Preparations for a search of the muon EDM at PSI

¹ Paul Scherrer Institute, 5232 Villigen, Switzerland
² CERN Beams Department, Esplanade des Particules 1, 1211 Meyrin, Switzerland
³ Instituto de Ingeniería, Universidad Nacional Autónoma de Mexico, 04510 Ciudad de Mexico, Mexico

^* e-mail: µphilipp.schmidt-wellenburg@psi.ch

Published online: 30 October 2023

Abstract

Electric dipole moments (EDMs) of fundamental particles violate time invariance and the combined symmetry of charge and parity (CP). The existence of a large muon EDM (muEDM) is made plausible by tensions with Standard Model predictions for semileptonic decays of heavy meson measured at LHCb, BaBar, and Belle, as well as the muon’s anomalous magnetic moment (AMM). A discovery of the muEDM would manifest CP and lepton flavor universality (LFU) violation, revealing physics beyond the SM (BSM). The most sensitive muEDM search to date provides an upper limit of 1.8 × 10⁻¹⁹ e cm (CL 95%), extracted from high-precision data collected to measure the muon AMM. At the Paul Scherrer Institute, we are setting up a dedicated search for the muEDM using, for the first time, the frozen-spin technique to target an ultimate sensitivity better than 6 × 10⁻²³ e cm. This novel technique increases the sensitivity to EDM-induced spin precession by cancelling the AMM-induced precession with the application of a precisely tuned electric field perpendicular to the muon momentum and the magnetic field. In this configuration, the dominant source of precession is the EDM coupling to the large relativistic electric field in the muon rest frame, generated by its motion in a strong 3 T uniform magnetic field. In a precursor experiment, we will apply the frozen-spin technique in a compact solenoid demonstrating a sensitivity of better than 3 × 10⁻²¹ e cm, probing uncharted and otherwise inaccessible territory in BSM theories.