Diquarks and the semileptonic decay of Λb in the hybrid scheme

In this work we use the heavy-quark-light-diquark picture to study the semileptonic decay Lambda(b)->Lambda(c)+l+(nu) over bar (l) in the so-called hybrid scheme. Namely, we apply the heavy quark effective theory (HQET) for larger q(2) (corresponding to small recoil), which is the invariant mass square of l+(nu) over bar, whereas the perturbative QCD approach for smaller q(2) to calculate the form factors. The turning point where we require the form factors derived in the two approaches to be connected, is chosen near rho(cut)=1.1. It is noted that the kinematic parameter rho which is usually adopted in the perturbative QCD approach, is in fact exactly the same as the recoil factor omega=v center dot v' used in HQET where v, v' are the four velocities of Lambda(b) and Lambda(c) respectively. We find that the final result is not very sensitive to the choice, so that it is relatively reliable. Moreover, we apply a proper numerical program within a small range around rho(cut) to make the connection sufficiently smooth and we parametrize the form factor by fitting the curve gained in the hybrid scheme. The expression and involved parameters can be compared with the ones gained by fitting the experimental data. In this scheme the end-point singularities do not appear at all. The calculated value is satisfactorily consistent with the data which is recently measured by the DELPHI collaboration within 2 standard deviations.