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

Recently, we observed an anomalous internal pair creation for the M1 transition depopulating the 18.15 MeV isoscalar 1(+) state in Be-8. 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/c(2). In order to clarify the interpretation, we re-investigated the Be-8 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 gamma-decay (B-x = 6(1) x 10(-6)). We investigated also the high-energy (21 MeV) J(pi) = 0(-) -> 0(+) transition in He-4 and got a consistent result for the X (17) particle.