Exploring Invisible New Physics with Exotic Pion Decays

TL;DR

This paper evaluates how past and future pion decay experiments can detect light, invisible dark sector particles, demonstrating that the upcoming PIONEER experiment could significantly improve current sensitivity to such exotic decays.

Contribution

It provides detailed sensitivity projections for the PIONEER experiment to detect invisible particles from pion decays, expanding the search for dark sector physics.

Findings

PIONEER can improve bounds on exotic pion decays by at least an order of magnitude.

Projected sensitivities surpass current limits from other experiments.

Weak-violating axion-like particles are identified as promising targets.

Abstract

We study the sensitivity of past and future stopped-pion experiments to light, invisible dark sector particles produced in exotic pion decays. We consider two-body decays involving sterile neutrinos, $\pi^+ \to \ell^+ N$, as well as three-body decays $\pi^+ \to \ell^+ \nu_\ell X$, with $X$ an invisible scalar, axion-like particle, or dark vector. We recast existing limits from the PIENU experiment and project the reach of the planned PIONEER experiment using detailed simulations based on the current detector design. We find that PIONEER can improve bounds on exotic pion branching ratios by at least one order of magnitude below current limits. We compare the projected sensitivities with complementary constraints from lepton anomalous magnetic moments, mono-photon searches, and beam-dump experiments, identifying weak-violating axion-like particles as a particularly well-motivated benchmark. Our results establish PIONEER as a powerful and complementary probe of light, invisible dark sectors.