New results on the ⁸Be anomaly

Abstract

Recently, we observed anomalous internal pair creation for the M1 transition depopulating the 18.15 MeV isoscalar 1$^+$ state in $^8$Be. We observed a significant (σ = 7.37) peak-like deviation from the predicted angular correlation of the e+ e pairs at θ = 140°. To the best of our knowledge no nuclear physics related description of such deviation can be made. However, the deviation can be described by assuming the creation and subsequent decay of a boson with mass of ≈17 MeV. In order to clarify the interpretation, we re-investigated the $^8$Be anomaly with an improved, and independent setup. We have confirmed the signal of the assumed X(17) particle and constrained its mass (m$_0$c $^2$ = 17.01(16) MeV) and branching ratio compared to the γ-decay (B$_x$ = 6(1) × 10$^{−6}$).