Inclusive semileptonic B(s)-meson decays from Lattice QCD

Abstract

The Standard Model (SM) is the most comprehensive framework to describe our current understanding of the known particle physics world. However, its limitations are well known as it cannot explain all observed phenomena. Discrepancies between measurements of SM observables and theoretical predictions guide us in the search for new physics and in extending the Standard Model. One intriguing puzzle is the long-standing tension between the inclusive and exclusive determinations of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements |V_cb| and |V_ub|, respectively. Semileptonic decays of B mesons constitute the main channel for the extraction of these parameters, and therefore represent crucial processes to address and investigate this tension. As B-physics experiments perform measurements and collect new data to study the B-meson properties, it becomes essential to address and understand the tension, and to provide more precise theoretical inputs to further improve our predictions. In this work we address the nonperturbative calculation of the inclusive decay rate of semileptonic B_(s)-meson decays from lattice QCD. This type of computation may eventually provide new insight into the aforementioned tension in the determination of |V_cb| and |V_ub|. We also address the computation of lepton energy and hadronic mass moments, which may provide ground for comparing lattice techniques with continuum approaches such as OPE and perturbation theory. We present results from a pilot lattice computation for B_s → X_c lν_l, where the initial b quark described by the relativistic-heavy-quark (RHQ) formalism on the lattice and other valence quarks discretised with domain-wall fermions are simulated approximately at their physical quark masses. We compare different methods for computing the decay rate and the moments from lattice data of Euclidean four-point functions, namely approaches based on expansions in Chebyshev polynomials, approaches based on the Backus-Gilbert technique and also combinations of both. We find that the results we obtain are in excellent agreement. We further test our setup studying the ground-state meson limit and how much it dominates the inclusive decay rate. We indicate possible strategies towards a computation with a more comprehensive error budget and leave a proper determination of the systematics effects for future studies.

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Identifiers

ORCID for Alessandro Barone: orcid.org/0000-0002-0036-2517

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

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