$R_K$ and $R_{K^{\ast }}$ beyond the Standard Model

Abstract

Measurements of the ratio of $B\to K^{\ast }\mu \mu$ to $B\to K^{\ast }ee$ branching fractions, $R_{K^{\ast }}$, by the LHCb collaboration strengthen the hints from previous studies with pseudoscalar kaons, $R_K$, for the breakdown of lepton universality, and therefore the Standard Model (SM), to $\sim 3.5\sigma$. Complementarity between $R_K$ and $R_{K^{\ast }}$ allows to pin down the Dirac structure of the new contributions to be predominantly SM-like chiral, with possible admixture of chirality-flipped contributions of up to $\mathcal{O}(\text{mbox}few10\%)$. Scalar and vector leptoquark representations $(S_3,V_1,V_3)$ plus possible (${\tilde{S}}_2,V_2$) admixture can explain $R_{K,K^{\ast }}$ via tree level exchange. Flavor models naturally predict leptoquark masses not exceeding a few TeV, with couplings to third generation quarks at $O(0.1)$, implying that this scenario can be directly tested at the LHC.

Comments: v1: 8 pages, v2: references added, typos corrected, matches published version