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

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 $q_{max}^2$. We then calculated the branching ratios and many other physical observables: $A_{FB}^l$, $F_L^{\varphi }$, $S_{3,4,7}$, $A_{5,6,8,9}$ and the clean angular observables $P_{1,2,3}$ and $P_{4,5,6,8}^{\mathrm{\prime }}$.From our studies, we find the following points: (a) the PQCD and “PQCD+Lattice” predictions of ${\cal B}( B_s \to \phi \mu^+ \mu^-)$areabout$7\times 10^{-7}$, 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 $R_{\varphi }^{e\mu }$ and $R_{\varphi }^{\mu \tau }$; © 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 $R_{\varphi }^{e\mu },R_{\varphi }^{\mu \tau },A_{FB}^l,P_{1,2,3}$ and $P_{4,5,6,8}^{\mathrm{\prime }}$, we suggest LHCb and Belle-II Collaboration to measure them in their experiments.