Observation of {Lambda}{sub b}{sup 0}{r_arrow}J/{psi}{Lambda} at the Fermilab proton-antiproton collider

The decay {Lambda}{sub b}{sup 0}{r_arrow}J/{psi}{Lambda} is observed in 110 pb{sup {minus}1} of p{bar p} collisions taken at {radical}(s)=1.8 TeV. These data are used to measure a {Lambda}{sub b}{sup 0} mass of 5621{plus_minus}4(stat){plus_minus}3(syst) MeV/c{sup 2}, and a mass difference between the {Lambda}{sub b}{sup 0} and the B{sup 0} of 340{plus_minus}5(stat){plus_minus}1(syst) MeV/c{sup 2}. The production cross-section times branching fraction for the decay {Lambda}{sub b}{sup 0}{r_arrow}J/{psi}{Lambda} relative to that for the decay B{sup 0}{r_arrow}J/{psi}K{sub S}{sup 0} has been measured to be 0.27{plus_minus}0.12(stat){plus_minus}0.05(syst). {copyright} {ital 1997} {ital The American Physical Society}