Neutral B -meson mixing from full lattice QCD at the physical point

Abstract

We calculate the bag parameters for neutral B-meson mixing in and beyond the Standard Model, in full four-flavor lattice QCD for the first time. We work on gluon field configurations that include the effect of u, d, s, and c sea quarks with the highly improved staggered quark (HISQ) action at three values of the lattice spacing and with three u/d quark masses going down to the physical value. The valence b quarks use the improved nonrelativistic QCD action and the valence light quarks, the HISQ action. Our analysis was blinded. Our results for the bag parameters for all five operators are the most accurate to date. For the Standard Model operator between Bs and Bd mesons we find B^Bs=1.232(53) and B^Bd=1.222(61). Combining our results with lattice QCD calculations of the decay constants using HISQ quarks from the Fermilab/MILC Collaboration and with experimental values for Bs and Bd oscillation frequencies allows determination of the Cabibbo-Kobayashi-Maskawa (CKM) elements Vts and Vtd. We find |Vts|=0.04189(93), |Vtd|=0.00867(23), and |Vts|/|Vtd|=0.2071(27). Our results agree well (within 2σ) with values determined from CKM unitarity constraints based on tree-level processes (only). Using a ratio to ΔMs,d in which CKM elements cancel in the Standard Model, we determine the branching fractions Br(Bs→μ+μ-)=3.81(18)×10-9 and Br(Bd→μ+μ-)=1.031(54)×10-10. We also give results for matrix elements of the operators R0, R1, and R1 that contribute to neutral B-meson width differences.