D* polarization vs. $$ {R}_{D^{\left(\ast \right)}} $$ anomalies in the leptoquark models

Abstract

Polarization measurements in $$ \overline{B}\to {D}^{\left(\ast \right)}\tau \overline{\nu} $$ B ¯ → D ∗ τ ν ¯ are useful to check consistency in new physics explanations for the R D and $$ {R}_{D^{*}} $$ R D * anomalies. In this paper, we investigate the D * and τ polarizations and focus on the new physics contributions to the fraction of a longitudinal D * polarization (F L D * ), which is recently measured by the Belle collaboration F L D * = 0.60 ± 0.09, in model-independent manner and in each single leptoquark model (R2, S1 and U1) that can naturally explain the $$ {R}_{D^{\left(\ast \right)}} $$ R D ∗ anomalies. It is found that ℬ(B c + → τ + ν) severely restricts deviation from the Standard Model (SM) prediction of F L,SM D * = 0.46±0.04 in the leptoquark models: [0.43, 0.44], [0.42, 0.48], and [0.43, 0.47] are predicted as a range of F L D * for the R2, S1, and U1 leptoquark models, respectively, where the current data of $$ {R}_{D^{\left(\ast \right)}} $$ R D ∗ is satisfied at 1σ level. It is also shown that the τ polarization observables can much deviate from the SM predictions. The Belle II experiment, therefore, can check such correlations between $$ {R}_{D^{\left(\ast \right)}} $$ R D ∗ and the polarization observables, and discriminate among the leptoquark models.

Charged current b→cτν¯τ anomalies in a general W′ boson scenario

“…Complementary test at the LHC searches of some of these scenarios have been also explored [44,47,52,104,105,109,114]. Furthermore, the polarizations of the τ lepton and D * are also useful observables to potentially distinguish the underlying NP [31,32,44,45]. The potential NP scenarios that could explain the R(D ( * ) ) and R(J/ψ) anomalies, would also affect the branching ratio associated to the leptonic decay B − c → τ −ν τ [121,122], since all of them are generated by the same quark level transition b → cτν τ .…”

“…Contribution to the charged current b → cτντ observables According to the above effective Lagrangian (7), a general W charged boson exchange will modify the observables associated with the semileptonic transition b → cτν τ . The ratios R(M ) (M = D, D * , J/ψ), and the D * and τ longitudinal polarizations can be parametrized in terms of the effective Wilson coefficients C LL V , C RL V , C LR V , and C RR V as follows[44,108,111]…”

Phenomenology of decays in a scalar leptoquark model *

“…+0.21 −0.16 (syst.) [7,8], they provide valuable information about the spin structure of the interaction involved in B → D ( * ) τν decays, and are good observables to test various NP scenarios [37][38][39][40][41][42]. Measurements of the angular observables in these decays will be considerably improved in the future [43,44].…”

With or without U(2)? Probing non-standard flavor and helicity structures in semileptonic B decays

“…As shown in Fig. 2, it is not possible in our setup to reach the current experimental central value for ∆F D * L (see [36,40] for a similar discussion).…”