Rare kaon decays offer new precision test of fundamental constants

Kaons decaying into leptons and antilepton pairs are suppressed processes that test the Standard Model's flavor structure. Researchers used lattice QCD to calculate the form factors governing these rare decays, predicting branching fractions for all electron and muon channels with 2–7% precision. The results match existing experiments and a preliminary NA62 measurement, and can be flipped to extract the Cabibbo angle—a fundamental mixing parameter—to better than 2% accuracy.