Exploring charmonia $η_c(6S,7S)$ in the $Λ_c^+\barΛ_c^-$ invariant mass spectrum of $B^0 \to K_S^0Λ_c^+\barΛ_c^-$

Abstract

We analyze the $B^0 \to K_S^0 Λ_c^+ \barΛ_c^-$ decay recently observed by the LHCb Collaboration. The $Λ_c^+\barΛ_c^-$ invariant-mass spectrum exhibits an accumulation near 4.63~GeV, which we investigate together with the accompanying $Λ_c^+ K_S^0$ spectrum constrained by the known $Ξ_c(2923)$ and $Ξ_c(2939)$ states. Our amplitude model includes a nonresonant term, the two established $Ξ_c$ contributions, and high-lying charmonium states in the $Λ_c^+\barΛ_c^-$ channel. Guided by screened Godfrey--Isgur and quark-pair-creation calculations, we test the 4.63~GeV enhancement with a fixed $η_c(6S)$ amplitude at $(4629.00, 24.20)$~MeV, {which improves the description of the 4.63~GeV region}, and examine a fixed $η_c(7S)$ contribution at $(4718.84, 22.84)$~MeV, which improves the higher-mass region of the spectrum. We also evaluate the $η_c(nS) \to Λ_c^+ \barΛ_c^-$ baryonic decays via a hadron-loop mechanism and estimate the $B^0 \to K_S^0 η_c(nS)$ production in naive factorization. Within the narrow width approximation, the fit yields a $\mathcal B(B^0\to K_S^0η_c(6S)) \mathcal B(η_c(6S)\toΛ_c^+\barΛ_c^-)$ of about $6.2 \times 10^{-6}$, while the factorization and hadron-loop calculations give rates roughly one to three orders of magnitude lower than the above fitted ratio. These results highlight the need for more precise data and a full amplitude analysis to determine the line shapes and clarify the nature of the higher-mass contribution.