Using a maximum-likelihood method we have extracted an upper limit (UL) at a confidence level CL = 90%. With known detector efficiencies and decay branching fractions, this UL has then been transformed into an UL on the mixing parameter ε2 as shown in Fig. 1 together with limits from the searches conducted by BaBar, KLOE-2, APEX, WASA at COSY, and A1 at MAMI. In particular at low masses (MU < 0.1 GeV/c2) we have improved on the recent result obtained by WASA, excluding now to a large degree the parameter range allowed by the muon g − 2 anomaly. At higher masses, the sensitivity of our search is compatible with, albeit somewhat lower than the KLOE-2 analysis of Φ decays.
 J. Behringer et al.. (Particle Data Group), Phys. Rev. D 86 (2012) 010001.
 G. Agakishiev et al. Phys. Lett. B 731 (2014) 265.