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

Electron-positron angular correlations were measured for the isovector magnetic dipole 17.6 MeV state ($J^pi=1^+$, $T=1$) $rightarrow$ ground state ($J^pi=0^+$, $T=0$) and the isoscalar magnetic dipole 18.15 MeV ($J^pi=1^+$, $T=0$) state $rightarrow$ ground state transitions in $^{8}$Be. Significant deviation from the internal pair creation was observed at large angles in the angular correlation for the isoscalar transition with a confidence level of $> 5sigma$. This observation might indicate that, in an intermediate step, a neutral isoscalar particle with a mass of 16.70$pm0.35 $ (stat)$pm 0.5 $ (sys) MeV$/c^2$ and $J^pi = 1^+$ was created.