Jet Reconstruction in Heavy Ion Collisions

Abstract

Measurements of strong suppression of inclusive hadron distributions and di-hadron correlations at high $p_{T}$, while providing evidence for partonic energy loss, also suffer from geometric biases due to the competition of energy loss and fragmentation. The measurements of fully reconstructed jets is expected to lack these biases as the energy flow is measured independently of the fragmentation details. In this article, we review the recent results from the heavy ion collisions collected by the STAR experiment at RHIC on direct jet reconstruction utilizing the modern sequential recombination and cone jet reconstruction algorithms together with their background subtraction techniques. In order to assess the jet reconstruction biases a comparison with the jet cross section measurement in $\sqrt{s}=200$ GeV p+p collisions scaled by the number of binary nucleon-nucleon collisions to account for nuclear geometric effects is performed. Comparison of the inclusive jet cross section obtained in central Au+Au events with that in $p+p$ collisions, published previously by STAR, suggests that unbiased jet reconstruction in the complex heavy ion environment indeed may be possible. Comment: Proceedings for the invited talk of the 25th Winter Workshop on Nuclear Dynamics 1-8 February 2009 Big Sky Montana

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arXiv:0905.1917v1 [nucl-ex] 12 May 2009
Proc. 25th Winter Workshop on
Nuclear Dynamics (2009) 000–000
25th Winter Workshop
on Nuclear Dynamics
Big Sky, Montana, USA
February 1–8, 2009
Jet Reconstruction in Heavy Ion Collisions
Sevil Salur1
1Lawrence Berkeley National Laboratory, 1 Cyclotron Road MS-70R0319,
Berkeley, CA 94720
Abstract. Measurements of strong suppression of inclusive hadron distribu-
tions and di-hadron correlations at high pT, while providing evidence for par-
tonic energy loss, also suffer from geometric biases due to the competition of
energy loss and fragmentation. The measurements of fully reconstructed jets
is expected to lack these biases as the energy flow is measured independently
of the fragmentation details. In this article, we review the recent results from
the heavy ion collisions collected by the STAR experiment at RHIC on di-
rect jet reconstruction utilizing the modern sequential recombination and cone
jet reconstruction algorithms together with their background subtraction tech-
niques. In order to assess the jet reconstruction biases a comparison with the
jet cross section measurement in√s = 200 GeV p+p collisions scaled by the
number of binary nucleon-nucleon collisions to account for nuclear geometric
effects is performed. Comparison of the inclusive jet cross section obtained
in central Au+Au events with that in p + p collisions, published previously
by STAR, suggests that unbiased jet reconstruction in the complex heavy ion
environment indeed may be possible.
Keywords: jet production, heavy ion collisions
PACS: 13.87.Ce, 25.75.Bh
1. Introduction
Jets are remnants of hard-scattered quarks and gluons and they are studied exten-
sively in high energy collisions of all kinds. Despite the naive expectations, jets are
not fundamental objects in Quantum Chromo Dynamics (QCD), but instead they
are artificial event properties defined by hand, i.e., well-defined and easy to measure
from the hadronic final-state, easy to calculate in pQCD from the partonic final-
state and closely related to the final-state quarks and gluons [ 1]. A well defined
jet definition allows us to study the fundamental objects of pQCD which are the
quarks and the gluons. But recently, many new areas outside the standard QCD in

