$\Lambda_b\to p$ transition form factors in perturbative QCD

TL;DR

This paper improves the calculation of $\Lambda_b o p$ transition form factors in perturbative QCD by including higher-twist effects, resulting in predictions consistent with experimental and lattice data, and enabling systematic studies of heavy baryon decays.

Contribution

It introduces higher-twist light-cone distribution amplitudes into PQCD calculations of $\Lambda_b o p$ form factors, resolving previous discrepancies with data.

Findings

Higher-twist LCDAs dominate the form factors.

Enhanced form factors match lattice QCD and experimental data.

Predicted decay rates are close to observed values.

Abstract

We reanalyze the $\Lambda_b\to p$ transition form factors in the perturbative QCD (PQCD) approach by including higher-twist light-cone distribution amplitudes (LCDAs) of a $\Lambda_b$ baryon and a proton. The previous PQCD evaluation performed decades ago with only the leading-twist $\Lambda_b$ baryon and proton LCDAs gave the form factors, which are two orders of magnitude smaller than indicated by experimental data. We find that the twist-4 $\Lambda_b$ baryon LCDAs and the twist-4 and -5 proton LCDAs contribute dominantly, and the enhanced form factors become consistent with those from lattice QCD and other nonperturbative methods. The estimated branching ratios of the semileptonic decays $\Lambda_b\to p\ell\bar{\nu}_\ell$ and the hadronic decay $\Lambda_b\to p\pi$ are also close to the data. It implies that the $b$ quark mass is not really heavy enough, and higher-power contributions play a crucial role, similar to the observation made in analyses of $B$ meson transition form factors. With the formalism established in this work, we are ready to study various exclusive heavy baryon decays systematically in the PQCD approach.