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

Abstract

Electron-positron angular correlations were measured for the isovector magnetic dipole 17.6 MeV state ($J^{\pi }=1^{+}$, $T=1$) $\to$ ground state ($J^{\pi }=0^{+}$, $T=0$) and the isoscalar magnetic dipole 18.15 MeV ($J^{\pi }=1^{+}$, $T=0$) state $\to$ 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 $>5\sigma$. This observation might indicate that, in an intermediate step, a neutral isoscalar particle with a mass of 16.70$\pm 0.35$ (stat)$\pm 0.5$ (sys) MeV$/c^2$ and $J^{\pi }=1^{+}$ was created.

5 pages, 5 figures

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