Searches for Axion-Like Particles, Hidden Sectors, and CP Violation with η and η ′ Mesons

Abstract

Light pseudoscalar mesons η and η′ provide a unique laboratory for searches of physics beyond the Standard Model (BSM), including axion-like particles (ALPs), hidden sector mediators, and sources of CP violation. Their suppressed decay widths, isoscalar quantum numbers, and rich radiative and leptonic decay channels make them ideal probes of weakly coupled new physics in the MeV-GeV scale. This work presents a review of present achievements, current experimental efforts, and prospects for future measurements. Particular attention is given to sensitivity estimates based on an ideal 4π detector with perfect efficiency and high intensity proton or pion beams, showing the fundamental reach possible in rare η and η′ decays.