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Jet Evolution and the "Underlying Event" in Run 2

Rick Field

April 1, 2003

Note: Click on the figures to enlarge the figure and to access an encapsulated postscript (EPS) version of the figure.

In this analysis the behavior of the "underlying event" in hard scattering proton-antiproton collisions at 1.96 TeV is studied and compared with PYTHIA Tune A and with the CDF published Run 1 analysis. The "underlying event" is studied using both "charged particle jets" (as was done in our Run 1 analysis) and using "calorimeter jets".
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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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Of course, from a certain point of view there is no such thing as an "underlying event" in a proton-antiproton collision. There is only an "event" and one cannot say where a given particle in the event originated. On the other hand, hard scattering collider "jet" events have a distinct topology. On the average, the outgoing hadrons "remember" the underlying the 2-to-2 hard scattering subprocess. An average hard scattering event consists of a collection (or burst) of hadrons traveling roughly in the direction of the initial beam particles and two collections of hadrons (i.e. "jets") with large transverse momentum. The two large transverse momentum "jets" are roughly back to back in azimuthal angle. One can use the topological structure of hadron-hadron collisions to study the "underlying event". In this analysis, the direction of the leading "charged particle jet" in each event or the direction of the leading "calorimeter jet" (JetClu, R = 0.7) 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 the "toward" region contains the leading "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".
In this analysis we will study the "underlying event" in the Run 2 Min-Bias and jet trigger data samples using both "charged particle jets" and "calorimeter jets" to define the "transverse" region. The goals of this analysis are the following:
  • Examine the evolution of "charged particle jets" and the "underlying event" in Run 2 and compare with the published Run 1 analysis. This involves studying charged particle correlations relative to the leading "charged particle jet" direction.
  • Examine the evolution of "calorimeter jets" and the "underlying event". This involves studying charged particle correlations relative to the leading "calorimeter jet" direction.
  • Examine correlations between the leading "charged particle jet" and the "calorimeter jets".
  • Examine the characteristics of the leading "charged particle jets" and the leading "calorimeter jets". This involves studying the charged particles within the leading "charged particle jets" and "calorimeter jets".

The Transverse Region Click to see more about the "transverse" region and the "underlying event" as defined by the leading "charged particle jet".
The Transverse Region Click to see more about the "transverse" region and the "underlying event" as defined by the leading "calorimeter jet".
Charged Particle Jets vs Calorimeter Jets Click to see more about the relationship between the leading "charged particle jet" and "calorimeter jets".
The Leading Jet Click to see more about the characteristics of the leading "charged particle jet" and the leading "calorimeter jet".

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