Are we overlooking lepton-flavor-universal new physics in $b\to s\ell \ell$ ?

The deviations with respect to the Standard Model that are currently observed in $b\to s\ell \ell$ transitions (the so-called flavor anomalies) can be interpreted in terms of different new physics (NP) scenarios within a model-independent effective approach. We reconsider the determination of NP in global fits from a different perspective by removing one implicit hypothesis of current analyses, namely, that NP is only lepton-flavor-universality violating (LFUV). We examine the roles played by LFUV NP and lepton-flavor-universal NP altogether, providing new directions to identify the possible theory beyond the SM responsible for the anomalies observed. New patterns of NP emerge due to the possibility of allowing at the same time large LFUV and lepton-flavor-universal NP contributions to ${\mathcal{C}}_{10\mu }$, which provides a different mechanism to obey the constraint from the $B_s\to {\mu }^{+}{\mu }^{-}$ branching ratio. In this landscape of NP, we discuss how to discriminate among these scenarios in the short term, thanks to current and forthcoming observables. While the update of $R_K$ will be a major milestone to confirm the NP origin of the flavor anomalies, additional observables, in particular the LFUV angular observable $Q_5$, turn out to be central to assessing the precise NP scenario responsible for the observed anomalies.

Figure 1

Confidence regions for scenario 6 in the plane (${\mathcal{C}}_{9\mu }^{\mathrm{V}}=-{\mathcal{C}}_{10\mu }^{\mathrm{V}}$, ${\mathcal{C}}_9^{\mathrm{U}}={\mathcal{C}}_{10}^{\mathrm{U}}$). The regions for different experimental subsets correspond to a confidence level of $3\sigma$, whereas the 1, 2, $3\sigma$ confidence regions are shown for the region associated with the global fit to all data.

Figure 2

Predictions for LFUV observables of interest under various hypotheses of LFU and LFUV NP contributions currently favored by the global $b\to s\ell \ell$. From left to right: the SM is followed by scenarios 1 to 8 as described in the main text. We plot $R_{K,K^{*}}-1$ to keep the figure compact.

Figure 3

Left: Correlation between ${\mathcal{C}}_{9\mu }^{\mathrm{V}}$ and ${\mathcal{C}}_9^{\mathrm{U}}$ from the scenario 7 fit. Right: Correlation between the parameters ${\mathcal{C}}_{9\mu }^{\mathrm{V}}=-{\mathcal{C}}_{10\mu }^{\mathrm{V}}$ and ${\mathcal{C}}_9^{\mathrm{U}}$ from the scenario 8 fit. The regions for different experimental subsets correspond to a confidence level of $3\sigma$, whereas the 1, 2, $3\sigma$ confidence regions are shown for the region associated with the global fit to all data.

Figure 4

$⟨R_K{⟩}_{[1,6]}$ as a function of $⟨Q_5{⟩}_{[1.1,6]}$ in the four of the scenarios analyzed. The solid blue and solid red lines correspond to $C_{9,10}^{\mathrm{U}}=0$, while the dotted blue and dotted red lines have LFU contributions ${\mathcal{C}}_9^{\mathrm{U}}=0.56$ and ${\mathcal{C}}_9^{\mathrm{U}}={\mathcal{C}}_{10}^{\mathrm{U}}=-0.44$, respectively.