Measurement of the Λb0 decay form factor

Abstract The form factor of Λ b 0 baryons is estimated using 3.46×10 6 hadronic Z decays collected by the DELPHI experiment between 1992 and 1995. Charmed Λ c + baryons fully reconstructed in the pK − π + , pK 0 S , and Λπ + π + π − modes, are associated to a lepton with opposite charge in order to select Λ b 0 →Λ c + l − ν l 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 ρ 2 =2.03±0.46( stat ) +0.72 −1.00 ( syst ). The exclusive semileptonic branching fraction Br(Λ b 0 →Λ c + l − ν l ) can be derived from ρ 2 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.