# New physics via pion capture and simple nuclear reactions

## TL;DR

This paper proposes using low-energy hadronic reactions, like pion capture and nuclear fusion, as effective methods to produce and detect light, beyond-the-standard-model particles such as dark photons and axion-like particles.

## Contribution

It introduces new experimental approaches utilizing simple nuclear reactions to search for and study exotic particles beyond the standard model.

## Key findings

- Pion capture process can be used to search for dark photons and axion-like particles.
- The $^7$Li + p reaction provides a decisive test for new particle production.
- Other nuclear processes like fusion are promising sources of exotic particles.

## Abstract

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 $\pi^- p \to X n \to e^+ e^- n$ 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 $^7{\rm Li}+p$ reaction. We also discuss a variety of other theoretically clean hadronic processes, such as $p+{\rm D(T)}$ fusion, as a promising source of $X$ particles.