Precision computation of the kaon bag parameter

Understand

Indirect CP violation in K \rightarrow {\pi}{\pi} decays plays a central role in constraining the flavor structure of the Standard Model (SM) and in the search for new physics.

• For many years the leading uncertainty in the SM prediction of this phenomenon was the one associated with the nonperturbative strong interaction dynamics in this process.
• Here we present a fully controlled lattice QCD calculation of these effects, which are described by the neutral kaon mixing parameter B_K .
• We use a two step HEX smeared clover-improved Wilson action, with four lattice spacings from a\approx0.054 fm to a\approx0.093 fm and pion masses at and even below the physical value.