Abstract
We present a model-independent global analysis of hadronic form factors for the semileptonic decays $b\rightarrow c\ell\nu$ that exploits lattice-QCD data, dispersion relations, and heavy-quark symmetries. The analysis yields predictions for the relevant form factors, within quantifiable bounds. These form factors are used to compute the semileptonic ratios $R(H_c)$ and various decay-product polarizations. In particular, we find $R(D_s^*)=0.20(3)$ and $R(J/\psi)=0.25(3)$, predictions that can be compared to results of upcoming LHCb measurements. In developing this treatment, we obtain leading-order NRQCD results for the nonzero-recoil relations between the $B_c^+ \rightarrow \{J/\psi , \eta_c \}$ form factors.
Highlights
While the Higgs interaction is the only source of lepton universality violations within the Standard Model (SM), the observation of neutrino masses implies that at least one form of beyond-SM modification exists, in the lepton sector
We extend the relations of Refs. [46,48] by deriving the leading-order nonrelativistic QCD (NRQCD) relations between the form factors and Δ at nonzero recoil
This work uses the existing lattice-QCD results for b → c form factors as input to our global analysis. These results have been produced by a number of different groups, and the determinations of the various form factors have been performed at varying numbers of momentumtransfer values t and with varying treatments of uncertainties
Summary
While the Higgs interaction is the only source of lepton universality violations within the Standard Model (SM), the observation of neutrino masses implies that at least one form of beyond-SM modification exists, in the lepton sector. Heavy-hadron semileptonic decay rates (both full and differential) producing distinct lepton flavors differ only due to factors of lepton mass that arise from kinematic and helicity-flip factors. Such dependences can be removed in a variety of ways [1,2,3,4,5,6,7,8,9]. Measurements from BABAR, Belle, and LHCb of the ratios RðDðÃÞÞ [10,11,12,13,14,15,16,17,18,19] of the heavy-light meson decays B → DðÃÞlν, with l 1⁄4 τ to l 1⁄4 μ, exhibit a combined 3.1σ discrepancy from the HFLAV-suggested SM values [20], which averages the results of Refs.
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