New results on the Be-8 anomaly

Recently, we measured the e+e− angular correlations in internal pair creation (IPC) for the M1 transitions depopulating the 17.6 and 18.15 MeV states in 8Be, and observed a peak-like deviation from the predicted IPC for the 18.15 MeV transition, but not for the 17.6 MeV one. To the best of our knowledge no nuclear physics related description of such deviation can be made. 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 m0c2=16.70±0.35(stat)±0.5(sys) MeV. The branching ratio of the e+e− decay of such a boson to the γ decay of the 18.15 MeV level of 8Be is found to be 5.8×10−6 for the best fit. The data can be explained by a 17.6 MeV vector gauge boson X that is produced in the decay of the excited state to the ground state, and then decays to e+e− pairs. In the present work we re-investigated the e+e− pair correlation in the 17.6 MeV transition of 8Be in which some smaller deviation was also expected. The branching ratio of the e+e− decay of such a boson to the γ decay was found to be (2±2)×10−6.