Anatomy of $b \to c \, \tau \, \nu$ anomalies

TL;DR

This paper analyzes the anomalies in B-meson decay ratios $R_D$ and $R_{D^*}$, exploring model-independent and composite Higgs model explanations, highlighting constraints from symmetries and collider searches.

Contribution

It provides a comprehensive analysis of $b o c au u$ anomalies within both model-independent and composite Higgs frameworks, emphasizing symmetry constraints and collider implications.

Findings

Constraints from $ m SU(2)_L imes U(1)_Y$ symmetry limit new physics scenarios.

Composite Higgs models face tension with electroweak precision tests due to low compositeness scale.

Composite leptoquarks could be detectable at the LHC.

Abstract

In recent times, one of the strongest hints of Physics Beyond the Standard Model (BSM) has been the anomaly in the ratios $R_D$ and $R_{D^\ast}$ measured in the charged current decays of $B$-mesons. In this work, we perform a comprehensive analysis of these decay modes, first in a model independent way and subsequently, in the context of composite Higgs models. We discuss in depth as to how linearly realised $\rm SU(2)_L \times U(1)_Y$ symmetry imposes severe constraint on the various scenarios because of correlations with other $\Delta F =1$ processes and $Z\,\tau\,\tau$ and $Z\,\nu\,\nu$ couplings. In the composite Higgs paradigm with partial compositeness, we show that, irrespective of the flavour structure of the composite sector, constraints from $\Delta F =2$ processes bring the compositeness scale down to $\sim 650$ GeV which is in tension with electroweak precision observables. In the presence of composite leptoquarks, the situation improves only marginally (a factor of $\sqrt{2}$ in the compositeness scale), thus making the new states soon discoverable by direct searches at the LHC. We also comment on the possible explanation of the $R_{K,K^*}$ anomalies within the composite Higgs framework.