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Measurement
of the Multiplicity Distribution of Charged Particles in pp(bar)
Collisions
at 1960 GeV |
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Authors:
Niccolo'
Moggi, Manuel Mussini, Franco Rimondi
University
and I.N.F.N., Bologna |
Abstract:
The multiplicity distribution of charged particles is measured in inelastic
nondiffractive pp(bar) collisions at 1.96 TeV. The data were collected using the
Collider Detector at Fermilab (CDF) experiment during RunII of the Fermilab
Tevatron collider. This analysis is part of a systematic and detailed set of measurements
of minimum-biased events. The data presented here have the highest precision and the
largest range extension ever reached in the pseudorapidity range |eta|<=1.
The
Analysis.
The multiplicity distribution in inelastic hadron interactions has
always been a complicated puzzle. It is probably the result of an interferring
superposition of various particle production effects that cannot be isolated.
A number of models have been proposed to describe this distribution, but none has
succesfully survived to more precise measures and/or to the advent of data from higher c.m.
energies. Also, effects due to multiple parton-parton interactions must be accounted for.
The data presented here have the highest precision and the largest
range
extension ever reached in the pseudorapidity range || <1. This makes of the
new measurement an important handle for the tuning of MC models and a base line for
their extrapolation to LHC energies. Likewise, it will contribute to more precise estimates of soft QCD background in high-pT measurements.
In
principle the measure is very simple and consists in counting the
charged particles observed in unbiased inelastic events.
This measure is based on a sample of 506pb-1 collected with a
"minimum-bias" trigger. To compensate for the vanishing cross section
in the high multiplicity
region of the spectrum, data from a special trigger are also employed
in this region.
The
data are corrected down to hadron level for the effects of
the trigger acceptance, the background of diffractive
collisions
and for tracking efficiency. The final state phase space region for
this measure is
limited to pT>0.4GeV/c
and || <1, where the effects of detector distorsions and
acceptance are minimal.
A
comparison with a Pythia simulation MonteCarlo (v6.2) shows
that
more efforts are needed to correctly reproduce the data.
Data numerical values may be
found here.
Blessed plots:
Probability P(Nch)
of observing a collision with Nch charged
particles in the final state.
The yellow
band represent the systematic uncertainty.
The
definition of primary particles was to consider all particles with mean
lifetime >0.3x10 e-10 s produced promptly in the pp interaction,
and
the decay products of those with shorter mean lifetimes. With this
definition strange hadrons are included among the primary particles,
and
those that are not reconstructed are corrected for. On the other hand,
their decay products (mainly pions from K0S
decays) are excluded, while those from heavier flavor hadrons are
included.
The average multiplicity is
4.510.02(stat.)0.2(syst.)
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| Statistical,
systematic and total uncertainties as a function of the multiplicity Nch |
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Comparison
with Pythia MonteCarlo Simulation.
The
CDF detector is fully simulated.
The
differences are better seen in the ratio of the two distributions
(below).
The Pythia MonteCarlo generator was
tuned with R.Field's "tuneA".
This tune was obtained from CDF underlying-event data and was shown to
be likely the best in reproducing minimum-bias data.
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Other
observables measured in the same data sample were published in
Phys. Rev. D 79, 112005 (2009) (arXiv:0904.1098
[hep-ex]).
| send comments to
moggi(at)bo.infn.it or rimondi(at)bo.infn.it |
Last updated 14 Sept. 2009, Niccolo' Moggi