A light Jπ = 1+ isoscalar neutral boson in nuclear transitions

Complete text of publication follows. Last year we could observe e+e- pairs from the 10.95 MeV 0- → 0+ forbidden transition in 16O. The angular correlation of the e+e- pairs was measured with standard Multi-wire Proportional Counters (MWPC), which were inserted between the ΔE and E detectors of the telescopes. The angular correlation of the pairs showed an unusual structure. Instead of the monotonically decreasing feature predicted by the IPC theory it shows a maximum at 103 deg. This behavior could be described by assuming the creation and the subsequent decay of a predicted light neutral boson with a mass of m = 8.5 MeV/c2, which value agrees well with the previously suggested value of 9 MeV/c2. The experimental value of the boson creation probability compared to the γ-decay probability ( Pb ∼ 5 x 10-5) turned out to be much smaller then the previously published value, which was observed in an M1 transition. If the boson has a Jπ of 1+ as predicted by Fayet , then we had to have an L = 1 emission in the 10.95 MeV 0- → 0+ transition, which explains the smaller creation probability. L = 1 emission of the Jπ = 1+ particle should then also happen in a M2 2- → 0+ transition. That was the reason we started to study the 8.87 MeV M2 transition in 16O. To get large cross-section for populating the 8.87 MeV 2- state we chose the 19F(p, αγ)16O reaction for the experiment. The γ and e+e- measurements have been performed et Ep = 5.8 MeV. The proton beam has been obtained from a 20-MeV isochronous cyclotron of the institute. The results are shown in Fig. 1. We could observe a peak at about θ = 139 deg, which nicely confirms our expectations. As the mass of the boson is only slightly smaller then the energy of the transition, their velocity is much smaller then in the 10.95 MeV transition. That is the reason of the larger correlation angle. With an average value of the mass of the boson (m = 8.4(2) MeV/c2) both twobody-decay pattern of the e+e- pair correlations can be explained. Moreover, the observation of the e+e- decay in both an M0 and in M2 transition supports the predicted Jπ = 1+ nature of the particle. (author)