Subjet multiplicity of gluon and quark jets reconstructed with the k(perpendicular) algorithm in pp-bar collisions

The D0 Collaboration has studied for the first time the properties of hadron-collider jets reconstructed with a successive-combination algorithm based on relative transverse momenta (k(perpendicular)) of energy clusters. Using the standard value D=1.0 of the jet-separation parameter in the k(perpendicular) algorithm, we find that the pT of such jets is higher than the ET of matched jets reconstructed with cones of radius R=0.7, by about 5 (8) GeV at pT≅90 (240) GeV. To examine internal jet structure, the k(perpendicular) algorithm is applied within D=0.5 jets to resolve any subjets. The multiplicity of subjets in jet samples at √(s)=1800 GeV and 630 GeV is extracted separately for gluons (Mg) and quarks (Mq), and the ratio of average subjet multiplicities in gluon and quark jets is measured as (-1)/(-1)=1.84±0.15 (stat)±0.180.22 (syst). This ratio is in agreement with the expectations from the HERWIG Monte Carlo event generator and a resummation calculation, and with observations in e+e- annihilations, and is close to the naive prediction for the ratio of color charges of CA/CF=9/4=2.25