Form Factor Counting and HQET Matching for New Physics in $\Lambda_b \to \Lambda_c^*l\nu$

TL;DR

This paper computes form factors and decay rates for $to c$ transitions involving excited $ Lambdac^*$ states, incorporating HQET corrections, and explores implications for new physics and lepton universality.

Contribution

It introduces a general method for counting and matching physical form factors in hadronic transitions within HQET, addressing a subtlety in higher excited states.

Findings

Preliminary HQET-based predictions for $R( Lambdac^*)$ beyond the SM.

Identification of a subtlety in form factor representation for $1/2^+ o 3/2^-$ transitions.

Analysis of potential conflicts between lattice QCD results and heavy-quark expansion expectations.

Abstract

We calculate the $\Lambda_b \to \Lambda_c^*(2595) l \nu$ and $\Lambda_b \to \Lambda_c^*(2625) l \nu$ form factors and decay rates for all possible $b \to c l \bar\nu$ 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 possible incompatibility of recent lattice QCD results with expectations from the heavy-quark expansion and available experimental data.