The 17 MeV Anomaly in Beryllium Decays and U(1) Portal to Dark Matter

Abstract

The experiment of Krasznahorkay et al observed the transition of a 8 Be excited state to its ground state and accompanied by an emission of e + e − pair with 17 MeV invariant mass. This 6.8σ anomaly can be fitted by a new light gauge boson. We consider the new particle as a U (1) gauge boson, Z , which plays as a portal linking dark sector and visible sector. In particular, we study the new U (1) gauge symmetry as a hidden or non-hidden group separately. The generic hidden U (1) model, referred to as dark Z model, is excluded by imposing various experimental constraints. On the other hand, a non-hidden Z is allowed due to additional interactions between Z and Standard Model fermions. We also study the implication of the dark matter direct search on such a scenario. We found the search for the DM-nucleon scattering cannot probe the parameter space that is allowed by 8 Be-anomaly for the range of DM mass above 500 MeV. However, the DM-electron scattering for DM between 20 and 50 MeV can test the underlying U (1) portal model using the future Si and Ge detectors with 5e − threshold charges.

Kinetic mixing, dark photons and an extra dimension. Part I

“…For the light DM that we consider here, ∼ 10 − 100 MeV, scattering off of electrons is likely to provide the greatest sensitivity [6]. Assuming a form factor of unity since m 2 DM m 2 e , this cross section can be expressed as (for either S or S † scattering) [71][72][73][74] …”

“…For representative parameter values SuperCDMS is likely to be able to probe this range of cross sections in the future but it now lies a few orders of magnitude below the current constraints [75]. The calculation of the thermal DM annihilation cross section into final state electrons (the most likely possible final state for typical DM masses) can be expressed in a similar fashion by writing σv rel =bv 2 rel , where the detailed kinematic information, including the sub-leading terms in the velocities, and (away from any resonances for simplicity) is contained in the parameterb which is given in the limit of a zero electron mass by [71][72][73] …”

Is there a sign of new physics in beryllium transitions?

“…The problem is that in such scenarios there exists a generic mass mixing between the new gauge boson and the Standard Model Z, severely constrained by atomic parity violation experiments [42]. This can be fixed, for example, by extending the gauge group by yet another U (1) and introducing sufficiently large mixing between them [78,79,80]. Several other models focused on the dark matter sector were constructed with the protophobic gauge boson mediating interactions between the dark sector and the Standard Model [81,82].…”

“…Finally, one can also construct the new gauge group using just the Standard Model symmetries (based on the charges Q, B, L) other than U(1) B or U(1) B−L [80,83] with sufficient kinetic mixing to explain the Atomki signal.…”

MeV dark matter complementarity and the dark photon portal

“…Several MeV dark matter studies have been conducted in the literature, e.g. in the context of a light dark Higgs [24], effective operators [25][26][27], radiative neutrino masses [28], sterile neutrinos [29], neutrino detectors such as Super-Kamiokande [30], dark radiation [31], interplay with gamma-rays [32,33], in connection to MeV anomalies at colliders [34], supernova physics [35][36][37], small scale structure [38], keV line emission [39][40][41][42], low energy colliders [43], Big Bang Nucleosynthesis [44] and coherent neutrino-nucleus scattering [45]. Despite the large number of existing studies in the context of MeV dark matter, our work is novel at a variety of levels:…”