Precision tests of Standard Model with leptonic and semileptonic kaon decays

Understand

Till the middle of 2004 it appeared that the unitarity relation Vud^2+Vus^2+Vub^2=1 might not hold at the 2.3sigma level.

• At that time, however, Vus was inferred from old experimental data.
• Since then, a large experimental and theoretical effort has been invested leading to a removal of the problem.
• Thanks to the new and improved measurements by BNL-E865, KLOE, KTeV, ISTRA+ and NA48, the Kl3 decay rate moved up so that Vus is now consistent with unitarity.