Observation of Anomalous Internal Pair Creation in Be 8: A Possible Indication of a Light, Neutral Boson

Abstract

Electron-positron angular correlations were measured for the isovector magnetic dipole 17.6 MeV (Jπ=1+, T=1) state→ground state (Jπ=0+, T=0) and the isoscalar magnetic dipole 18.15 MeV (Jπ=1+, T=0) state→ground state transitions in Be8. Significant enhancement relative to the internal pair creation was observed at large angles in the angular correlation for the isoscalar transition with a confidence level of >5σ. This observation could possibly be due to nuclear reaction interference effects or might indicate that, in an intermediate step, a neutral isoscalar particle with a mass of 16.70±0.35(stat)±0.5(syst) MeV/c2 and Jπ=1+ was created.