Recent experimental results of RðD ðÃÞ Þ deviate from the standard model (SM) by 3.1σ, suggesting a new physics (NP) that affects the b → cτν τ transition. Motivated by this, we investigate the possible NP effects in the Λ b → Λ c τν τ decay. For this purpose, assuming the neutrinos are left-handed, we calculate in detail the helicity amplitudes of Λ b → Λ c lν l (l ¼ e − , μ − , and τ − ) decays with all possible four-fermion operators. Within the latest results of Λ b → Λ c form factors from lattice QCD calculations, we study these decays in a model-independent manner. The differential and total branching fractions, the longitudinal polarizations of final leptons and hadrons, the forward-backward asymmetries in the lepton side, the convexity parameters, and the ratio RðΛ c Þ are calculated. In SM, we obtain the ratio RðΛ c Þ ¼ 0.33 AE 0.01. Supposing that NP only affects the third generation fermions, we present the correlations among RðDÞ, RðD à Þ, and RðΛ c Þ, as the Λ b → Λ c τν τ and B → D ðÃÞ τν τ are all induced by b → clν l . We perform a minimum χ 2 fit of the Wilson coefficient of each operator to the latest experimental data of different observables, including the ratios RðD ðÃÞ Þ and RðJ=ψÞ and the τ polarization P τ ðD à Þ. It is found that the left-handed scalar operator O SL affects the branching fraction remarkably, and the ratio RðΛ c Þ can be enhanced by 30%. For other operators, the ratio amounts to 0.38 AE 0.02, which is larger than prediction of SM by 20%. Using the fitted values of the Wilson coefficients of the single NP operators, we also give a prognosis for the physical observables of Λ b → Λ c τν τ , including the ratio RðΛ c Þ, forward-backward asymmetry, and other polarized observables as well as the differential branching fraction. Furthermore, we also study the effects of three typical NP models on the ratio and the differential branching fraction of Λ b → Λ c τν τ . We hope our results can be tested in the current LHCb experiment and the future high energy experiments.
Anomalies in decays induced by ( ) transitions may imply lepton flavor universality violations, which raises questions on such phenomena in the D decays induced by transitions. Current measurements of the pure leptonic and semi-leptonic D decays agree with the standard model (SM) predictions, and such agreements can be used to constrain the new physics (NP) contributions. In this work, we extend SM by assuming general effective Hamiltonians describing the transitions including the full set of the four-fermion operators. With the latest experimental data, we perform a least fit of the Wilson coefficient corresponding to each operator. The results indicate that the Wilson coefficients of tensor and scalar operators in the muon sector are in the order of while others are in the order of . The lepton flavor universality could be violated by interactions with the scalar operators. We also determine that the pure leptonic decays are significantly sensitive to scalar operators. The effects of NP on the semi-leptonic decays with electron final state are negligible; however, for the decays with the muon final state, the effects of scalar and tensor operators will appear in the forward-backward asymmetries and the muon helicity asymmetries of decays. The future measurements of these decays in the BESIII and Belle II experiments will facilitate the evaluation of NP effects.
In $$ {B}_c^{-} $$ B c − → J/ψ(→ μ+μ−)τ−$$ {\overline{\nu}}_{\tau } $$ ν ¯ τ decay, the three-momentum $$ {\boldsymbol{p}}_{\tau^{-}} $$ p τ − cannot be determined accurately due to the decay products of τ− inevitably include an undetected ντ. As a consequence, the angular distribution of this decay cannot be measured. In this work, we construct a measurable angular distribution by considering the subsequent decay τ− → π−ντ. The full cascade decay is $$ {B}_c^{-} $$ B c − → J/ψ(→ μ+μ−)τ−(→ π−ντ)$$ {\overline{\nu}}_{\tau } $$ ν ¯ τ , in which the three-momenta $$ {\boldsymbol{p}}_{\mu^{+}},{\boldsymbol{p}}_{\mu^{-}} $$ p μ + , p μ − , and $$ {\boldsymbol{p}}_{\pi^{-}} $$ p π − can be measured. The five-fold differential angular distribution containing all Lorentz structures of the new physics (NP) effective operators can be written in terms of twelve angular observables ℐi(q2, Eπ). Integrating over the energy of pion Eπ, we construct twelve normalized angular observables $$ {\hat{\mathrm{\mathcal{I}}}}_i $$ ℐ ̂ i (q2) and two lepton-flavor-universality ratios $$ R\left({P}_{L,T}^{J/\psi}\right) $$ R P L , T J / ψ (q2). Based on the Bc → J/ψ form factors calculated by the latest lattice QCD and sum rule, we predict the q2 distribution of all $$ {\hat{\mathrm{\mathcal{I}}}}_i $$ ℐ ̂ i and $$ R\left({P}_{L,T}^{J/\psi}\right) $$ R P L , T J / ψ both within the Standard Model and in eight NP benchmark points. We find that the benchmark BP2 (corresponding to the hypothesis of tensor operator) has the greatest effect on all ℐi and $$ R\left({P}_{L,T}^{J/\psi}\right) $$ R P L , T J / ψ , except $$ {\hat{\mathrm{\mathcal{I}}}}_5 $$ ℐ ̂ 5 . The ratios $$ R\left({P}_{L,T}^{J/\psi}\right) $$ R P L , T J / ψ are more sensitive to the NP with pseudo-scalar operators than the ℐi. Finally, we discuss the symmetries in the angular observables and present a model-independent method to determine the existence of tensor operators.
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