Abstract
Electron-positron angular correlations were remeasured for the 17.6 MeV ( J π = 1 + → 0 + ) ground state transition in 8 Be using an improved setup compared to the one we used previously. Significant deviations from the internal pair creation was observed at large angles in the angular correlations, which supports that, in an intermediate step, a neutral isoscalar particle with a mass of 17.0 ±0.2(stat) ±0.5(sys) MeV/ c 2 and J π = 1 + was created.
Highlights
We measured the e+e− angular correlation in internal pair creation for the M1 transition depopulating the 18.15 MeV state in 8Be, and observed a peak-like deviation from the predicted IPC [1]
The data can be explained by a 17 MeV vector gauge boson X that is produced in the decay of the excited state to the ground state, and decays to e+e− pairs [2]
The X boson would mediate a fifth force with a characteristic range of 12 fm and would have millicharged couplings to up and down quarks and electrons, and a proton coupling that is suppressed relative to neutrons [2]. This protophobic 17 MeV gauge boson can mediate isovector transitions, so there is no dynamical suppression for the decay from the 17.64 MeV 1+ state to the ground state
Summary
We measured the e+e− angular correlation in internal pair creation for the M1 transition depopulating the 18.15 MeV state in 8Be, and observed a peak-like deviation from the predicted IPC [1]. The X boson would mediate a fifth force with a characteristic range of 12 fm and would have millicharged couplings to up and down quarks and electrons, and a proton coupling that is suppressed relative to neutrons [2]. This protophobic 17 MeV gauge boson can mediate isovector transitions, so there is no dynamical suppression for the decay from the 17.64 MeV 1+ state to the ground state.
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