Novel technique to measure Lepton Universality ratio $R_K$ at NA62

Lepton universality is one of the key properties of the Standard Model. In the kaon sector, lepton universality is tested measuring the ratio $R_K = \Gamma(K^+\to e^+ \nu(\gamma))/ \Gamma(K^+ \to \mu^+ \nu (\gamma))$, which has a very precise theoretical prediction. $R_K$ has been measured collecting both channels with the same trigger stream and extracting $BR(K^+ \to \mu^+ \nu (\gamma))$ from events with muon decays in flight. The sample collected is three times larger than in previous measurements, with a 4 parts per thousand systematic uncertainty. This new approach will pave the way to reach experimentally the same precision as the theoretical prediction.