 |
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.
(Return to the Introduction) |
|