Lepton-flavor universality limits in warped space

TL;DR

This paper investigates how warped extra dimension theories can naturally cause lepton-flavor universality violation, analyzing experimental data and predicting observable effects that could confirm or constrain such models.

Contribution

It demonstrates that warped space models can explain LFU violation signals and provides specific parameter ranges consistent with current data.

Findings

Agreement with $R_{K^{(*)}}$ and $R_{D^{(*)}}$ data at 95% CL

Predicted lower bound for $R_{K^{(*)}}$ is approximately 0.79

Predicted $B o K uar u$ branching ratio exceeds Standard Model predictions

Abstract

We explore the limits on lepton-flavor universality (LFU) violation in theories where the hierarchy problem is solved by means of a warped extra dimension. In those theories LFU violation, in fermion interaction with Kaluza-Klein modes of gauge bosons, is provided ab initio when different flavor of fermions are differently localized along the extra dimension. As this fact arises from the mass pattern of quarks and leptons, LFU violation is natural in this class of theories. We analyze the experimental data pointing towards LFU violation, as well as the most relevant electroweak and flavor observables, and the LFU tests in the $\mu/e$ and $\tau/\mu$ sectors. We find agreement with $R_{K^{(\ast)}}$ and $R_{D^{(\ast)}}$ data at 95% CL, provided the third generation left-handed fermions are composite ($0.14 < c_{b_L} < 0.28$ and $0.27 < c_{\tau_L} < 0.33$), and find the absolute limits $R_{K^{(\ast)}}\gtrsim 0.79$ and $R_{D^{(\ast)}}/R_{D^{(\ast)}}^{\rm SM}\lesssim 1.13$. Moreover we predict $\mathcal B( B\to K\nu\bar\nu)\gtrsim 1.14\times 10^{-5}$ at 95% CL, smaller than the present experimental upper bound but a few times larger than the Standard Model prediction.