  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.
