Test of lepton flavour universality in K+→ℓ+ν decays

A precision test of lepton flavour universality has been performed by measuring the ratio RK of kaon leptonic decay rates K+→e+ν and K+→μ+ν in a sample of 59 813 reconstructed K+→e+ν candidates with (8.71±0.24)% background contamination. The result RK=(2.487±0.013)×10−5 is in agreement with the Standard Model expectation.