New physics via pion capture and simple nuclear reactions

Light, beyond-the-standard-model particles X in the 1-100 MeV mass range can be produced in nuclear and hadronic reactions but would have to decay electromagnetically. We show that simple and well-understood low-energy hadronic processes can be used as a tool to study X production and decay. In particular, the pion capture process π⁻p → Xn → e⁺e⁻n can be used in a new experimental setup to search for anomalies in the angular distribution of the electron-positron pair, which could signal the appearance of dark photons, axion-like particles and other exotic states. This process can be used to decisively test the hypothesis of a new particle produced in the ⁷Li + p reaction. We also discuss a variety of other theoretically clean hadronic processes, such as p + D(T) fusion, as a promising source of X particles.