Measurement of the $\Lambda^0_b$ decay form factor

The form factor of Λ0 b baryons is estimated using 3.46×106 hadronic Z decays collected by the DELPHI experiment between 1992 and 1995. Charmed Λ+c baryons fully reconstructed in the pK−π+, pK0 S, and Λπ+π+π− modes, are associated to a lepton with opposite charge in order to select Λ0 b →Λ+c l−¯νl decays. From a combined likelihood and event rate fit to the distribution of the Isgur–Wise variable w, and using the Heavy Quark Effective Theory (HQET), the slope of the b-baryon form factor is measured to be ˆ ρ2 = 2.03 ± 0.46(stat)+0.72−1.00(syst). The exclusive semileptonic branching fraction Br(Λ0 b →Λ+c l−¯νl ) can be derived from ˆ ρ2 and is found to be (5.0+1.1−0.8(stat)+1.6−1.2 (syst))%. Limits on other branching fractions are also obtained.