Diquark Explanation of $b\to s{\ell }^{+}{\ell }^{-}$

Abstract

The discrepancies between $b\to s{\ell }^{+}{\ell }^{-}$ data and the corresponding Standard Model predictions point to the existence of new physics with a significance at the $5\sigma$ level. While previously a lepton flavour universality violating effect was preferred, the new $R(K^{(\ast )})$ and $B_s\to {\mu }^{+}{\mu }^{-}$ measurements are now compatible with the Standard Model, favouring a lepton flavour universal beyond the Standard Model contribution to $C_9$. Since heavy new physics is generally chiral, and because of the stringent constraints from charged lepton flavour violation, this poses a challenge for model building. In this article, we point out a novel possibility: a diquark, i.e. a coloured scalar, induces the Wilson coefficient of the $(\bar{s}{\gamma }^{\mu }{P}_{L}b)(\bar{c}{\gamma }_{\mu }{P}_{L}c)$ operator at tree-level, which then mixes into $O_9$ via an off-shell photon penguin. This setup allows for a lepton flavour universal effect of $C_9\approx -0.5$, without violating bounds from $\Delta M_s$, $\Delta \Gamma$, $B\to X_s\gamma$ and $D^0-{\bar{D}}^0$ mixing. This scenario predicts a small and negative $C_9^{\prime }$ and a light diquark, preferably with a mass around $500$GeV, as compatible with the CMS di-di-jet analysis, and a deficit in the inclusive $b\to c\bar{c}s$ rate.

12 pages, 1 figure. v2: Journal version - added references, fixed typos, moved some large equations to appendix

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