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Abstract
Recently, we observed an anomalous peak-like excess of internal e+e− pairs at around 140° for the M1 transition depopulating the 18.15 MeV isoscalar 1+ state in 8Be. The deviation from the theoretical prediction can be described by GEANT simulations assuming the creation and subsequent decay of a new, light boson with a mass of 16.7 MeV/c2. In order to reduce the possible systematic errors from the experimenntal data, we re-investigated the 8Be anomaly with an improved setup and confirmed the anomaly within the statistical uncertainties. We also studied the angular correlation of the electron-positron pairs created in the M0 transition depopulating the 21.01 MeV 0− state in 4He, and observed an anomalous excess of e+e− pairs at a significantly smaller angle of 115°. Since the transition energy was higher in this case, the observed anomaly could be described by assuming the creation and subsequent decay of the same light particle in the simulations.
Highlights
We measured electron-positron angular correlations for the 17.6 MeV and 18.15 MeV, Jπ = 1+ → Jπ = 0+ M1 transitions in 8Be and an anomalous angular correlation, a significant peak-like enhancement relative to the internal pair creation was observed at large angles in the angular correlation spectrum of the 18.15 MeV transition [1]
We studied the angular correlation of the electron-positron pairs created in the M0 transition depopulating the 21.01 MeV 0− state in 4He, and observed an anomalous excess of e+e− pairs at a significantly smaller angle of 115◦
Results for the 4He transitions Fig.3 shows our experimental results for the angular correlation of e+e− pairs gated by the total energy for the signal region (19.5 MeV≤ Etot ≤22.0 MeV), and the asymmetry parameter (−0.5 ≤ y ≤ 0.5) as defined in Ref.[1]
Summary
Zhang and Miller [3] investigated the possibility to explain the anomaly within nuclear physics. They explored the nuclear transition form factor as a possible origin of the anomaly, and found the required form factor to be unrealistic for the 8Be nucleus.
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