Dynamical Evidence For a Fifth Force Explanation of the ATOMKI Nuclear Anomalies

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.

32 pages, 6 figures; Corrected small typos and added references

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• Secluded WIMP Dark Matter
• The WIMPless Miracle: Dark Matter Particles without Weak-scale Masses or Weak Interactions
• Particle Physics Models for the 17 MeV Anomaly in Beryllium Nuclear Decays
• The light U boson as the mediator of a new force, coupled to a combination of Q, B, L and dark matter
• New evidence supporting the existence of the hypothetic X17 particle
• Protophobic Light Vector Boson as a Mediator to the Dark Sector
• Atomki Anomaly in Family-Dependent $U(1{)}^{\prime }$ Extension of the Standard Model
• Is There a Sign of New Physics in Beryllium Transitions?

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