A Mundane Explanation for the ``ATOMKI Anomaly''

Using the electron-positron pair spectrometer at the 5 MV Van de Graaff-accelerator at the Institute for Nuclear Research, Hungarian Academy of Sciences (ATOMKI), Krasznahorkay {\it et al.} announced data deviating from the Standard Model of particle physics. The authors claim a 6.8 $\sigma$ excess in internal pair creation at high relative angles for the particle pair released in the isoscalar transition $^{8}\mathrm{Be}^*\to ^{8}\mathrm{Be}e^+e^-$ , indicative of a particle of mass circa 16.7 MeV. A similar excess has now been seen in the $^4$He system. A hypothetical gauge boson, a carrier of a fifth force, has been proposed as an explanation for the excesses. We show that a more mundane explanation may lie in the presence of additional nonresonant decay amplitudes, such as a second-order electromagnetic transition.