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Register a new userEffects of vector leptoquarks on $ Lambda_b rightarrow Lambda_c~ell ~ overline{ u}_ell$ decay
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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 ........
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We present the first lattice-QCD determination of the form factors describing the semileptonic decays $Lambda_b to Lambda_c^*(2595)ell^-bar{ u}$ and $Lambda_b to Lambda_c^*(2625)ell^-bar{ u}$, where the $Lambda_c^*(2595)$ and $Lambda_c^*(2625)$ are the lightest charm baryons with $J^P=frac12^-$ and $J^P=frac32^-$, respectively. These decay modes provide new opportunities to test lepton flavor universality and also play an important role in global analyses of the strong interactions in $bto c$ semileptonic decays. We determine the full set of vector, axial vector, and tensor form factors for both decays, but only in a small kinematic region near the zero-recoil point. The lattice calculation uses three different ensembles of gauge-field configurations with $2+1$ flavors of domain-wall fermions, and we perform extrapolations of the form factors to the continuum limit and physical pion mass. We present Standard-Model predictions for the differential decay rates and angular observables. In the kinematic region considered, the differential decay rate for the $frac12^-$ final state is found to be approximately 2.5 times larger than the rate for the $frac32^-$ final state. We also test the compatibility of our form-factor results with zero-recoil sum rules.
We study the implications for $Lambda_b to Lambda_c^*ellbar{ u}_ell$ and $Lambda_b to Lambda_c^*pi^-$ $[Lambda_c^*=Lambda_c(2595)$ and $Lambda_c(2625)]$ decays that can be deduced from heavy quark spin symmetry (HQSS). Identifying the odd parity $Lambda_c(2595)$ and $Lambda_c(2625)$ resonances as HQSS partners, with total angular momentum--parity $j_q^P=1^-$ for the light degrees of freedom, we find that the ratios $Gamma(Lambda_brightarrowLambda_c(2595)pi^-)/Gamma(Lambda_brightarrowLambda_c(2625)pi^-)$ and $Gamma(Lambda_brightarrow Lambda_c(2595) ell bar{ u}_ell)/ Gamma(Lambda_brightarrowLambda_c(2625) ell bar{ u}_ell)$ agree, within errors, with the experimental values given in the Review of Particle Physics. We discuss how future, and more precise, measurements of the above branching fractions could be used to shed light into the inner HQSS structure of the narrow $Lambda_c(2595)$ odd-parity resonance. Namely, we show that such studies would constrain the existence of a sizable $j^P_q=0^-$ component in its wave-function, and/or of a two-pole pattern, in analogy to the case of the similar $Lambda(1405)$ resonance in the strange sector, as suggested by most of the approaches that describe the $Lambda_c(2595)$ as a hadron molecule. We also investigate the lepton flavor universality ratios $R[Lambda_c^*] = {cal B}(Lambda_b to Lambda_c^* tau,bar u_tau)/{cal B}(Lambda_b to Lambda_c^* mu,bar u_mu)$, and discuss how $R[Lambda_c(2595)]$ may be affected by a new source of potentially large systematic errors if there are two $Lambda_c(2595)$ poles.
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.
Based on the standard model (SM) of particle physics, we study the decays $Lambda_b to Lambda ell^+ ell^-$ in light of the available inputs from lattice and the data from LHCb. We fit the form-factors of this decay mode using the available theory and experimental inputs after defining different fit scenarios and checking their consistencies. The theory inputs include the relations between the form-factors in heavy quark effective theory (HQET) and soft collinear effective theory (SCET) at the endpoints of di-lepton invariant mass squared $q^2$. Utilizing the fit results, we have predicted a few observables related to this mode. We have also predicted the observable $R_{Lambda} = Br(Lambda_b to Lambda ell_i^+ell_i^- )/Br(Lambda_b to Lambda ell_j^+ell_j^-)$ where $ell_{i}$ and $ell_j$ are charged leptons of different generations ($i e j$). At the moment, we do not observe noticeable differences in the extracted values of the observables in fully data-driven and SM like fit scenarios.
The decay $Lambda_b^0 to Lambda_c^+ p overline{p} pi^-$ is observed using $pp$ collision data collected with the LHCb detector at centre-of-mass energies of $sqrt{s}=$ 7 and 8 TeV, corresponding to an integrated luminosity of 3 $fb^{-1}$. The ratio of branching fractions between $Lambda_b^0 to Lambda_c^+ p overline{p} pi^-$ and $Lambda_b^0 to Lambda_c^+ pi^-$ decays is measured to be begin{equation*} frac{mathcal{B}(Lambda_b^0 to Lambda_c^+ p overline{p}pi^-)}{mathcal{B}(Lambda_b^0 to Lambda_c^+ pi^-)} = 0.0540 pm 0.0023 pm 0.0032. end{equation*} Two resonant structures are observed in the $ Lambda_c^+ pi^-$ mass spectrum of the ${Lambda_b^0 to Lambda_c^+ poverline{p} pi^-}$ decays, corresponding to the $Sigma_c(2455)^0$ and $Sigma_c^{*}(2520)^0$ states. The ratios of branching fractions with respect to the decay $Lambda_b^0 to Lambda_c^+ p overline{p} pi^-$ are begin{align*} frac{mathcal{B}(Lambda_b^0 to Sigma_c^0 poverline{p})timesmathcal{B}(Sigma_c^0to Lambda_c^+ pi^-)}{mathcal{B}(Lambda_b^0 to Lambda_c^+ p overline{p}pi^-)} = 0.089pm0.015pm0.006, frac{mathcal{B}(Lambda_b^0 to Sigma_c^{*0} poverline{p})timesmathcal{B}(Sigma_c^{*0}to Lambda_c^+ pi^-)}{mathcal{B}(Lambda_b^0 to Lambda_c^+ p overline{p}pi^-)} = 0.119pm0.020pm0.014. end{align*} In all of the above results, the first uncertainty is statistical and the second is systematic. The phase space is also examined for the presence of dibaryon resonances. No evidence for such resonances is found.
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