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2 S. Salur
high energy physics are utilizing jet physics to answer major questions. For example
at RHIC, jets are put to a new use to study the hot QCD matter through their
interaction and energy loss in the medium (“jet quenching”).
Direct jet measurements in p+p collisions at RHIC have been carried out since
the third year of RHIC operations [ 2]. However until now, to avoid the complex
backgrounds of heavy ion events, inclusive hadron distributions and di-hadron cor-
relations at high transverse momentum are utilized and indirect measurements of jet
quenching have been made at RHIC. However, these measurements of jet fragmen-
tation particles are biased towards the population of jets that has the least inter-
action with the medium. Since 2006, the STAR barrel electromagnetic calorimeter
(BEMC) has been operated with full azimuthal coverage (φ) and large pseudora-
pidity (η) acceptance. This detector upgrade together with the increased beam
luminosities of RHIC and data recording capabilities of STAR, enables the study
of full jet reconstruction in heavy ion collisions for the first time at RHIC [ 3].
This article discusses results from a recent new approach of full jet reconstruction
measurement in heavy ion collisions, utilizing the high luminosity Au+Au data set
collected by the STAR experiment from 2007 RHIC run. The experimental details
can be found in [ 4] for the direct measurement of jets and [ 5, 6, 7] for the ac-
companying jet fragmentation studies in heavy ion collisions utilizing the STAR
experiment.
2. Jet Reconstruction Analysis
During the last 20 years, various jet reconstruction algorithms have been developed
for both leptonic and hadronic colliders. For a detailed overview of jet algorithms
in high energy collisions, see [ 4, 8] and the references therein. Here we will briefly
discuss the algorithms used for the STAR analysis. Two kinds of jet reconstruction
algorithms are utillized; seeded cone (leading order high seed cone (LOHSC)) and
sequential recombination (kTand Cambridge/Aachen).
The cone algorithm is based on the simple picture that a jet consists of a large
amount of hadronic energy in a small angular region. Therefore, the main method
for the cone algorithm is to combine particles in η − φ space with their neighbors
within a cone of radius R (R =
?∆φ2+ ∆η2). The sequential recombination algo-
rithms combine objects in relative to the closeness of their pT. Particles are merged
into a new cluster via successive pair-wise recombination. In the sequential recom-
bination algorithm, arbitrarily shaped jets are allowed to follow the energy flow
resulting in less bias on the reconstructed jet shape than with the cone algorithm
[ 9]. Algorithmic details of cone and sequential recombination can be found in [
10, 11, 12, 13] and the references therein.
Most recently a new approach to jet reconstruction and background subtraction,
motivated by the need of precision jet measurements in the search for new physics in
high luminosity p+p collisions at the LHC is developed by M. Cacciari, G. Salam and
G. Soyez [ 9, 14]. A key feature of their approach is a new QCD inspired algorithm

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Jet Reconstruction in Heavy Ion Collisions3
for separating jets from the large backgrounds due to pile up. As it turns out from
simulations, these improved techniques can also be used in heavy ion environments
where the background subtraction is essential for jet measurements. Sequential
recombination algorithms (kT, anti-kTand Cambridge/Aachen (CAMB)) encoded
in the FastJet suite of programs [ 9, 15], along with an alternative seeded cone
algorithm (labeled LOHSC) are utilized to search for jets in the Au+Au collisions.
A seedless infrared-safe cone algorithm (SISCone) [ 16] which is also available in
the FastJet suite of programs as a plug-in, is already used in p + p collisions at
√s = 200 GeV and the first results can be found in [ 7].
3. Results
Figure 1 shows an example of an identified di-jet event for central Au+Au collisions,
using both the neutral energy from the BEMC and charged particles from the Time
Projection Chamber of the STAR experiment. In order to assess the bias of the
heavy ion jet measurements, the inclusive jet cross section is compared to that from
p + p collisions presented in reference [ 2]. Figure 2 shows the comparison of the
inclusive jet spectrum for central Au+Au collisions (taken with a Minimum Bias
online trigger “MB-Trig” ) to the NBinscaled p+p spectrum, for the kTand CAMB
jet reconstruction algorithms. Jets in p + p collisions are measured the same way
as in Au+Au collisions, utilizing the STAR Time Projection Chamber and BEMC
and correcting for missing and double counted energy [ 2]. However for the p + p
case, a mid-point cone jet algorithm with splitting and merging steps is used. The
inclusive jet spectrum from p+p collisions agrees well with the Next-to-leading order
perturbative QCD calculation [ 17]. The same comparison for the jets reconstructed
with the LOHSC algorithm is presented in Figure 3. For both figures, to account
for nuclear geometric effects, the p + p spectrum is scaled by NBin, the number of
binary nucleon+nucleon collisions (NBin) equivalent to a central Au+Au collisions,
as calculated by a Glauber model [ 18].
In the case of jet reconstruction, NBinscaling is expected if the reconstruction
is unbiased, i.e. the jet energy is recovered independent of the fragmentation, even
in the presence of strong jet quenching. This scaling is analogous to the cross
section scaling of high pT direct photon production in heavy ion collisions, observed
by the PHENIX experiment [ 19].At present, the total systematic systematic
uncertainty on the normalisation of the inclusive p+p jet spectrum is around 50%.
Figure 2 and Figure 3 show that the heavy ion jet spectrum agrees well with the
scaled p + p measurement within the systematic uncertainty. Figure 4 is for the
jet spectra obtained with the kTalgorithm by using different threshold cuts on the
track momenta and calorimeter tower energies (pcut
between the binary scaled p + p spectra and the Au+Au measurement is worse for
larger pcut
T. This suggests that the threshold cuts introduce biases which are not
fully corrected with the current procedure that uses fragmentation models that are
developed for e++e−and p+p collisions. It could also be an indication of modified
T). It is found that the agreement

