Kaon semileptonic vector form factor with Nf = 2 + 1 + 1 Twisted Mass fermions

Abstract

We investigate the vector form factor relevant for the Kℓ3 semileptonic decay using maximally twisted-mass fermions with 4 dynamical flavours (Nf=2+1+1). Our simulations feature pion masses ranging from 210 MeV to approximately 450 MeV and lattice spacing values as small as 0.06fm. Our main result for the vector form factor at zero 4-momentum transfer is f+(0)=0.9683(65) where the uncertainty is both statistical and systematic. By combining our result with the experimental value of f+(0)|Vus| we obtain |Vus|=0.2234(16), which satisfies the unitarity constraint of the Standard Model at the permille level.