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Jet-hadron correlations in pp and Pb-Pb collisions with ALICE

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Megan Connors
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Jet quenching has been observed at both RHIC and LHC energies, indicating that partons lose energy as they traverse the medium. To probe the effects of this partonic energy loss, measurements of the angular correlations between fully reconstructed jets and charged tracks in Pb-Pb collisions are studied. Fully reconstructing a jet provides access to the kinematics of the initial hard scattering while allowing us to study the distribution of hadrons on the away side from the modified recoil jet. Here we present first measurements of jet-hadron correlations in pp collisions at =2.76 TeV and an outlook for Pb-Pb collisions. The jets in this analysis are reconstructed from the 2011 data set using both charged tracks and neutral energy measured in the ALICE tracking system and the electromagnetic calorimeter respectively.
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2013 J. Phys.: Conf. Ser. 446 012009
(http://iopscience.iop.org/1742-6596/446/1/012009)
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Jet-Hadron Correlations in pp and Pb-Pb Collisions
with ALICE
Megan Connors
Yale University, New Haven, CT 06520
E-mail: Megan.Connors@Yale.edu
Abstract. Jet quenching has been observed at both RHIC and LHC energies, indicating that
partons lose energy as they traverse the medium. To probe the e↵ects of this partonic energy loss,
measurements of the angular correlations between fully reconstructed jets and charged tracks in
Pb-Pb collisions are studied. Fully reconstructing a jet provides access to the kinematics of the
initial hard scattering while allowing us to study the distribution of hadrons on the away side
from the modified recoil jet. Here we present first measurements of jet-hadron correlations in
pp collisions at ps=2.76 TeV and an outlook for Pb-Pb collisions. The jets in this analysis are
reconstructed from the 2011 data set using both charged tracks and neutral energy measured
in the ALICE tracking system and the electromagnetic calorimeter respectively.
1. Introduction
Jets are the result of a hard scattering process in the initial phase of the collision. In heavy ion
collisions the partons from the hard scattering process are modified by the presence of the quark-
gluon plasma (QGP). This modification has been observed as a suppression of high-momentum
particles at both RHIC and LHC energies [1, 2, 3, 4]. This suppression has also been observed
for di-jets via two-particle correlations. A high-momentum particle is used as a trigger particle
and is paired with all other particles in the event, referred to as associated particles. In pp
collisions, the distribution of associated particles shows a peak on the near side at'= 0 and
on the away side at'=⇡,where'is the azimuthal angle between the two particles. The
near-side peak results from the jet particles associated with the same jet as the trigger particle.
The away-side jet peak results from the opposing jet created in the hard scattering. In heavy ion
collsions the away-side jet peak is suppressed [5, 6]. In addition to the suppression on the away
side, modifications to the shape have also been observed on both the near and away side. In this
analysis we reconstruct the jet and use the reconstructed jet axis to measure'distributions
with associated charged tracks, instead of using a single trigger particle as a proxy for the jet.
Fully reconstructed jets are very versatile triggers because there are several variables
associated with jet reconstruction that can be adjusted. In principle, by varying the di↵erent
cuts such as minimum constituent pT, we can vary how surface biased the trigger jet is and
thereby adjust the pathlength traversed by the opposing jet. This would allow us to map out
energy loss as a function of path length and do true jet tomography. However, lowering the
constituent cut to probe deeper into the medium also introduces more background from the
underlying event which would need to be handled with care. By fully reconstructing the jet
close to the QGP surface, we can better approximate the initial energy of the parton on the
Hot Quarks 2012 IOP Publishing
Journal of Physics: Conference Series 446 (2013) 012009 doi:10.1088/1742-6596/446/1/012009
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opposing side. While biasing the jet used as the trigger in our correlations to have a shorter
pathlength through the medium, we also bias its fragmentation but the opposing jet is not biased
by these cuts.