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4 S. Salur
Fig. 1. 21 GeV di-jet reconstructed from a central Au+Au event at√sNN= 200
GeV in the STAR detector [ 4, 5].
Fig. 2. Jet yield per event vs transverse jet energy (ET) for the central Au+Au
collisions obtained by the sequential recombination (kT, CAMB) algorithms [ 2, 4].
Triangle symbols are from MB-Trig and corrected for efficiency, acceptance and
energy resolution. Only statistical error bars are shown for the Au+Au data. Solid
black squares are the distribution from p + p collisions, scaled by NBinary. The
systematic uncertainty of the p + p jet spectra normalization is ∼ 50%.
fragmentation due to jet quenching.

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Jet Reconstruction in Heavy Ion Collisions5
Fig. 3. Jet yield per event vs transverse jet energy (ET) for the central Au+Au
collisions obtained by the Leading Order High Seed Cone (LOHSC) algorithm [
2, 4]. Triangle symbols are from MB-Trig and corrected for efficiency, acceptance
and energy resolution. Only statistical error bars are shown for the Au + Au data.
Solid black squares are the distribution from p + p collisions, scaled by NBinary.
The systematic uncertainty of the p + p jet spectra normalization is ∼ 50%.
4. Conclusions
Unbiased reconstruction of jets in central heavy ion collisions at RHIC energies
would be a breakthrough to investigate the properties of the matter produced at
RHIC. The study shown here indicates that unbiased reconstruction of jets may be
possible in heavy ion events. However, spectrum corrections are currently based on
model calculations using PYTHIA fragmentation [ 20]. This aspect, together with
the spectrum variations due to cuts and reconstruction algorithms, must be inves-
tigated further in order to assess the systematic uncertainties of this measurement.
Further, we utilize the reconstructed jets and study the jet shapes to test the
underlying QCD theory [ 21, 22]. The results from the intra-jet energy distributions,
jet-jet and hadron-jet correlation studies will be available in the coming months and
will enable us to study the medium properties produced at RHIC. Also a copious
production of very energetic jets, well above the heavy ion background is predicted
to occur at the LHC [ 23, 24]. The large kinematic reach of high luminosity running
at RHIC and at the LHC may provide sufficient lever-arm to map out the QCD
evolution of jet quenching. The comparison of full jet measurements in the different
physical systems generated at RHIC and the LHC will provide unique and crucial
insights into our understanding of jet quenching and the nature of hot QCD matter.