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We benefit from the lattice QCD determination by the HPQCD of the StandardModel (SM) form factors for the $\bar B_s\to D_s$ [Phys. Rev. D 101, 074513(2020)] and the SM and tensor ones for the $\bar B_s\to D_s^{*}$(arXiv:2304.03137 [hep-lat]) semileptonic decays, and the heavy quark effectivetheory (HQET) relations for the analogous $B\to D^{(*)}$ decays obtained byF.U. Bernlochner et al. in Phys. Rev. D 95, 115008 (2017), to extract theleading and sub-leading Isgur-Wise functions for the $\bar B_s\to D_s^{(*)}$decays. Further use of the HQET relations allows us to evaluate thecorresponding scalar, pseudoscalar and tensor form factors needed for aphenomenological study of new physics (NP) effects on the $\bar B_s\toD_s^{(*)}$ semileptonic decay. At present, the experimental values for theratios ${\cal R}_{D^{(*)}}=\Gamma[\bar B\toD^{(*)}\tau^-\bar\nu_\tau]/\Gamma[\bar B\to D^{(*)}e^-(\mu^-)\bar\nu_{e(\mu)}]$are the best signal in favor of lepton flavor universality violation (LFUV)seen in charged current (CC) $b\to c$ decays. In this work we conduct a studyof NP effects on the $\bar B_s\to D_s^{(*)}\tau^-\bar\nu_\tau$ semileptonicdecays by comparing tau spin, angular and spin-angular asymmetry distributionsobtained within the SM and three different NP scenarios. As expected from SU(3)light-flavor symmetry, we get results close to the ones found in a similaranalysis of the $\bar B\to D^{(*)}$ case. The measurement of the $\bar B_s\toD_s^{(*)}\ell\bar\nu_\ell$ semileptonic decays, which is within reach ofpresent experiments, could then be of relevance in helping to establish or ruleout LFUV in CC $b\to c$ transitions.

Study of new physics effects in $\bar B_s\to  D^{(*)}_s\tau^-\bar\nu_\tau$ semileptonic decays using lattice QCD form  factors and heavy quark effective theory