Semileptonic decays of heavy Λ baryons in a quark model -: art. no. 035201

The semileptonic decays of Lambda(c) and Lambda(b) are treated in the framework of a constituent quark model. Both nonrelativistic and semirelativistic Hamiltonians are used to obtain the baryon wave functions from a fit to the spectra, and the wave functions are expanded in both the harmonic-oscillator and the Sturmian bases. The latter basis leads to form factors in which the kinematic dependence on q(2) is in the form of multipoles, and the resulting form factors fall faster as a function of q(2) in the available kinematic ranges. As a result, decay rates obtained in the two models with the Sturmian basis are significantly smaller than those obtained with the harmonic-oscillator basis. In the case of the Lambda(c), decay rates calculated with the Sturmian basis are closer to the experimentally reported rates. However, we find a semileptonic branching fraction for the Lambda(c) to decay to excited Lambda(*) states of 11 to 19%, in contradiction to what is assumed in available experimental analyses. Our prediction for the Lambda(b) semileptonic decays is that decays to the ground state Lambda(c) provide a little less than 70% of the total semileptonic decay rate. For the decays Lambda(b) -> Lambda(c), the analytic form factors we obtain satisfy the relations expected from heavy-quark effective theory at the nonrecoil point, at leading and next-to-leading orders in the heavy-quark expansion. In addition, some features of the heavy-quark limit are shown to naturally persist as the mass of the heavy quark in the daughter baryon is decreased.