QUARK AND GLUON JET DIFFERENCES FROM SYMMETRIC Z° EVENTS

摘要

Symmetric three-jet events are selected from hadionic Z° decays such that the two lower energy jets are each produced at an angle of about 150° with respect to the highest energy jet. In some cases, a displaced secondary vertex is re-constructed in one of the two lower energy jets, which permits the other lower energy jet to be identified as a gluon jet through anti-tagging with about 93% purity. Comparing these gluon jets to the inclusive sample of lower energy jets from the symmetric data set yields direct, model independent measurements of quark and gluon jet properties, for which the quark jets have a normal mix-ture of flavors for Z° decays. The energies of the jets being studied are about 24 GeV. Clear differences are observed between the properties of the quark and gluon jets. In other cases, the highest energy jet is tagged as a b jet or as a light quark (uds) jet using secondary vertex or track impact parameter and momen-tum information. Comparing the two lower energy jets of the events with a tag in the highest energy jet to the anti-tagged gluon jets yields a direct comparison of b, uds and gluon jets. The b and gluon jets are observed to have similar properties. The uds and gluon jets exhibit large differences. The measurements are well described by QCD Monte Carlo event generator programs.