Abstract
Recent anomalies in $^8$Be and $^4$He nuclear decays can be explained by postulating a fifth force mediated by a new boson $X$. The distributions of both transitions are consistent with the same $X$ mass, 17 MeV, providing kinematic evidence for a single new particle explanation. In this work, we examine whether the new results also provide dynamical evidence for a new particle explanation, that is, whether the observed decay rates of both anomalies can be described by a single hypothesis for the $X$ boson's interactions. We consider the observed $^8$Be and $^4$He excited nuclei, as well as a $^{12}$C excited nucleus; together these span the possible $J^P$ quantum numbers up to spin 1. For each transition, we determine whether scalar, pseudoscalar, vector, or axial vector $X$ particles can mediate the decay, and we construct the leading operators in a nuclear physics effective field theory that describes them. Assuming parity conservation, the scalar case is excluded and the pseudoscalar case is highly disfavored. Remarkably, however, the protophobic vector gauge boson, first proposed to explain only the $^8$Be anomaly, also explains the $^4$He anomaly within experimental uncertainties. We predict signal rates for other closely related nuclear measurements, which, if confirmed, will provide overwhelming evidence that a fifth force has been discovered.
Highlights
In the standard model of particle physics, particles interact through the electromagnetic, strong, and weak forces
Nuclear physics provides a fruitful hunting ground for new forces. This was obviously true in the past, given the central role of nuclear physics in elucidating the strong and weak forces, but it remains true today
The transitions between energy levels of light nuclei are a natural laboratory to explore ∼MeV mass particles with ultraweak interactions with the standard model. These nuclear transitions are able to probe models that are interesting from the point of view of theories of dark matter and dark forces, and they provide information complementary to searches for such particles from accelerator and astrophysical sources
Summary
In the standard model of particle physics, particles interact through the electromagnetic, strong, and weak forces. All possible spin-parity assignments for the X particle were examined, several explanations were excluded, and the possibility that the X particle is a vector gauge boson mediating a protophobic fifth force was shown to be a viable explanation Following these studies, other new physics explanations were examined in detail, with a host of implications for nuclear, particle, and astrophysical observations; see, for example, Refs. III we determine how the observation or exclusion of various decays constrains the possible X spin-parity assignments
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