Kl3 form factors at the physical point on a (10.9??fm)3 volume

概要

We present the calculation of the Kl3 form factors with Nf=2+1 nonperturbatively O(a)-improved Wilson quark action and Iwasaki gauge action at the physical point on a large volume of (10.9??fm)3 at one lattice spacing of a=0.085??fm. We extract the form factors from 3-point functions with three different time separations between the source and sink operators to confirm suppression of excited state contributions. The form factors are calculated in very close to the zero momentum transfer, q2=0, thanks to the large volume, so that stable interpolations to q2=0 are carried out. Using our form factors, we obtain the form factor at q2=0, f+(0)=0.9603(16)(?+14?4)(44)(19)(1), where the first, second, and fifth errors are statistical, systematic errors from fit functions and the isospin breaking effect, respectively. The third and fourth errors denote the finite lattice spacing effects estimated from the renormalization factor and contribution beyond the leading order SU(3) chiral perturbation theory. The result of f+(0) yields the Cabibbo-Kobayashi-Maskawa (CKM) matrix element, |Vus|=0.2255(13)(4), where the first error comes from our calculation and the second from the experiment. This value is consistent with the ones determined from the unitarity of the CKM matrix and the Kl2 decay within one standard deviation, while it is slightly larger than recent lattice calculations by at most 1.5σ. Furthermore, we evaluate the shape of the form factors and the phase space integral from our results. We confirm that those results are consistent with the experiment, and also |Vus| determined with our phase space integral agrees with the one in the above.