Physical Review D Aritcle |
Charged Jet Evolution and the Underlying Event in Proton-Antiproton CollisionsThe CDF CollaborationDecember 7, 2001 |
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| Note: Click on the figures to enlarge the figure and to access an encapsulated postscript (EPS) version of the figure and data files. | |||||||||
In a proton-antiproton collision a large transverse momentum outgoing parton manifests itself as a cluster of particles (both charged and neutral) traveling in roughly the same direction. These clusters are referred to as "jets". This analysis examines the charged particle component of "jets" in proton-antiproton collisions at 1.8 TeV. Using a simple jet algorithm, clusters of charged particles refered to as "charged particle jets" are studied. The transverse momentum of a "charged particle jet" is defined to be the scalar sum of the transverse momenta of the charged particles making up the jet. The properties of the leading (highest transverse momentum) "charged particle jet" are examined and compared with the QCD hard scattering Monte-Carlo models HERWIG, ISAJET, and PYTHIA. The method of comparing the QCD Monte-Carlo models with data is to select a region where the data is very clean so that corrections for experimental effects are small. For this reason, throughout this analysis only charged particles measured by the CDF central tracking chamber (CTC) in the region PT > 0.5 GeV/c and |eta| < 1, where the track finding efficiency is high and uniform, are considered. |
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| The above drawing illustrates of the way the QCD Monte-Carlo models simulate a proton-antiproton collision in which a hard 2-to-2 parton scattering with transverse momentum, PT(hard), has occurred. | |||||||||
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| The resulting event (shown above) contains particles that originate from the two outgoing partons (plus initial and final-state radiation) and particles that come from the breakup of the proton and antiproton ("beam-beam remnants"). The "underlying event" is everything except the two outgoing hard scattered "jets" and consists of the "beam-beam remnants" plus initial and final-state radiation. The "hard scattering" component consists of the outgoing two "jets" plus initial and final-state radiation. | |||||||||
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| The "beam-beam remnants" are what is left over after a parton is knocked out of each of the initial two beam hadrons. It is the reason hadron-hadron collisions are more "messy" than electron-positron annihilations and no one really knows how it should be modeled. In the QCD Monte-Carlo models the "beam-beam remnants" are an important component of the "underlying event". Also, it is possible that multiple parton scattering contributes to the "underlying event". The above drawing shows the way PYTHIA models the "underlying event" in proton-antiproton collision by including multiple parton interactions. In addition to the hard 2-to-2 parton-parton scattering and the "beam-beam remnants", sometimes there is a second "semi-hard" 2-to-2 parton-parton scattering that contributes particles to the "underlying event". | |||||||||
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| The above figure shows the average total number of charged particles in the event with PT > 0.5 GeV/c and |eta| < 1 (including chgjet1) as a function of the transverse momentum of the leading charged particle jet, PTJ1, for the Min-Bias and JET20 data. The JET20 data connect smoothly to the Min-Bias data and there is a small overlap region where the Min-Bias and JET20 data agree. There is sharp rise in the overall charged multiplicity at low PTJ1 and then a more gradual rise at high PTJ1. We now investigate where these charged particles are located relative to the direction of the leading charged particle jet. | |||||||||
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The above figures show the distribution of charged particles and the distribution of the scalar PTsum
of charged particles in the azimuthal angle  relative to the direction of the leading
charged particle jet. The kink in data around 40 degrees arises from the
choice of R = 0.7 in defining charged particle jets.
In the above figures the region || < 60 degrees
is labeled as the
"toward" region and the region || > 120 degrees
is labeled as the "away" region.
The "transverse" region is defined by
60 deg <|| <120 deg. The data show a rapid growth in
the "toward" and "away" region as PTJ1 increases since the "toward" region contains the
leading charged particle jet, while the "away" region, on the average, contains the away-side jet.
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The direction of the leading "charged particle jet" in each event is used to define three regions of
eta-phi space,
where eta is the pseudo-rapidity measured along the beam axis and
is the azimuthal angle relative to the
leading charged jet. As shown above "toward" region contains the
leading "charged particle jet",
while the "away" region, on the average, contains the away-side jet. The
"transverse" region is perpendicular to the
plane of the hard 2-to-2 scattering and is
very sensitive to the "underlying event" component of the QCD Monte-Carlo models.
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| The above figure shows the data on the average number of charged particles as a function of transvere momentum of the leading "charged particle jet", PTJ1, for the three regions of eta-phi space. Each point corresponds to the "toward", "transverse", or "away" <Nchg> in a 1 GeV/c bin. The solid points are Min-Bias data and the open points are JET20 data. The data define the average event "shape". | |||||||||
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| For example, for a proton-antiproton collider event at 1.8 TeV with PTJ1 = 20 GeV/c there are, on the average, 8.7 charged particles in the "toward" region (including the particles in chgjet1), 2.5 in the "transverse" region, and 4.9 in the "away" region, as illustrated in the above figure. Of course, in all three regions <Nchg> is forced to go to zero as PTJ1 goes to zero. If the leading charged particle jet has no particles then there are no charged particles anywhere. | |||||||||
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| The above figure shows the data on the average scalar PTsum of charged particles as a function of transvere momentum of the leading "charged particle jet", PTJ1, for the three regions of eta-phi space. Each point corresponds to the "toward", "transverse", or "away" <PTsum> in a 1 GeV/c bin. | |||||||||
The "transverse" region in the above figures is roughly normal to the plane of the 2-to-2 hard parton scattering and therefore contains, on the average, considerably fewer charged particles than the "toward" and "away" region. However, there is a lot more activity in the "transverse" region than one might naively expect. If we suppose that the "transverse" multiplicity is uniform in azimuthal angle and pseudo-rapidity, the observed 2.3 charged particles at PTJ1 = 20 GeV/c translates into 3.8 charged particles per unit pseudo-rapidity with PT > 0.5 GeV/c (multiply by 3 to get 360 degrees, divide by 2 for the two units of pseudo-rapidity covered in this analysis, multiply by 1.09 to correct for the track finding efficiency). We know from previous CDF studies that if we include all PT > 50 MeV/c that there are, on the average, about four charged particles per unit rapidity in a "soft" proton-antiproton collision at 1.8 TeV. The data above imply that in the "underlying event" of a hard scattering there are, on the average, about 3.8 charged particles per unit rapidity with PT > 0.5 GeV/c. Extrapolating to low PT assuming the form exp(-2PT) (which roughly fits the PT distribution in the "transverse region") implies that there are roughly 10 charged particles per unit pseudo-rapidity with PT > 0 in the "underlying event" (factor of e). Since we examine only those charged particles with PT > 0.5 GeV/c, we cannot accurately extrapolate to low PT = 0, however, it is clear that the "underlying event" in a hard scattering process has a charged particle density that is at least a factor of two larger than the four charged particles per unit rapidity observed in "soft" proton-antiproton collisions at this energy. |
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