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Disentangling lepton flavor universal and lepton flavor universality violating effects in bs+ transitions

Physical review(2022)

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摘要
In this paper we propose a strategy for discerning if new physics in the Wilson coefficient ${\mathcal{C}}_{9\ensuremath{\mu}}$ is dominantly lepton flavor universality violating or if it contains a sizable lepton flavor universal component (${\mathcal{C}}_{9}^{\mathrm{U}}$). Distinguishing among these two cases, for which the model independent fit of the related scenarios exhibits similar pulls with respect to the Standard Model, is crucial to advance our understanding of the $B$ anomalies. We first identify the origin of the degeneracy of these two cases and point out the key observables that can break it. In particular, while the observables measured so far that test lepton flavor universality exhibit similar dependencies to all the relevant Wilson coefficients, the forthcoming measurement of ${Q}_{5}={P}_{5}^{\ensuremath{'}\ensuremath{\mu}}\ensuremath{-}{P}_{5}^{\ensuremath{'}e}$ is particularly sensitive to ${\mathcal{C}}_{9\ensuremath{\mu}}\ensuremath{-}{\mathcal{C}}_{9e}$. In fact, if ${Q}_{5}$ were found to be small (i.e., close to its Standard Model value), this would imply a small ${\mathcal{C}}_{9\ensuremath{\mu}}\ensuremath{-}{\mathcal{C}}_{9e}$ but a sizable ${\mathcal{C}}_{9}^{\mathrm{U}}$, given the preference of global fits for a large negative new physics contribution in ${\mathcal{C}}_{9\ensuremath{\mu}}$. We discuss the possible origins of ${\mathcal{C}}_{9}^{\mathrm{U}}$, in particular how it could originate from new physics. Here, a promising scenario that could even link $b\ensuremath{\rightarrow}s{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ to ${R}_{{D}^{(*)}}$ is the one in which ${\mathcal{C}}_{9}^{\mathrm{U}}$ is generated from a tau loop via an off shell photon penguin diagram. This setup predicts the branching ratios of ${B}_{s}\ensuremath{\rightarrow}{\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$ and $B\ensuremath{\rightarrow}{K}^{(*)}{\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$ to lie within the reach of LHCb, CMS, and Belle II. Alternatively, in case of a nonobservation of these tauonic processes, we show that the most natural possibility to generate ${\mathcal{C}}_{9}^{\mathrm{U}}$ is a ${Z}^{\ensuremath{'}}$ with partially lepton flavor universal couplings.
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关键词
lepton flavor universal,transitions
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