CP Violation and Non-perturbative Renormalization

CP violation parameter epsilon_K is very well known from experiments. Hence, the calculation of epsilon_K on the lattice is very important to test the standard model. Our current estimate of epsilon_K has 3.4 sigma tension with the experiment. In our calculation, one of the dominant source of error comes from the matching factor for B_K. One-loop perturbative matching has 4.4% systematic error for B_K. The non-perturbative renormalization (NPR) method can reduce this error down to 2% level. We present the first results of NPR for B_K with staggered quarks in this talk.