Jet-hadron correlations have been studied by the STAR collaboration at RHIC in Au+Au
collisions at 200 GeV [7]. The results show that the suppression of pairs at high pTis
compensated by the production of additional low momentum particles. An apparent increase in
the away-side width in the heavy ion environment compared to pp collsions was also observed
but the significance of this di↵erence is limited by the uncertainties due to the ambiguity in the
jet vn. Only very recently, ATLAS and STAR presented measurements of the jet v2for central
collisions at 2.76 TeV and 200 GeV respectively [8, 9]. The impact of these measurements on
the jet-hadron correlations is still being explored.
Making similar measurements at the LHC is important to help constrain theories which can
teach us about how the partons lose energy in the QGP. These proceedings present a measure
of the baseline jet-hadron correlations from pp data at 2.76 TeV and discuss the status of the
jet-hadron measurement in PbPb collisons at ALICE.
2. The Data Sample
The data used in this analysis were collected by ALICE during the 2.76 TeV pp and 2.76 Pb-
Pb LHC runs in 2011. The pp data include events that were triggered by using the ALICE
electromagnetic calorimeter (EMCal). If a shower contained at least 3 GeV of energy, the event
was accepted by the EMCal trigger. The statistics for the jet analysis is enhanced by using
the triggered data sample. However, tracks from the minimum bias events are used for event
mixing to exclude the correlations due to the EMCal trigger. The Pb-Pb analysis shown in these
proceedings use a combination of the minimum bias, central and semi central triggered events.
This results in a relatively flat centrality distribution for the 0-10% most central events.
3. Jet Reconstruction
Jets are reconstructed using both clusters in the EMCal and charged particles reconstructed in
the ALICE tracking system. Contribution from the deposition of energy by charged tracks in
the EMCal has been removed from the clusters by subtracting up to 100% of the momentum of
the tracks matched to a given cluster. If the sum of the momentum of matched tracks is greater
than the energy in the cluster, the cluster is simply excluded. We use the anti-kTalgorithm
with a resolution parameter, R=0.4, from the FastJet package [10]. Only jets which are
fully contained within the area of the EMCal are considered. To avoid contributions from the
underlying event in heavy ion collisions, we require the jets to have an area A>0.4 [11] and
use a minimum constituent cut of 3 GeV/cfor both clusters and tracks. The constituent cut
also helps bias the trigger jet closer to the surface [12]. Note that a 3 GeV/ccut for jets in the
range 20-60 GeV has a more significant e↵ect on the surface bias than a 1 GeV/ccut has on
a 100 GeV jet as shown in [13]. To increase the surface bias and reject more background, we
require that the leading particle in the jet satifies a pT>6 GeV/ccut.
4. Correlations Measurement
4.1. pp
A correlation function is defined as the number of same-event pairs over the number of
mixed-event pairs where the mixed-event pair distribution is normalized such that the bin at
⌘== 0 is 1. For two-particle correlations the yield is typically expressed as number of
pairs per trigger, Ntrig , as in Eqn. 1, where ✏is the efficiency of the associated particles.
Hot Quarks 2012 IOP Publishing
Journal of Physics: Conference Series 446 (2013) 012009 doi:10.1088/1742-6596/446/1/012009
2
1
Ntrig
dN
d'=1
✏Ntrig
dNsame
pairs
d'
dNmixed
pairs
d'
(1)
Since the ALICE tracking detectors have full azimuthal coverage, the mixed-event pairs are
approximately flat as a function of'while the finite ⌘acceptance causes a drop towards
large⌘. However, for jet-hadron correlations the⌘dependence is rather flat in the range
|⌘|<0.4 since the range in ⌘for the jets is more restricted than the ⌘range of the associated
tracks. To remove the contribution of the underlying event from the correlation function, a flat
pedestal is subtracted. The resulting jet functions for three di↵erent associated momentum bins
are shown in Fig. 1. The pedestal is determined from a fit which consists of two Gaussians plus
the flat pedestal. The uncertainty on the pedestal subtraction is shown as a gray band around
zero. There is an overall uncertainty of 7% on the normalization due to the uncertainty on the
efficiency correction.
