Study of $B_s\to \phi \ell^+ \ell^-$ decays in the PQCD factorization approach with lattice QCD input

TL;DR

This study systematically analyzes the $B_s o \,\phi l^+ l^-$ decays using PQCD and lattice QCD inputs, providing predictions for branching ratios, angular observables, and CP asymmetries, and comparing them with experimental data.

Contribution

It introduces a combined PQCD and lattice QCD approach to improve form factor extrapolation and provides comprehensive predictions for decay observables, aiding future experimental tests.

Findings

Predicted branching ratio for $B_s \to \phi \mu^+ \mu^-$ agrees with LHCb data.

Calculated ratios of branching ratios $R_\phi^{e\mu}$ and $R_\phi^{\mu\tau}$.

Predictions for angular observables are consistent with current experimental measurements.

Abstract

In this paper, we studied systematically the semileptonic decays $B_s \to \phi l^+ l^-$ with $l^-=(e^-,\mu^-,\tau^-)$ by using the perturbative QCD (PQCD) and the "PQCD+Lattice" factorization approach, respectively. We first evaluated all relevant form factors $F_i(q^2)$ in the low $q^2$ region using the PQCD approach, and we also took the available lattice QCD results at the high-$q^2$ region as additional input to improve the extrapolation of $F_i(q^2)$ from the low-$q^2$ region to the endpoint $q^2_{max}$. We then calculated the branching ratios and many other physical observables: $A_{FB}^{l}$, $F_L^{\phi}$, $S_{3,4,7}$, $A_{5,6,8,9}$ and the clean angular observables $P_{1,2,3} $ and $P^{\prime}_{4,5,6,8}$.From our studies, we find the following points: (a) the PQCD and "PQCD+Lattice" predictions of ${\cal B}( B_s \to \phi \mu^+ \mu^-)$ are about $7\times 10^{-7}$, which agree well with the LHCb measured values and the QCD sum rule prediction within still large errors; (b) we defined and calculated the ratios of the branching ratios $R_\phi^{e\mu}$ and $R_\phi^{\mu\tau}$; (c) the PQCD and "PQCD+Lattice" predictions of the longitudinal polarization $F_L$, the CP averaged angular coefficients $S_{3,4,7} $ and the CP asymmetry angular coefficients $A_{5,6,8,9}$, agree with the LHCb measurements in all considered bins within the still large experimental errors; and (d) for those currently still unknown observables $R_\phi^{e\mu}, R_\phi^{\mu\tau}, A_{FB}^{l}, P_{1,2,3}$ and $P^{\prime}_{4,5,6,8}$, we suggest LHCb and Belle-II Collaboration to measure them in their experiments.