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Motivated by the recent observation of anomalous electron-positron angularcorrelations in the decay of the 18.15 MeV 1+ excited states in 8Be, wereexamine in detail the Standard Model expectations for these angularcorrelations. The 18.15 MeV state is above particle threshold, and severalmultipoles can contribute to its $e^+e^-$ decay. We present the generaltheoretical expressions for $e^+e^-$ angular distributions for nuclear decay byC0, C1, C2 M1, E1, and E2 multipoles, and we examine their relativecontribution to the $e^+e^-$ decay of 8Be at 18.15 MeV. We find that thisresonance is dominated by M1 and E1 decay, and that the ratio of M1 to E1strength is a strong function of energy. This is in contract to the originalanalysis of the $e^+e^-$ angular distributions, where the M1/E1 ratio wasassumed to be a constant over the energy region Ep = 0:8-1:2 MeV. We find thatthe existence of a `bump' in the measured angular distribution is stronglydependent on the assumed M1/E1 ratio, with the present analysis finding themeasured large-angle contributions to the $e^+e^-$ angular distribution to belower than expectation. Thus, in the current analysis we find no evidence foraxion decay in the 18.15 MeV resonance region of 8Be.

The Angular Correlations in the $e^+e^-$ Decay of Excited States in 8Be