Study of new physics effects in B_s→D_s^(∗)τ^-ν_τ semileptonic decays using lattice QCD form factors and heavy quark effective theory

We benefit from the lattice QCD determination by the HPQCD of the Standard Model (SM) form factors for the B_s→D_s [ Phys. Rev. D 101 (2020) 074513] and the SM and tensor ones for the B_s→D_s^∗ ( arXiv:2304.03137 [hep-lat]) semileptonic decays, and the heavy quark effective theory (HQET) relations for the analogous B → D (*) decays obtained by F.U. Bernlochner et al. in Phys. Rev. D 95 (2017) 115008, to extract the leading and sub-leading Isgur-Wise functions for the B_s→D_s^(∗) decays. Further use of the HQET relations allows us to evaluate the corresponding scalar, pseudoscalar and tensor form factors needed for a phenomenological study of new physics (NP) effects on the B_s→D_s^(∗) semileptonic decay. At present, the experimental values for the ratios ℛ_D^(∗)=Γ[B→D^(∗)τ^-ν_τ]/Γ[B→D^(∗)e^-(μ^-)ν_e(μ)] are the best signal in favor of lepton flavor universality violation (LFUV) seen in charged current (CC) b → c decays. In this work we conduct a study of NP effects on the B_s→D_s^(∗)τ^-ν_τ semileptonic decays by comparing tau spin, angular and spin-angular asymmetry distributions obtained 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 similar analysis of the B→D^(∗) case. The measurement of the B_s→D_s^(∗)ℓν_ℓ semileptonic decays, which is within reach of present experiments, could then be of relevance in helping to establish or rule out LFUV in CC b → c transitions.