Test of Lepton Flavour Universality in K+ --> l+nu Decays

A precision test of lepton flavour universality has been performed by measuring the ratio RK of kaon leptonic decay rates K+ --> e+nu and K+ --> mu+nu in a sample of 59813 reconstructed K+ --> e+nu 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.