Test of lepton flavour universality in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"><mml:msup><mml:mi>K</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:mo>→</mml:mo><mml:msup><mml:mi>ℓ</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:mi>ν</mml:mi></mml:math> decays

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