Future Muon Physics Experiments: Muon Beams and Experimental Apparatus

Abstract

Muons are powerful probes of fundamental physics at the precision and intensity frontiers. We review the status and planned upgrades of muon-beam facilities worldwide, together with the experimental apparatus required across three experimental categories. Charged-lepton flavor violation (CLFV) searches test charged-lepton flavor conservation through rare muon decays, muon-to-electron conversion in nuclei, and muonium--antimuonium conversion. Precision measurements of the muon magnetic and electric dipole moments, muonium spectroscopy, and muonic-atom spectroscopy test the Standard Model and bound-state QED and determine fundamental constants and nuclear charge and magnetization distributions. Muon-scattering programs include the MUonE determination of hadronic vacuum polarization, NA64$μ$ missing-momentum searches for invisible dark sectors, and the phased PKMu program, which investigates muonphilic dark matter, light mediators, LFV muon--electron processes, and atomic effects in resonant annihilation. We emphasize the beam intensity, timing, purity, and phase-space control, as well as beamline and detector design, background suppression, and experimental techniques needed to achieve next-generation sensitivity and precision.