Constraining axion-like particles from rare pion decays

TL;DR

This paper derives new constraints on axion-like particles lighter than the neutral pion by analyzing rare pion decay data, revealing the potential to test ALP models through existing experimental results.

Contribution

It introduces a novel method to constrain ALP-pion mixing angles using existing rare pion decay measurements, extending the reach of experimental bounds.

Findings

Strong bounds on ALP-pion mixing angle from PIENU data

Constraints surpass previous limits for ALPs below the pion mass

Demonstrates the sensitivity of rare pion decays to ALP properties

Abstract

Ultraviolet completions for axion-like particles (ALPs) lighter than the neutral pion generically induce ALP-neutral pion mixing, and are therefore sensitive to direct constraints on the mixing angle. For ALPs below the pion mass, we demonstrate that strong and novel bounds on the ALP-pion mixing angle can be extracted from existing rare pion decay data, measured by the PIENU and PIBETA experiments.