K -> pion Semileptonic Form Factors from Two-Flavor Lattice QCD

TL;DR

This paper reports lattice QCD calculations of K to pion semileptonic form factors, providing precise values for f+(0) that impact the determination of the CKM matrix element |V_{us}| and tests of unitarity.

Contribution

It presents new two-flavor lattice QCD results for semileptonic form factors with light pion masses, improving the precision of |V_{us}| and unitarity tests.

Findings

f+(0) = 0.9560 (84) from lattice QCD

|V_{us}| = 0.2267 (5)_exp (20)_f+(0)

Unitarity sum = 1.0004 (15)

Abstract

We present new lattice results of the K -> pion 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 Kl3 decays, leads to |V_{us}| = 0.2267 (5)_exp (20)_f+(0). Using the PDG(2008) determinations of |Vud| and |Vub| our result implies for the unitarity relation |Vud|**2 + |Vus|**2 + |Vub|**2 = 1.0004 (15). For the O(p**6) term of the chiral expansion of f+(0) we get Df = f+(0) - 1 - f2 = -0.0214 (84).