Abstract
We perform an analysis of the b→cτν data, including R(D(⁎)), R(J/ψ), Pτ(D⁎) and FLD⁎, within and beyond the Standard Model (SM). We fit the B→D(⁎) hadronic form factors in the HQET parametrization to the lattice and the light-cone sum rule (LCSR) results, applying the general strong unitarity bounds corresponding to JP=1−, 1+, 0− and 0+. Using the obtained HQET relations between helicity amplitudes, we give the strong unitarity bounds on individual helicity amplitudes, which can be used in the BGL fits. Using the fitted form factors and taking into account the most recent Belle measurement of R(D(⁎)) we investigate the model-independent and the leptoquark model explanations of the b→cτν anomalies. Specifically, we consider the one-operator, the two-operator new physics (NP) scenarios and the NP models with a single R2, S1 or U1 leptoquark which is supposed to be able to address the b→cτν anomalies, and our results show that the R2 leptoquark model is in tension with the limit B(Bc→τν)<10%. Furthermore, we give predictions for the various observables in the SM and the NP scenarios/leptoquark models based on the present form factor study and the analysis of NP.
Highlights
In recent years several discrepancies between the measurements and the Standard Model (SM) predictions have been observed in the bottom sector, the R(D(∗)) anomalies in the charged current transition b → cτν and the RK(∗) anomalies in the neutral current transition b → s + − ( = e, μ), which both hint the violation of lepton flavour universality (LFU)1
All these parameterizations are based on the analytic properties of the QCD correlation functions, the dispersion relation and the crossing symmetry, with the difference being that in the CLN(-like) parametrizations the heavy quark symmetry is employed to relate the form factors while in the BGL(-like) parametrizations the Bc pole(s) are separated from the analytic parts that are expanded in z and truncated at a certain level
We have mainly studied two nontrivial aspects: the determination of the hadronic form factors and the new physics analysis
Summary
The deviation between the current world average [32] of the measurements [23, 40,41,42,43,44,45,46,47] of R(D(∗)) and the SM prediction is still at the level of 3 − 4σ depending on the determination of hadronic form factors, it is well worth reexamining the NP explanations of the b → cτν anomalies along with the form factor study. Using the fitted B → D(∗) form factors and taking into account the most recent experimental data, we update the model-independent analysis of the NP effects in the b → cτν transition within the framework of the weak effective theory. We study the models with a single LQ which can explain the b → cτν anomalies, including models with a scalar LQ R2, S 1 or a vector LQ U1.
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