6533b825fe1ef96bd1283155

RESEARCH PRODUCT

Reassessing the discovery potential of theB→K*ℓ+ℓ−decays in the large-recoil region: SM challenges and BSM opportunities

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We critically examine the potential to disentangle the Standard Model (SM) and new physics (NP) in $B\ensuremath{\rightarrow}{K}^{*}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ and $B\ensuremath{\rightarrow}{K}^{*}{e}^{+}{e}^{\ensuremath{-}}$ decays, focusing on (i) the LHCb anomaly, (ii) the search for right-handed currents, and (iii) lepton-universality violation. Restricting ourselves to the large-recoil region, we advocate a parametrization of the hadronic matrix elements that separates model-independent information about nonperturbative QCD from the results of model calculations. We clarify how to estimate corrections to the heavy-quark limit that would generate a right-handed (virtual) photon in the $b\ensuremath{\rightarrow}s\ensuremath{\gamma}$ contribution to the decay. We then apply this approach to the discussion of various sets of observables of increasing theoretical cleanness. First, we show that angular observables in the optimized ${P}_{i}^{(\ensuremath{'})}$ basis are, in general, still not robust against the long-distance QCD effects, both numerically and by examining analytically the dependence on corrections to the (model-independent) heavy-quark limit. As a result, while a fit to data favors a NP contribution to the semileptonic operators of the type $\ensuremath{\delta}{C}_{9}\ensuremath{\simeq}\ensuremath{-}1.5$, this comes at a relatively small statistical significance of $\ensuremath{\lesssim}2\ensuremath{\sigma}$ once such power corrections are properly accounted for. Second, two of these observables, ${P}_{1}$ and ${P}_{3}^{CP}$, are particularly clean at very low ${q}^{2}$ and sensitive probes of right-handed quark currents. We discuss their potential to set stringent bounds on the Wilson coefficient ${C}_{7}^{\ensuremath{'}}$, especially using data of the electronic mode, and we update the bounds with current angular data in the muonic channel. Finally, in light of the recent hint of lepton-universality violation in ${B}^{+}\ensuremath{\rightarrow}{K}^{+}\ensuremath{\ell}\ensuremath{\ell}$, we introduce and investigate new lepton-universality observables involving angular observables of the muonic and electronic modes and their zero crossings and show that, if the effect is of the size suggested by experiment, these can clearly distinguish between different NP explanations in terms of underlying semileptonic operators.