CDF Note 6403
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Jet Evolution and the "Underlying Event"
in Run 2
Rick Field
April 1, 2003
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Note: Click on the figures to enlarge the figure and to access an encapsulated postscript (EPS) version of the
figure.
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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".
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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".
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Click to see more about the "transverse" region and the "underlying event" as defined by the
leading "charged particle jet". |
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Click to see more about the "transverse" region and the "underlying event" as defined by the
leading "calorimeter jet". |
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Click to see more about the relationship between the leading "charged particle jet" and
"calorimeter jets". |
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Click to see more about the characteristics of the leading "charged particle jet" and the leading "calorimeter jet". |
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