$Λ_b$ decays into $Λ_c^*\ell\barν_\ell$ and $Λ_c^*π^-$ $[Λ_c^*=Λ_c(2595)$ \& $Λ_c(2625)]$ and heavy quark spin symmetry

Abstract

We study the implications for $Λ_b \to Λ_c^*\ell\barν_\ell$ and $Λ_b \to Λ_c^*π^-$ $[Λ_c^*=Λ_c(2595)$ and $Λ_c(2625)]$ decays that can be deduced from heavy quark spin symmetry (HQSS). Identifying the odd parity $Λ_c(2595)$ and $Λ_c(2625)$ resonances as HQSS partners, with total angular momentum--parity $j_q^P=1^-$ for the light degrees of freedom, we find that the ratios $Γ(Λ_b\rightarrowΛ_c(2595)π^-)/Γ(Λ_b\rightarrowΛ_c(2625)π^-)$ and $Γ(Λ_b\rightarrow Λ_c(2595) \ell \barν_\ell)/ Γ(Λ_b\rightarrowΛ_c(2625) \ell \barν_\ell)$ 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 $Λ_c(2595)$ 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 $Λ(1405)$ resonance in the strange sector, as suggested by most of the approaches that describe the $Λ_c(2595)$ as a hadron molecule. We also investigate the lepton flavor universality ratios $R[Λ_c^*] = {\cal B}(Λ_b \to Λ_c^* τ\,\barν_τ)/{\cal B}(Λ_b \to Λ_c^* μ\,\barν_μ)$, and discuss how $R[Λ_c(2595)]$ may be affected by a new source of potentially large systematic errors if there are two $Λ_c(2595)$ poles.