Constraint on parity-violating muonic forces

Using the nonobservance of missing mass events in the leptonic kaon decay $K \to \mu X$, we place a strong constraint on exotic parity-violating gauge interactions of the right-handed muon. By way of illustration, we apply it to an explanation of the proton size anomaly that invokes such a new force; scenarios in which the gauge boson decays invisibly or is long-lived are constrained.