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Register a new userForm Factor Counting and HQET Matching for New Physics in $Lambda_b to Lambda_c^*l u$
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We calculate the $Lambda_b to Lambda_c^*(2595) l u$ and $Lambda_b to Lambda_c^*(2625) l u$ form factors and decay rates for all possible $b to c l bar u$ four-Fermi interactions in and beyond the Standard Model (SM), including nonzero charged lepton masses and terms up to order $mathcal{O}(alpha_s, 1/m_{c,b})$ in the heavy quark effective theory (HQET). We point out a subtlety involving the overcompleteness of the representation of the spin-parity $1/2^+ to 3/2^-$ antisymmetric tensor form factors, relevant also to other higher excited-state transitions, and present a general method for the counting of the physical form factors for any hadronic transition matrix element and their matching onto HQET. We perform a preliminary fit of a simple HQET-based parametrization of the $Lambda_b to Lambda_c^*$ form factors at $mathcal{O}(alpha_s, 1/m_{c,b})$ to an existing quark model, providing preliminary predictions for the lepton universality ratios $R(Lambda_c^*)$ beyond the SM. Finally, we examine the putative incompatibility of recent lattice QCD results with expectations from the heavy-quark expansion and available experimental data.
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We evaluate the partial decay widths for the semileptonic $Lambda_b to bar u_l l Lambda_c(2595)$ and $Lambda_b to bar u_l l Lambda_c(2625)$ decays from the perspective that these two $Lambda^*_c$ resonances are dynamically generated from the $DN$ and $D^*N$ interaction with coupled channels. We find that the ratio of the rates obtained for these two reactions is compatible with present experimental data and is very sensitive to the $D^* N$ coupling, which becomes essential to obtain agreement with experiment. Together with the results obtained for the $Lambda_b to pi^- Lambda^*_c$ reactions, it gives strong support to the molecular picture of the two $Lambda^*_c$ resonances and the important role of the $D^*N$ component neglected in prior studies of the $Lambda_c(2595)$ from the molecular perspective.
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 semileptonic decay of $Lambda_c$ to $ u l^+$ and $Lambda(1405)$, where the $Lambda(1405)$ is seen in the invariant mass distribution of $pi Sigma$. We perform the hadronization of the quarks produced in the reaction in order to have a meson baryon pair in the final state and then let these hadron pairs undergo final state interaction from where the $Lambda(1405)$ is dynamically generated. The reaction is particularly suited to study this resonance because we show that it filters I=0. It is also free of tree level $pi Sigma$ production, which leads to a clean signal of the resonance with no background. This same feature has as a consequence that one populates the state of the $Lambda(1405)$ with higher mass around 1420 MeV, predicted by the chiral unitary approach. We make absolute predictions for the invariant mass distributions and find them within measurable range in present facilities. The implementation of this reaction would allow us to gain insight into the existence of the predicted two $Lambda(1405)$ states and their nature as molecular states.
We present the first lattice-QCD calculation of the form factors governing the charm-baryon semileptonic decays $Lambda_c to Lambda^*(1520)ell^+ u_ell$. As in our previous calculation of the $Lambda_b to Lambda^*(1520)$ form factors, we work in the $Lambda^*(1520)$ rest frame, but here we use four different heavy-baryon momenta instead of just two. Because of the lower mass of the $Lambda_c$, the moderately-sized momenta used here are sufficient to determine the form factors in the full kinematic range of the semileptonic decay. We also update the analysis of our lattice results for the $Lambda_b to Lambda^*(1520)$ and $Lambda_b to Lambda_c^*(2595,2625)$ form factors by imposing exact relations among the different form factors at zero recoil that follow from rotational symmetry. Imposing these relations ensures the correct behavior of the angular observables near the endpoint.
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.
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