# $\Lambda_b$ decays into $\Lambda_c^*\ell\bar{\nu}_\ell$ and $\Lambda_c^*\pi^-$ $[\Lambda_c^*=\Lambda_c(2595)$ \& $\Lambda_c(2625)]$ and heavy quark spin symmetry

## TL;DR

This paper analyzes $ ext{Lambda}_b$ decays into excited $ ext{Lambda}_c$ states using heavy quark spin symmetry, comparing theoretical predictions with experimental data, and discusses how future measurements could reveal the internal structure of these resonances.

## Contribution

It provides a theoretical framework linking heavy quark spin symmetry to decay ratios and explores the internal structure of $ ext{Lambda}_c(2595)$, including potential two-pole patterns and molecular components.

## Key findings

- Decay ratios agree with experimental data within errors.
- Future measurements can constrain the internal structure of $ ext{Lambda}_c(2595)$.
- Potential large systematic errors in lepton flavor universality ratios if multiple poles exist.

## Abstract

We study the implications for $\Lambda_b \to \Lambda_c^*\ell\bar{\nu}_\ell$ and $\Lambda_b \to \Lambda_c^*\pi^-$ $[\Lambda_c^*=\Lambda_c(2595)$ and $\Lambda_c(2625)]$ decays that can be deduced from heavy quark spin symmetry (HQSS). Identifying the odd parity $\Lambda_c(2595)$ and $\Lambda_c(2625)$ resonances as HQSS partners, with total angular momentum--parity $j_q^P=1^-$ for the light degrees of freedom, we find that the ratios $\Gamma(\Lambda_b\rightarrow\Lambda_c(2595)\pi^-)/\Gamma(\Lambda_b\rightarrow\Lambda_c(2625)\pi^-)$ and $\Gamma(\Lambda_b\rightarrow \Lambda_c(2595) \ell \bar{\nu}_\ell)/ \Gamma(\Lambda_b\rightarrow\Lambda_c(2625) \ell \bar{\nu}_\ell)$ agree, within errors, with the experimental values given in the Review of Particle Physics. We discuss how future, and more precise, measurements of the above branching fractions could be used to shed light into the inner HQSS structure of the narrow $\Lambda_c(2595)$ odd-parity resonance. Namely, we show that such studies would constrain the existence of a sizable $j^P_q=0^-$ component in its wave-function, and/or of a two-pole pattern, in analogy to the case of the similar $\Lambda(1405)$ resonance in the strange sector, as suggested by most of the approaches that describe the $\Lambda_c(2595)$ as a hadron molecule. We also investigate the lepton flavor universality ratios $R[\Lambda_c^*] = {\cal B}(\Lambda_b \to \Lambda_c^* \tau\,\bar\nu_\tau)/{\cal B}(\Lambda_b \to \Lambda_c^* \mu\,\bar\nu_\mu)$, and discuss how $R[\Lambda_c(2595)]$ may be affected by a new source of potentially large systematic errors if there are two $\Lambda_c(2595)$ poles.