Buskulic, D (reprint author), INST NATL PHYS NUCL & PHYS PARTICULES, CNRS, PHYS PARTICULES LAB, F-74019 ANNECY LE VIEUX, FRANCE.
In a data sample of approximately four million hadronic Z decays recorded with the ALEPH detector from 1990 to 1995, a search for the strange b baryon Xi(b) is performed with a study of Xi-lepton correlations. Forty-four events with same sign Xi(-)l(-) combinations are found whereas 8.4 are expected based on the rate of opposite sign Xi(-)l(+) combinations. This significant excess is interpreted as evidence for Xi(b) semileptonic decays. The measured product branching ratio is Br(b-->Xi(b))xBr(Xi(b)-->X(c)Xl(-)<(nu)over bar (l)>)xBr(X(c)-->Xi(-)X')=(5.4+/-1.1(stat)+/-0.8(syst))x10(-4) per lepton species, averaged over electrons and muons, with X(c) a charmed baryon. The Xi(b) lifetime is measured to be tau(Xi b)=1.35(-0.28)(+0.37)(stat)(-0.17)(+0.15)(syst) ps.