The $\Lambda_b\to pl{\bar\nu}$ decay in perturbative QCD

We develop perturbative QCD factorization theorems for the exclusive semileptonic heavy baryon decay $\Lambda_b \to pl{\bar\nu}$, in which the relevant hadronic form factor is expressed as the convolution of a hard subamplitude with the $\Lambda_b$ baryon wave function and the proton wave function. The specific evolution scale for the proton wave function, determined from the best fit to the data of the proton form factor, is adopted. The Sudakov resummation for a heavy-light system (the $\Lambda_b$ baryon) and the quark-level decay diagrams with at least one hard gluon attaching the $b$ quark, which were ignored in the literature, are incorporated. It turns out that these additional ingredients are important: the neglect of the former and the latter reduces the results of the form factor by factors 1.5 and 3, respectively. We present the predictions of the form factor and of the proton energy spectrum for the various choices of the parameter involved in the $\Lambda_b$ baryon wave function.