On the $X$(17) Light-particle Candidate Observed in Nuclear Transitions

Abstract

Recently, we observed an anomalous internal pair creation for the M1 transition depopulating the 18.15 MeV isoscalar 1+ state in 8Be. The deviation from the theoretical prediction can be described by assuming the creation and subsequent decay of a new, light boson with a mass of 16.7 MeV/c2. 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 (m0c2= 17.01(16) MeV) and branching ratio compared to the γ-decay (Bx=6(1)x10−6). We investigated also the high-energy (21 MeV) J=0−->0+ transition in 4He and got a consistent result for the X(17) particle.