CDF Run II Monte-Carlo tunes Page: 3 of 11
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and jet#3) as "leading jet" events. Events with at least two jets with PT > 15 GeV/c
where the leading two jets are nearly "back-to-back" ( A4 b2 > 1500) with
PT(jet#2)/PT(jet#1) > 0.8 and PT(jet#3) < 15 GeV/c are referred to as "back-to-back"
events. "Back-to-back" events are a subset of the "leading jet" events. The idea here
is to suppress hard initial and final-state radiation thus increasing the sensitivity of the
"transverse" region to the "beam-beam remnant" and the multiple parton scattering
component of the underlying event.
Fig. 3 compares the data on the density of charged particles and the charged PTsum
density in the "transverse" region for "leading jet" and "back-to-back" events with
PYTHIA Tune A (with multiple parton interactions) and HERWIG (without multiple
parton interactions). As expected, the "leading jet" and "back-to-back" events behave
quite differently. For the "leading jet" case the densities rise with increasing
PT(jet#1), while for the "back-to-back" case they fall slightly with increasing
PT(jet#1). The rise in the "leading jet" case is, of course, due to hard initial and final-
state radiation, which has been suppressed in the "back-to-back" events. The "back-
to-back" events allow for a more close look at the "beam-beam remnant" and multiple
parton scattering component of the "underlying event" and PYTHIA Tune A does a
better job describing the data than HERWIG. PYTHIA Tune A was determined by
fitting the CDF Run 1 "underlying event" data .
Lepton-Pair Production ,,eo
Lepton-Pair Production AionLePt. ITd.
I ilState R datinon
Unadrying Event Unadrying Event
Fig. 4. Illustration of the way QCD Monte-Carlo models simulate Drell-Yan lepton-pair production. The "hard
scattering" component of the event consists of the two outgoing leptons plus particles that result from initial-state
radiation. The "underlying event" consists of particles that arise from the "beam-beam remnants" and from
multiple parton interactions.
As illustrated in Fig. 4, Drell-Yan lepton-pair production provides an excellent
place to study the underlying event. Here one studies the outgoing charged particles
(excluding the lepton pair) as a function of the lepton-pair invariant mass. After
removing the lepton-pair everything else results from the beam-beam remnants,
multiple parton interactions, and initial-state radiation. Unlike high pT jet production
(Fig. 1) for lepton-pair production there is no final-state gluon radiation.
Fig. 5 shows that PYTHIA Tune A does not fit the CDF Run 1 Z-boson pT
distribution . PYTHIA Tune A was determined by fitting the Run 1 "underlying
event" data and, at that time, we did not consider the Z-boson data. PYTHIA Tune
AW fits the Z-boson pT distribution as well as the "underlying event" at the Tevatron
. PYTHIA Tune DW is very similar to Tune AW except PARP(67) = 2.5, which is
the preferred value determined by DO in fitting their dijet A$ distribution . It
determines the maximal parton virtuality allowed in time-like showers. HERWIG
does a fairly good job fitting the Z-boson pT distribution without additional tuning, but
does not fit the CDF "underlying event" data.
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Field, Rick & U., /Florida. CDF Run II Monte-Carlo tunes, article, November 1, 2006; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc889398/m1/3/: accessed September 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.