B-Physics in Lattice QCD and at Belle II

Abstract

Theory and experiment both have key roles to play in our understanding of the Universe. In flavour physics, semileptonic and leptonic decays of B mesons let us access CKM matrix elements, and anomalies in these decay processes offer tantalising hints of new physics. New measurements of these processes with improved statistical and systematic errors are expected over the coming years using data from the Belle II experiment. New lattice calculations (with a greater understanding of QCD effects in the Standard Model) will be needed to support this increase in experimental precision. Precise measurement of rare processes with unreconstructed energy from neutrinos such as B→D(*)τν and B→τν requires the entire BB‾ event to be constrained. This is achieved via reconstruction of the companion B meson in the decay, the so-called tag B. In this work, we describe Belle II's reconstruction process for the tag B, and prepare the skimming to collect data for analysts ahead of Belle II data production. Measurements of B→τν can be used to resolve the anomaly between inclusive and exclusive measurements of CKM matrix elements, if a suffciently precise value of fB is available from the lattice QCD community. As fB is often calculated on the lattice via the ratio fBs/fB, it is important to understand and control SU(3) breaking effects in the light and strange quarks, and study how these affect extrapolations of fBs/fB. In this work, we compute fB and fBs using a set of gauge field configurations that break SU(3) flavour in a controlled way, keeping the average of the lighter quark masses held fixed at the physical value.