Form factors and phenomenology of $\boldsymbol{B_{(s)}}$ and $\boldsymbol{D_{(s)}}$ semileptonic decays to $\boldsymbol{\eta}$ and $\boldsymbol{\eta^\prime}$

Abstract

Motivated by more precise recent measurements of the $B \to \eta \ell \nu$ and $D_{(s)} \to \eta^{(\prime)}\ell \nu$ decays, we employ state-of-the-art parametrizations to describe the $B_{(s)}, D_{(s)} \to \eta^{(\prime)}$ form factors across the full $q^2$ range, fitting them to the latest light-cone sum rule results. Using these results, we compute the branching ratios for all relevant decays and compare them with experimental data, finding good agreement. Additionally, we examine the validity and precision of the extracted $\eta$-$\eta'$ mixing angle in the context of heavy meson decays. By combining our predictions for the $B, D, D_s \to \eta^{(\prime)}$ form factors with recent semileptonic decay measurements, we extract the CKM elements $|V_{ub}|$, $V_{cs}$, and $V_{cd}$. We also provide tests of the lepton flavour universality in the $B, D, D_s \to \eta^{(\prime)} \ell \nu$ decays and present results for the forward-backwards asymmetries in these decays. Our findings indicate that the extracted values are becoming comparable in precision to those obtained from more conventional semileptonic decay analyses.