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

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 8Be. 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. [ABSTRACT FROM AUTHOR]