Abstract
A calculation of the ratio of leptonic decay constants makes possible a precise determination of the ratio of Cabibbo-Kobayashi-Maskawa (CKM) matrix elements in the standard model, and places a stringent constraint on the scale of new physics that would lead to deviations from unitarity in the first row of the CKM matrix. We compute numerically in unquenched lattice QCD using gauge-field ensembles recently generated that include four flavors of dynamical quarks: up, down, strange, and charm. We analyze data at four lattice spacings , 0.09, 0.12, and 0.15 fm with simulated pion masses down to the physical value 135 MeV. We obtain , where the errors are statistical and total systematic, respectively. This is our first physics result from our ensembles, and the first calculation of from lattice-QCD simulations at the physical point. Our result is the most precise lattice-QCD determination of , with an error comparable to the current world average. When combined with experimental measurements of the leptonic branching fractions, it leads to a precise determination of where the errors are theoretical and experimental, respectively.
- Received 30 January 2013
DOI:https://doi.org/10.1103/PhysRevLett.110.172003
© 2013 American Physical Society