B → πlν and Bs → Klν decays in the continuum limit of lattice QCD

Abstract

The Standard Model of particle physics is our current best model of the fundamental mechanics of nature. However, it cannot explain all observed phenomena, and clearly there must be new physics to uncover. At some level of precision, new physics effects must enter standard model predictions, and present themselves as discrepancies between observation and theory. Finding such clues at the precision frontier will provide valuable input on the advancement of our theoretical understanding of nature. We see intriguing tensions between theoretical expectations and collider experiments for semileptonic heavy-light decays. The expected unitarity of the Cabbibo-Kobayashi-Maskawa (CKM) matrix is in tension with a combination of theoretical calculations and experimental evidence at a 3σ level for the first row of the matrix, and a similar 2 − 3σ discrepancy is present between determinations of the |V_ub| matrix element from exclusive B → πlν decays and inclusive B → X_ulν decays. Investigating CKM matrix elements such as |V_ub| requires the theoretical calculation of QCD form factors, which must be calculated non-perturbatively in the low energy regime of QCD, in combination with observations of decay rates from the B-factories and LHCb. Here we present our work on determining the standard model QCD form factors for B → πlν and B_s → Klν using the non-perturbative lattice QCD technique. Our calculations of these quantities are more precise than a previous 2015 determination currently used in theoretical averages, and by updating these results we anticipate a corresponding increase in precision in these averages. In addition, we investigate a modification to standard lepton flavour universality ratios, which we demonstrate to provide more precise predictions. We suggest that these ratios may be used as an additional way to monitor lepton flavour universality.

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ORCID for Andreas Juttner: orcid.org/0000-0002-3978-0927

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