Bound on the curvature of the Isgur-Wise function of the baryon semileptonic decay Lambda_b -> Lambda_c + l + nu

Abstract

In the heavy quark limit of QCD, using the Operator Product Expansion, the formalism of Falk for hadrons or arbitrary spin, and the non-forward amplitude, as proposed by Uraltsev, we formulate sum rules involving the Isgur-Wise function $ξ_Λ (w)$ of the baryon transition $Λ_b \to Λ_c \ell \overlineν_{\ell}$, where the light cloud has $j^P=0^+$ for both initial and final baryons. We recover the lower bound for the slope $ρ_Λ^2 = - ξ'_Λ(1) \geq 0$ obtained by Isgur et al., and we generalize it by demonstrating that the IW function $ξ_Λ (w)$ is an alternate series in powers of $(w-1)$, i.e. $(-1)^n ξ_Λ^{(n)} (1) \geq 0$. Moreover, exploiting systematically the sum rules, we get an improved lower bound for the curvature in terms of the slope, $σ_Λ^2 = ξ"_Λ(1) \geq {3 \over 5} [ρ_Λ^2 + (ρ_Λ^2)^2]$. This bound constrains the shape of the Isgur-Wise function and it will be compelling in the analysis of future precise data on the differential rate of the baryon semileptonic decay $Λ_b \to Λ_c \ell \overlineν_{\ell}$, that has a large measured branching ratio, of about 5%.