Assessing Lepton Flavor Universality Violations in Semileptonic Decays

Abstract: In light of recent measurements suggesting potential lepton flavor universality violations in semileptonic decays at collider experiments, this article provides a concise study of tree- and loop-level $B$-hadron semileptonic decays, $b \to c l \nu_l$ and $b \to s l^+ l^-$. We provide predictions for lepton flavor violating observables, $\mathcal{R}{J/\psi}$ and $\mathcal{R}{\eta_c}$, across the entire $q^2$ range. Our study employs the Relativistic Independent Quark Model (RIQM), highlighting a model-dependent approach to these observables. We compare our model's predictions with existing lattice predictions, demonstrating the strong applicability of the RIQM framework in describing $B_c$ decays. Additionally, we reassess global averages for $\mathcal{R}{D(D^*)}$ and $\mathcal{R}{K(K^*)}$ in semileptonic transitions. With the upcoming experimental upgrades and the anticipated Run 3 data on $B_c$ meson decays, rapid confirmation of these quantities could indicate significant evidence of physics beyond the Standard Model, thereby opening new pathways for understanding the complex flavor dynamics in $B$ meson decays.