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CDF Note 10874

The Energy Dependence of Min-Bias and the Underlying Event at CDF

Rick Field, Craig Group, and David Wilson
for the CDF Collaboration

Updated November 4, 2012

We study charged particles production in proton-antiproton collisions at 300 GeV, 600 GeV, and 1.96 TeV. The 300 GeV and 900 GeV data are a result of the "Tevatron Energy Scan" which was performed just before the Tevatron was shut down. Min-bias (MB) is a generic term which refers to events that are selected with a "loose" trigger that accepts a large fraction of the overall inelastic cross section. All triggers produce some bias and the term “min-bias” is meaningless until one specifies the precise trigger used to collect the data. The CDF MB trigger requires at least one charged particle in the forward region 3.2 < eta < 5.9 and simultaneously at least one charged particle in the backward region -5.9 < eta < -3.2. The underlying event (UE) consists of the beam-beam remnants (BBR) and the multiple parton interactions (MPI) that accompany a hard scattering. The UE is an unavoidable background to hard-scattering collider events. MB and UE are not the same object! The majority of MB collisions are "soft" while the UE is studied in events in which a hard-scattering has occurred.
Hard Scattering Process
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. The resulting event 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.
Mulitple Parton Interactions
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 are additional "semi-hard" 2-to-2 parton-parton scatterings that contribute particles to the underlying event.
PTmax Direction PTmax Direction Eta-Phi Space
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, PTmax, 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 usually 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 component of the hard scattering. In this analysis we study MB and the underlying event at 300 GeV, 600 GeV, and 1.96 TeV using data collected by the CDF MB trigger. The goal is to study the energy dependence MB and the UE. The data are corrected to the particle level with errors that include both the statistical error and the systematic uncertainty and are compared with PYTHIA 6.4 Tune Z1 at the generator level.

Click on the icons below to see the data and the QCD Monte-Carlo comparisons.

Min-Bias Collisions Click icon to see results for Min-Bias collisions at 300 GeV, 900 GeV, and 1.96 TeV.
The Transverse Region Click icon to see results for the "transverse" region as defined by the leading charged particle, PTmax, at 300 GeV, 900 GeV, and 1.96 TeV.
Transverse Ratios Click icon to see the "transverse" region center-of-mass energy ratios.
Tune Z1 Comparisons Click icon to see comparisons with PYTHIA 6.4 Tune Z1.

Rick Field for the CDF Collaboration