Study of the subjet structure of quark and gluon jets

Abstract Measurements of the subjet structure of quark and gluon jets in hadronic Z decays are presented. The analysis is based on one million hadronic events recorded by the Alephdetector. Roughly symmetric three-jet eventsare selected with a coarse jet-resolution cut-off, y 1 . Gluon jets are identified with a purity of 94.6% in those events where evidence of long-lived heavy-flavour hadrons in the other two jets is found. The jets are then analyzed using a smaller cut-off y 0 ( y 1 ) so that subjets are resolved. The properties of the jets (subjet multiplicities ( N q ), ( N g ) and rates R n g ( q ) for n = 1, 2, 3, 4) are determined and are found to be in good agreement with the expectations of perturbative QCDas long as the subjet resolution parameter y 0 is sufficiently large to keep non-perturbative effects small. In particular, the ratio (N g − 1) (N q − 1) , which to leading order in QCD is given by the ratio of colour factors C A C F = 9 4 , is measured to be 1.96 ± 0.15 for y 0 = 2 · 10 −3 , but falls to 1.29 ± 0.03 for y 0 = 1.6 · 10 −5 .