Figure 1. Jet-hadron'correlations from pp data. The underlying event has been subtracted.
The measured jet-hadron correlations qualitatively agree with expectations. The peaks of the
near-side and away-side jets are clearly observed. The away-side peak appears broader and is
lower than the near-side peak due to the e↵ects of kT, the intristic tranverse momentum [14, 15],
which causes the two jets to not be directly back to back. Since we limit the correlations to be
within |⌘|<0.4, we do not capture all the jet particles.
4.2. Pb-Pb
In heavy ion collisions, the underlying event in the correlation functions is modulated by flow.
To measure the e↵ects of the QGP on the properties of the jet, we must remove the contribution
to the correlation function from flow as in Eqn. 2. This requires a measure or an assumption of
jet vn.
1
Ntrig
dN
d'=1
✏Ntrig
dNsame
pairs
d'
dNmixed
pairs
d'
b0(1 +⌃vtrig
nvassoc
ncos(n')) (2)
A first look at the jet-hadron correlations from the Pb-Pb data is shown in Fig. 2 as a
function of'and⌘. Correlations due to flow have not been removed and the normalization
scale has not been corrected for efficiency. There is a clear near-side jet peaked observed near
'=⌘= 0 but nothing more about the jets can be concluded until the flow is removed. Once
this is done, one can compare the shapes and yields of the distributions observed in Pb-Pb to
pp to study any potential modication due to the QGP.
Quantifying the modification of the jet-hadron correlations at LHC energies, will provide
additional constraints on energy loss models and will lead to a better understanding of the
energy loss mechanisms and properties of the QGP.
Hot Quarks 2012 IOP Publishing
Journal of Physics: Conference Series 446 (2013) 012009 doi:10.1088/1742-6596/446/1/012009
3
(rad.)
ϕ
∆
-1 01234
η∆
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
)
-1
(rad.η∆/dϕ∆dN/d
trig
1/N
4
5
6
7
8
= 2.76 TeV 0-10%
NN
s
PbPb at
Uncorrected Correlation Function
10/10/2012
<60 GeV/c
ch+em
T,j et
20<p
<3 GeV/c
assoc
T,t rac k
2<p
Figure 2. Jet-hadron⌘'correlations from 0-10% most central Pb-Pb collisions at 2.76
TeV. Contributions from flow are not removed and efficiency corrections have not been applied.
5. References
[1] J. Adams et al. [STAR Collaboration], Phys. Rev. Lett. 91, 172302 (2003)
[2] K. Adcox et al. [PHENIX Collaboration], Phys. Lett. B 561, 82 (2003)
[3] K. Aamodt et al. [ALICE Collaboration], Phys. Lett. B 696, 30 (2011)
[4] S. Chatrchyan et al. [CMS Collaboration], Eur. Phys. J. C 72, 1945 (2012)
[5] K. Aamodt et al. Phys. Rev. Lett. 108, 092301 (2012)
[6] C. Adler et al. [STAR Collaboration], Phys. Rev. Lett. 90, 082302 (2003)
[7] Alice Ohlson (for the STAR collaboration) Quark Matter 2011 Proceedings arXiv:1106.6243
[8] Alice Ohlson (for the STAR collaboration) Quark Matter 2011 Proceedings
[9] ATLAS Collaboration ATLAS-CONF-2012-116
[10] M. Cacciari, G. P. Salam and G. Soyez, JHEP 0804, 063 (2008)
[11] Peter Jacobs, Nucl. Phys. A855, 299 (2011).
[12] Thorsten Renk arXiv:1210.1330 [hep-ph]
[13] Thorsten Renk arXiv:1202.4579v2 [hep-ph]
[14] S. S. Adler et al. [PHENIX Collaboration], Phys. Rev. D 74, 072002 (2006)
[15] R. P. Feynman, R. D. Field, and G. C. Fox, Nucl. Phys. B128, 1 (1977).
Hot Quarks 2012 IOP Publishing
Journal of Physics: Conference Series 446 (2013) 012009 doi:10.1088/1742-6596/446/1/012009
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