Abstract
In a 2016 paper, an anomaly in the internal pair creation on the M1 transition depopulating the 18.15 MeV isoscalar 1+ state on 8Be was observed. This could be explained by the creation and subsequent decay of a new boson, with mass mXc2 = 16.70 MeV. Further experiments of the same transition with an improved and independent setup were performed, which constrained the mass of the X17 boson (mXc2) and its branching ratio relative to the γ-decay of the 8Be excited state (BX), to mXc2 = 17.01(16) MeV and BX = 6(1) 10−6, respectively. Using the latter setup, the e+e− pairs depopulating the 21 MeV Jπ = 0− 0+ transition in 4He were investigated and a resonance in the angular correlation of the pairs was observed, which could be explained by the same X17 particle, with mass mXc2 = 16.98 ± 0.16(stat) ± 0.20(syst) MeV.
Highlights
A recent measurement of the angular correlation of e+e− pairs from the 18.15 MeV Jπ = 1+ → 0+ M1 transition of 8Be revealed an anomalous peak-like enhancement relative to the internal pair creation (IPC) at large e+e−separation angles [1]
The expected angular correlation for e+e−pairs from the X17 boson in the decay of the 21.0 MeV 4He state is at around 110◦, instead of the 140◦ observed on the 8Be experiment, due to the higher energy of the 4He transition
Since the expected angular correlation for the e+e−pairs for the boson is peaked around 110◦, the energy sum spectra was taken for pairs 60◦ and 120◦ apart
Summary
A recent measurement of the angular correlation of e+e− pairs from the 18.15 MeV Jπ = 1+ → 0+ M1 transition of 8Be revealed an anomalous peak-like enhancement relative to the internal pair creation (IPC) at large e+e−separation angles [1]. This was interpreted as the creation and subsequent decay of a new boson with a mass of mXc2 = 16.70 ± 0.35(stat) ± 0.5(syst) MeV. The possibility that the anomaly could be explained without a new particle, but within nuclear physics, with an improved model of the reaction or by introducing a nuclear transition form factor was explored by Zhang and Miller [3]. Some of the recent possible explanations for the anomaly shall be discussed
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