Constraints to new physics models for the proton charge radius puzzle from the decay $K^+ \rightarrow \mu^+ +\nu + e^- + e^+$

Understand

A possible explanation for the discrepancy between electronic and muonic hydrogen measurements of the proton charge radius are new, lepton-universality violating interactions.

• Several new couplings and particles have been suggested that account for this discrepancy.
• At present, these explanations are poorly constrained.
• Experiments such as the upcoming kaon decay experiment at JPARC may constrain or eliminate some explanations by sensitivity to the decay channel $K^+ \rightarrow \mu^+ +\nu + e^- + e^+$.