Lepton-flavor universality violation in $R_K$ and $R_{D^{(\ast )}}$ from warped space

Abstract

Some anomalies in the processes $b\to s\ell \ell$ ($\ell =\mu ,e$) and $b\to c\ell {\bar{\nu }}_{\ell }$ ($\ell =\tau ,\mu ,e$), in particular in the observables $R_K$ and $R_{D^{(\ast )}}$, have been found by the BaBar, LHCb and Belle Collaborations, leading to a possible lepton flavor universality violation. If these anomalies were confirmed they would inevitably lead to physics beyond the Standard Model. In this paper we try to accommodate the present anomalies in an extra dimensional theory, solving the naturalness problem of the Standard Model by means of a warped metric with a strong conformality violation near the infra-red brane. The $R_K$ anomaly can be accommodated provided that the left-handed bottom quark and muon lepton have some degree of compositeness in the dual theory. The theory is consistent with all electroweak and flavor observables, and with all direct searches of Kaluza-Klein electroweak gauge bosons and gluons. The fermion spectrum, and fermion mixing angles, can be reproduced by mostly elementary right-handed bottom quarks, and tau and muon leptons. Moreover the $R_{D^{(\ast )}}$ anomaly requires a strong degree of compositeness for the left-handed tau leptons, which turns out to be in tension with experimental data on the $g_{{\tau }_L}^Z$ coupling, possibly unless some degree of fine-tuning is introduced in the fixing of the CKM matrix.

40 pages, 20 figures; v2: Extended version with analyses of non-observation of FUV in the mu/e sector, LFU tests in tau decays, B->K nu nu and B->K tau tau; v3: Revised version where radiative corrections to observables $\tau \to \ell \nu \nu$ and the $Z\bar{\ell }\ell$ couplings ($\ell =\tau ,\mu$), as well as the LHC dilepton resonance from bottom-bottom fusion, are considered. Some conclusions changed

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