Lepton universality test with K+ --> l+ nu decays

The NA62 experiment at CERN collected a large sample of $K^+\to e^+\nu$ decays during a dedicated run in 2007, which allows a precise test of lepton universality by measurement of the helicity suppressed ratio $R_K = \Gamma(K^+\to e^+\nu)/\Gamma(K^+\to\mu^+\nu)$. The preliminary result of the analysis of a partial data sample of 51089 $K^+\to e^+\nu$ candidates is $R_K=(2.500\pm0.016)\times10^{-5}$, consistent with the Standard Model expectation.