New-physics searches in $C\!P$-violating $η$ muonic decays

Abstract: In this work we investigate the possibility to observe $C!P$-violation in $\eta$ decays containing muons at the proposed REDTOP experiment. Employing the SMEFT to parametrize the new-physics $C!P$-violating effects, we find that a single operator exists for which is possible to observe $C!P$-violation at REDTOP in $\eta\to\mu^+\mu^-$ decays, while evading bounds from the neutron electric dipole moment and $D^-\to\mu\bar{\nu}$