Studying the underlying event at CDF Page: 4 of 4
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allowed in time-like showers. Tune DW and
Tune DWT are identical at 1.96 TeV, but Tune
DW and DWT extrapolate differently to the
LHC. Tune DWT uses the ATLAS energy
dependence, PARP(90) = 0.16, while Tune
DW uses the Tune A value of PARP(90) =
0.25. The ATLAS Tune is the default tune
currently used by ATLAS at the LHC. All the
tunes use the CTEQ5L structure functions.
"Transverse" Charged Particle Density: dN/d,1d$
4 eV
A 0.5
Ch~argedl Particles(n<1 0, PT>0 5 GeVlc)
0 250 500 750 1000 1250 1500 1750 2000
PT(particlejet#1) (GeV/c)
"Transverse" PTsum Density: dPT/drld
RDF Prelimnary PY n_ DW.
0- - -- - - PY-ATLAS --
6.0 - - -
L-d-yet (1,1<2)
-HE Charged Particles (n1<10, PT>0 s GeV )
0 250 500 750 1000 1250 1500 1750 2000
PT(particle jet4l) (GeV/c)
Fig. 4. Predictions at 14 TeV of PYTHIA Tune DW, Tune
DWT, HERWIG (without MPI), and the ATLAS Tune for
the density of charged particles, dN/dTdy (top), and the
charged PTsum density, dPT/ drdy (bottom), with PT > 0.5
GeV/c and I < 1 in the overall "transverse" region for
"leading jet" events as a function of the leading jet PT.
PHYTIA Tune A agrees fairly well with
the CDF Run 2 "underlying event" data for
"leading jet" events and Tune DW roughly
agrees with Tune A PYTHIA Tune A, Tune
DW, and the ATLAS PYTHIA Tune predict
about the same density of charged particles in
the "transverse" region with PT > 0.5 GeV/c for
"leading jet" events at the Tevatron. However,
the ATLAS Tune has a much softer PT
distribution of charged particles resulting in a
much smaller average PT per particles. Fig. 3
shows that the softer PT distribution of the
ATLAS Tune does not agree with the CDF
data.Fig. 4 shows the predictions of PYTHIA
Tune DW, Tune DWT, HERWIG (without
MPI), and the ATLAS Tune for the density of
charged particles and the PTsum density in the
"transverse" region for "leading jet" events at
the LHC. The PYTHIA Tunes (with MPI)
predict a large increase in the charged particle
density in going from the Tevatron to the LHC.
HERWIG (without MPI) does not increase as
much. At the LHC PYTHIA Tune DWT and
the ATLAS Tune both predict about the same
charged particle density with PT > 0.5 GeV/c,
however, the ATLAS Tune predicts a smaller
PTsum density than Tune DWT (i.e. the
ATLAS Tune produces a softer PT distribution
here as well).
In my opinion the best PYTHIA 6.2 tune
at present is Tune DW or DWT. These tunes
are identical at the 1.96 TeV and they do a
good job fitting the CDF Run 2 "underlying
event" data. More work will have to be done
in studying the "universality" of these tunes.
For example, we do not know if Tune DW will
correctly describe the underlying event in top
quark production.
References
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Field, R.D. Studying the underlying event at CDF, article, October 1, 2006; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc890467/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.