K→πlν semileptonic form factors from two-flavor lattice QCD

We present new lattice results of the K ->pi l nu semileptonic form factors obtained from simulations with two flavors of dynamical twisted-mass fermions, using pion masses as light as 260 MeV. Our main result is f(+)(0)=0.9560(84), which, combined with the latest experimental data for K-l3 decays, leads to |V-us|=0.2267(5)(exp)(20)(f+)(0). Using the PDG(2008) determinations of |V-ud| and |V-ub| our result implies for the unitarity relation |V-ud|(2)+|V-us|(2)+|V-ub|(2)=1.0004(15). For the O(p(6)) term of the chiral expansion of f(+)(0) we get Delta f equivalent to f(+)(0)-1-f(2)=-0.0214(84).