New results on the ⁸Be anomaly

Recently, we observed anomalous internal pair creation for the M1 transition depopulating the 18.15 MeV isoscalar 1⁺ state in ⁸Be. 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 ⁸Be 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). (paper)