The Leading Charged Particle Jet
|The above figure shows the average number of charged particles (PT > 0.5 GeV/c) within the leading charged particle jet as a function of of its transverse momentum, PTJ1. The solid points are Min-Bias data and the open points are the JET20 data. The JET20 data connect smoothly to the Min-Bias data and this allows us to study observables over the range 0.5 GeV/c < PTJ1 < 50 GeV/c. The data show a sharp rise in the leading charged jet multiplicity at low PTJ1 and then a more gradual rise at high PTJ1. The data are compared with the QCD Monte-Carlo model predictions of HERWIG, ISAJET, and PYTHIA.|
|Although charged particle jets are defined as circular regions in eta-phi space with radius R = 0.7, this is not necessarily the "size" of the jet. The size of a jet can be defined in many ways. Here we define the size of a jet in two ways, according to particle number or according to transverse momentum. The first corresponds to the radius in eta-phi space that contains 80% of the charged particles in the jet and the second corresponds to the radius in eta-phi space that contains 80% of the jet transverse momentum. The data on the average jet "size" of the leading charged particle jet are compared with the QCD Monte-Carlo model predictions of HERWIG, ISAJET, and PYTHIA in the above figure. A leading 20 GeV/c charged jet has 80% of its charged particles contained, on the average, within a radius in eta-phi space of about 0.33, and 80% of its transverse momentum contained, on the average, within a radius of about 0.20. The data clearly show the "hot core" of charged jets. The radius containing 80% of the transverse momentum is smaller than the radius that contains 80% of the particles. Furthermore, the radius containing 80% of the transverse momentum decreases as the overall transverse momentum of the jet increases due to limited momentum perpendicular to the jet direction.|
We can study the radial distribution of charged particles and transverse momentum within the
leading charged particle jet by examining the distribution of <Nchg> and <PTsum> as a
function of the distance in eta-phi space from the leading jet direction. The above figures
compare data on the radial particle distribution and the radial transverse momentum distribution
with the QCD Monte-Carlo model predictions.
For an average charged jet with PTJ1 > 5 GeV/c
(> 30 GeV/c), 80% of the jet PTsum lies within R = 0.36 (0.18). Note that because of the
nature of QCD fluctuations the average jet size is not exactly the same as the
size of an average jet. A given charged jet rarely looks like an
average charged jet and at low PTJ1 the average jet size is slightly smaller than the size of
an average jet.
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