Precision tests of fundamental physics with $\eta$ and $\eta^\prime$ mesons

TL;DR

This paper reviews how $\, ext{eta}$ and $\, ext{eta}^ ext{'}$ meson decays serve as a crucial platform for testing low-energy QCD, determining quark mass ratios, and searching for new physics beyond the Standard Model.

Contribution

It provides an updated overview of theoretical progress, experimental opportunities, and future research directions in using $\, ext{eta}$ and $\, ext{eta}^ ext{'}$ mesons for fundamental physics tests.

Findings

New experimental data will refine quark mass ratios.

Meson decays can probe hidden photons and axion-like particles.

The review highlights upcoming experimental and theoretical developments.

Abstract

Decays of the neutral and long-lived $\eta$ and $\eta'$ mesons provide a unique, flavor-conserving laboratory to test low-energy Quantum Chromodynamics and search for new physics beyond the Standard Model. They have drawn world-wide attention in recent years and have inspired broad experimental programs in different high-intensity-frontier centers. New experimental data will offer critical inputs to precisely determine the light quark mass ratios, $\eta$-$\eta'$ mixing parameters, and hadronic contributions to the anomalous magnetic moment of the muon. At the same time, it will provide a sensitive probe to test potential new physics. This includes searches for hidden photons, light Higgs scalars, and axion-like particles that are complementary to worldwide efforts to detect new light particles below the GeV mass scale, as well as tests of discrete symmetry violation. In this review, we give an update on theoretical developments, discuss the experimental opportunities, and identify future research needed in this field.