This thesis describes a measurement of the ΛB(5624) lifetime in the exclusive decay mode Λb → J/ψΛ, with J/ψ → μ +μ− and Λ → p +π−. This is the first measurement of the Λ B lifetime in a mode where the ΛB is fully reconstructed. As a control sample, we use the decay mode B0 → J/ψ, with J/ψ → μ+μ − and → π+π−.; We obtain 46 ± 9 ΛB and 256 ± 15 B0 candidate decays in 65 pb −1 of pp¯ collision data collected by the CDF II experiment at Fermilab. We obtain the value tLB =1.25±0.26stat.± 0.14syst.ps. ... |