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Register a new userThe Forward-Backward Asymmetry in $Bto D^{*}ell u$: One more hint for Scalar Leptoquarks?
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In recent years, intriguing hints for the violation of lepton flavour universality have accumulated. In particular, deviations from the Standard-Model (SM) predictions in $Bto D^{(*)}tau u/Bto D^{(*)}ell u$, in the anomalous magnetic moment of the muon and {in} $bto sell^+ell^-$ data were observed with a significance of $!>3,sigma$, $>!4,sigma$ and $>!5,sigma$, respectively. Furthermore, in a recent re-analysis of 2018 Belle data, it was found that the forward-backward asymmetry of $bar B to D^{*}mubar u$ vs $bar Bto D^{*}ebar u$ disagrees with the SM prediction by $approx!!4,sigma$ which would be an additional sign of lepton flavour universality violation. Since one naturally expects muon-related new effects to also emerge at some point in $b to cmu u$ decays, the above putative deviation might share a common origin with the other flavour anomalies. We show that a tensor operator is necessary to significantly improve the global fit w.r.t. the SM, which can only be induced (at tree-level in a renormalizable model) by a scalar leptoquark. Interestingly, among the two possible representations, the $SU(2)_L$-singlet $S_1$ and the doublet $S_2$, which can both also account for the anomalous magnetic moment of the muon, only $S_1$ can provide a good fit as it naturally gives rise to the scenario $C_{VL}, C_{SL}=-4 C_T$. While the constraints from (differences of) other angular observables prefer a smaller value $Delta A_{rm FB}$, this scenario is significantly preferred ($approx 3 sigma$) over the SM hypothesis and compatible with constraints such as $Bto K^* u u$ and electroweak precision bounds.
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After improving the knowledge about residua of the semileptonic form factor at its first two poles we show that $f_+^{Dpi}(q^2)$ is not saturated when compared with the experimental data. To fill the difference we approximate the rest of discontinuity by an effective pole and show that the data can be described very well with the position of the effective pole larger than the next excitation in the spectrum of $D^ast$ state. The results of fits with experimental data also suggest the validity of superconvergence which in the pole models translates to a vanishing of the sum of residua of the form factor at all poles. A similar discussion in the case of $Bto pi ell u_ell$ leads to the possibility of extracting $vert V_{ub}vert$, the error of which appears to be dominated by $g_{B^ast Bpi}$, which can be nowadays computed on the lattice. In evaluating the residua of the form factors at their nearest pole we needed the vector meson decay constants $f_{D^ast}$ and $f_{B^ast}$, which we computed by using the numerical simulations of QCD on the lattice with $N_{rm f}=2$ dynamical quarks. We obtain, $f_{D^ast}/f_D=1.208(27)$ and $f_{B^ast}/f_B=1.051(17)$.
Experimental data on $ R(D^{(*)}) $, $ R(K^{(*)}) $ and $ R(J/psi) $, provided by different collaborations, show sizable deviations from the SM predictions. To describe these anomalies many new physics scenarios have been proposed. One of them is leptoquark model with introducing the vector and scalar leptoquarks coupling simultaneously to the quarks and leptons. To look for similar possible anomalies in baryonic sector, we investigate the effects of a vector leptoquark $U_3 (3,3, frac{2}{3})$ on various physical quantities related to the tree-level $ Lambda_b rightarrow Lambda_c ell ~ overline{ u}_ell$ decays ($ ell=mu, ~tau $), which proceed via $ b rightarrow c~ell ~ overline{ u}_ell$ transitions at quark level. We calculate the differential branching ratio, forward-backward asymmetry and longitudinal polarizations of lepton and $Lambda_{c}$ baryon at $ mu $ and $ tau $ lepton channels in leptoquark model and compare their behavior with respect to $ q^2 $ with the predictions of the standard model (SM). In the calculations we use the form factors calculated in full QCD as the main inputs and take into account all the errors coming from the form factors and model parameters. It is observed that ........
We update the standard model (SM) predictions of $R(D^*)$ using the latest results on the decay distributions in $B to D^* ell u_{ell}$ ($ell = mu, e$) by Belle collaboration, while extracting $|V_{cb}|$ at the same time. Depending on the inputs used in the analysis, we define various fit scenarios. Although the central values of the predicted $R(D^*)$ in all the scenarios have reduced from its earlier predictions in 2017, the results are consistent with each other within the uncertainties. In this analysis, our prediction of $R(D^*)$ is consistent with the respective world average at $sim 3sigma$. We have also predicted several angular observables associated with $B to D^* tau u_{tau}$ decays. We note that the predicted $F_L(D^*)$ is consistent with the corresponding measurement at 2$sigma$. Utilizing these new results, we fit the Wilson coefficients appearing beyond the standard model of particle physics (BSM). To see the trend of SM predictions, we have utilized the recently published preliminary results on the form-factors at non-zero recoil by the lattice groups like Fermilab-MILC and JLQCD and predicted the observables in $B to D^* ell u_{ell}$, and $B to D^* tau u_{tau}$ decays.
Using the most general effective Hamiltonian comprising scalar,vector and tensor type interactions, we have written the branching ratio, the forward-backward (FB) asymmetry and the normalized FB asymmetry as functions of the new Wilson coefficients. It is found that the branching ratio depends on all new coefficients,but the dependence of asymmetries on coefficients could be analyzed only for one Wilson coefficient.
The mass and coupling of the scalar tetraquark $T_{bb;overline{u}overline{d }}^{-}$ (hereafter $T_{b:overline{d}}^{-} $) are calculated in the context of the QCD two-point sum rule method. In computations we take into account effects of various quark, gluon and mixed condensates up to dimension ten. The result obtained for the mass of this state $m=(10135pm 240)~mathrm{MeV} $ demonstrates that it is stable against the strong and electromagnetic decays. We also explore the dominant semileptonic $T_{b:overline{d}}^{-} to widetilde{Z}_{bc;bar{u}bar{d}}^{0}loverline{ u }_{l}$ and nonleptonic decays $T_{b:overline{d}}^{-} to widetilde{Z}_{bc;bar{u}bar{ d}}^{0}M$, where $widetilde{Z}_{bc;bar{u}bar{d}}^{0}$ is the scalar tetraquark composed of color-sextet diquark and antidiquark, and $M$ is one of the final-state pseudoscalar mesons $pi^{-}, K^{-}, D^{-}$ and $D_s^{-}$ , respectively. The partial widths of these processes are calculated in terms of the weak form factors $G_{1(2)}(q^2)$, which are determined from the QCD three-point sum rules. Predictions for the mass, full width $Gamma _{mathrm{full}} =(10.88pm 1.88)times 10^{-10}~mathrm{MeV}$, and mean lifetime $tau=0.61_{-0.09}^{+0.13}~mathrm{ps}$ of the $T_{b:overline{d} }^{-}$ obtained in the present work can be used in theoretical and experimental studies of this exotic state.
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