Decoding (Pseudo)-Scalar Operators in Leptonic and Semileptonic $B$ Decays

TL;DR

This paper develops a method to determine New Physics contributions and the CKM element |V_{ub}| in leptonic and semileptonic B decays, using recent experimental data and theoretical calculations to constrain and predict decay observables.

Contribution

It introduces a strategy to extract short-distance New Physics coefficients and |V_{ub}| from B decay data, including constraints on scalar operators and CP phases.

Findings

Constraints on New Physics coefficients from current data

Predictions for unmeasured decay observables

Impact of CP-violating phases on decay processes

Abstract

We consider leptonic $B^-\to \ell^- \bar\nu_\ell$ and semileptonic $\bar B \to \pi \ell^- \bar\nu_\ell$, $\bar B \to \rho \ell^- \bar\nu_\ell$ decays and present a strategy to determine short-distance coefficients of New-Physics operators and the CKM element $|V_{ub}|$. As the leptonic channels play a central role, we illustrate this method for (pseudo)-scalar operators which may lift the helicity suppression of the corresponding transition amplitudes arising in the Standard Model. Utilising a new result by the Belle collaboration for the branching ratio of $B^-\to \mu^- \bar\nu_\mu$, we explore theoretically clean constraints and correlations between New Physics coefficients for leptonic final states with $\mu$ and $\tau$ leptons. In order to obtain stronger bounds and to extract $|V_{ub}|$, we employ semileptonic $\bar B \to \pi \ell^- \bar\nu_\ell$ and $\bar B \to \rho \ell^- \bar\nu_\ell$ decays as an additional ingredient, involving hadronic form factors which are determined through QCD sum rule and lattice calculations. In addition to a detailed analysis of the constraints on the New Physics contributions following from current data, we make predictions for yet unmeasured decay observables, compare them with experimental constraints and discuss the impact of CP-violating phases of the New-Physics coefficients.