Recently, we measured the e+e− angular correlation in internal pair creation for the M1 transition depopulating the 18.15 MeV 1+ state in 8Be, and observed a peak-like deviation from the predicted IPC . 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. 8x10-6 for the best fit .
The data can be explained by a 17 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 . The X boson would mediate a fifth force with a characteristic range of 12 fm and would have millicharged couplings to up and down quarks and electrons, and a proton coupling that is suppressed relative to neutrons .
Recently we reinvestigated the anomaly observed previously by using a new Tandetron accelerator of our Institute. The multi-wire proportional counters were replaced with silicon DSSD detectors, as well as the complete electronics and data acquisition system was changed from CAMAC to VME.
We have measured the e+e− angular correlation in internal pair creation for the M1 transition depopulating the 17.64 MeV 1+ state in 8Be, and observed a peak-like deviation from the predicted IPC as well. It is a smaller deviation than we observed from the decay of the 18.15 MeV 1+ state, and appeared at larger angles corresponding to the mass of:
m0c2 = 17.0± 0. 3(stat) ± 0. 5(sys) MeV.
The branching ratio of the e+e− decay of such a boson to the γ-decay agrees well with the prediction of Feng et al. .
 A.J. Krasznahorkay et al., Phys. Rev. Lett. 1 16 042501 (2016)
 J. Feng et al., Phys. Rev. Lett. 1 17, 071803 (2016)