$e^{+}{e}^{-}$ pairs from a nuclear transition signaling an elusive light neutral boson

Abstract

Electron-positron pairs have been observed in the 10.95-MeV $0^{-}\to 0^{+}$ decay in ${}^{16}$O. The branching ratio of the e${}^{+}$e${}^{-}$ pairs compared to the 3.84-MeV $0^{-}\to 2^{+}$ $\gamma$ decay of the level is deduced to be $20(5)\times 10^{-5}$. This magnetic monopole (M0) transition cannot proceed by $\gamma$-ray decay and is, to first order, forbidden for internal pair creation. However, the transition may also proceed by the emission of a light neutral $0^{-}$ or $1^{+}$ boson. Indeed, we do observe a sharp peak in the $e^{+}{e}^{-}$ angular correlation with all the characteristics belonging to the intermediate emission of such a boson with an invariant mass of 8.5(5) MeV/c${}^2$. It may play a role in the current quest for light dark matter in the universe.

6 pages