LABORATORI NAZIONALI DI FRASCATI
Speaker:A.J. Krasznahorkay (Institute for Nuclear Research) Recently, several experimental anomalies were discussed as possible signatures for a new light particle. In order to search for such signatures, we have measured the $e^+e^-$ angular correlation in internal pair creation (IPC) for the the M1 transitions depopulating the 17.6 and 18.15 MeV states in $^8$Be, and observed anomalous IPC in the latter transition. The observed deviations from the M1 IPC in the case of the 17.6 MeV transition could be explained by the contribution of the background, which has E1 multipolarity. However, E1 or any other mixing cannot explain the measured peak-like deviation in the case of the 18 MeV transition. The deviation between the experimental and theoretical angular correlations is significant and can be described by assuming the creation and subsequent decay of a boson with mass $m_0c^2$ =16.70 MeV. The branching ratio of the $e^+e^-$ decay of such a boson to the $\gamma$ decay of the 18.15 MeV level of $^8$Be is found to be $5.8\times10^{-6}$ for the best fit. Such a boson might be a good candidate for the relatively light U(1)$_d$ gauge boson, or the light mediator of the secluded WIMP dark matter scenario, or the dark photon, or the dark Z (Z$_d$) suggested for explaining the muon anomalous magnetic moment. I am going to show the reliability of the data obtained, which will place such a particle into context with other experimental results, and discuss their implications.

DATA: 19-05-2016