Abstract
We consider leptonic B^-rightarrow ell ^- {bar{nu }}_ell and semileptonic {bar{B}} rightarrow pi ell ^- {bar{nu }}_ell , {bar{B}} rightarrow 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^-rightarrow 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}} rightarrow pi ell ^- {bar{nu }}_ell and {bar{B}} rightarrow 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.
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