Further search for a neutral boson with a mass around 9 MeV/c²

Two dedicated experiments on internal pair conversion (IPC) of isoscalar M1 transitions were carried out in order to test a 9 MeV/c² X-boson scenario. In the ⁷Li(p,e⁺e^-)⁸Be reaction at 1.1 MeV proton energy to the predominantly T = 0 level at 18.15 MeV, a significant deviation from IPC was observed at large pair correlation angles. In the ¹¹B(d,ne⁺e^-)¹²C reaction at 1.6 MeV, leading to the 12.71 MeV 1⁺ level with pure T = 0 character, an anomaly was observed at ~9 MeV/c². The compatibility of the results with the scenario is discussed.