Jet fragmentation functions in PbPb and pp collisions at
2.76 TeV with CMS
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Citation
Ma, Frank. “Jet Fragmentation Functions in PbPb and Pp
Collisions at 2.76 TeV with CMS.” Nuclear Physics A 904–905
(May 2013): 740c–743c. © CERN for the benefit of the CMS
Collaboration
As Published
http://dx.doi.org/10.1016/j.nuclphysa.2013.02.123
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Elsevier
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Final published version
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Thu May 26 09:14:42 EDT 2016
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http://hdl.handle.net/1721.1/90557
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Nuclear Physics A 904–905 (2013) 740c–743c
www.elsevier.com/locate/nuclphysa
Jet fragmentation functions in PbPb and pp collisions at 2.76 TeV
with CMS
Frank Ma (on behalf of the CMS Collaboration)1
Massachusetts Institute of Technology, Cambridge, MA, USA
Abstract
The jet fragmentation function of inclusive jets with pT > 100 GeV/c in PbPb collisions is measured for reconstructed charged particles with pT > 1 GeV/c within the jet cone. A data sample
√
of PbPb collisions collected in 2011 at a center-of-mass energy of sNN = 2.76 TeV corresponding to an integrated luminosity of 129 μb−1 is used. The results for PbPb collisions as a function
of collision centrality are compared to reference distributions based on pp data collected at the
same collision energy. A centrality-ordered modification of the fragmentation function is revealed. For the most central collisions a significant rise of the PbPb/pp fragmentation function
ratio for the softest fragmentation products with pT < 3 GeV/c is observed.
1. Introduction
High transverse momentum partons produced by hard scatterings in heavy-ion collisions
are expected to lose energy as they travel through the hot and dense medium created in these
interactions [1]. Suppression of high-pT particles (as first seen at RHIC and more recently at the
LHC, including CMS [2]) was taken as evidence for this effect. CMS data have been used to
directly demonstrate the existence of this energy loss using the difference in momentum between
pairs of jets [3, 4] and also jets paired with photons [5]. Unbalanced di-jets and photon-jet pairs
were found to be much more prevalent in the most central PbPb collisions. Some theoretical
models have been proposed that explain the observed suppression of high-pT particles in heavyion collisions using the assumption that partons fragment differently inside the bulk medium
created during the collision [6, 7, 8]. Measurements of jet fragmentation properties provide an
experimental constraint which is complementary to the measurement of jet energy.
The goal of this analysis is to measure the partitioning of the jet energy into particles (the
fragmentation function) in heavy-ion collisions. As a first approach to addressing this issue, the
higher-momentum (pT > 4 GeV/c) component of the fragmentation function was found to be
qualitatively similar to that for jets in pp collisions, for which the medium is absent [9]. Taking
advantage of data from the higher integrated luminosity heavy-ion run in 2011, the analysis
described in this report expands on the previous result by measuring the fragmentation functions
for tracks down to pT > 1 GeV/c and in more differential centrality bins.
1A
list of members of the CMS Collaboration and acknowledgements can be found at the end of this issue.
© CERN for the benefit of the CMS Collaboration.
0375-9474/ © 2013 CERN Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.nuclphysa.2013.02.123
F. Ma / Nuclear Physics A 904–905 (2013) 740c–743c
741c
2. Experimental Setup and Analysis Procedure
This analysis was performed using the data collected in 2011 from PbPb collisions at a
√
nucleon-nucleon center-of-mass energy of sNN = 2.76 TeV at the Compact Muon Solenoid
(CMS) detector [10]. A jet-triggered dataset corresponding to approximately 129 μb−1 of PbPb
collisions was used. Centrality of the heavy-ion collisions was determined from minimum bias
events based on the total energy from both forward hadronic calorimeters [3].
For both pp and PbPb collisions, the analysis is based on jets reconstructed using the anti-kT
jet algorithm, with a radius parameter (R) of 0.3, utilizing particle-flow (PF) objects that combine
tracking and calorimetric information [11]. In the PbPb data, the contribution of the underlying
heavy-ion event is removed using an iterative pileup subtraction method [12]. Charged particles
in PbPb collisions are reconstructed from hits in the CMS silicon pixel and strip detectors. The
reconstructed algorithm is performed similarly to what is done in pp collisions [13]. For PbPb
collisions, some criteria have been fine-tuned to cope with the challenges presented by the much
higher hit density (as similarly done in [2]). In this analysis, only jets with |η| < 2 are considered,
where η is the pseudorapidity. Events are selected with a minimum corrected jet pT > 100 GeV/c.
For the jet selection used in this analysis the triggers used are more than 99% efficient. Within
this event sample all jets above pT > 100 GeV/c are selected for analysis.
Jet fragmentation functions are measured by correlating reconstructed charged-particle tracks
falling within the jet cones of the respective jet. As done in previous measurements at hadron
colliders [14], the fragmentation function is presented as a function of the variable
ξ = ln
ptrack
1
; z = jet ,
z
p
(1)
is the momentum component of the track along the jet axis, and pjet is the magnitude
where ptrack
of the jet momentum. The tracks in a cone of ΔR = (Δφ)2 + (Δη)2 < 0.3 around the jet axis are
selected for analysis. The fragmentation functions, defined as 1/Njet dNtrack /dξ, are normalised
to the total number jets (Njet ). Due to the high level of underlying event activity coming from the
heavy-ion collisions, many tracks that are not associated with the jet fragmentation can fall inside
the jet cone. The background contribution can be estimated by selecting charged hadrons that lie
in a control region jet cone obtained by reflecting the original jet cone around η = 0 while keeping
the same φ coordinate. For each signal jet, the background distribution is subtracted from the
raw distribution obtained from the jet cone. For this procedure, jets in the region |η| <0.3 are
excluded to avoid overlap between the signal jet region and the region used for background
estimation. Tracking efficiency and fake rates are derived from PYTHIA embedded heavy-ion
simulations and are corrected for before subtraction. Finally in order to quantify the mediumrelated effects, the analysis was repeated in pp data at the same center-of-mass collision energy.
The pp data was collected by CMS with a jet trigger corresponding to an integrated luminosity
of 212 nb−1 . For constructing a proper reference distribution, the pp jet momenta are smeared
and reweighted to match the PbPb jet distributions for a given analysis selection.
3. Results
The final corrected fragmentation functions in pp and PbPb data for tracks with pT above
1 GeV/c are shown in Fig. 1. The figure shows that the fragmentation function of jets is modified
increasingly with the collision centrality. In the 50-100% bin, the ratio of PbPb/pp is flat at unity
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F. Ma / Nuclear Physics A 904–905 (2013) 740c–743c
which means no modification. However, an excess in high ξ is observed for more central events.
In the most central 0–10% collisions and for the lowest charged particle momenta studied, the
PbPb/pp fragmentation function ratio rises to 2.2 ± 0.3(stat.) ± 0.7(syst.). This implies that for
central collisions the spectrum of particles in a jet has an enhanced contribution of soft particles
compared to pp collisions.
CMS Preliminary
1/N jet dN track /dξ
10
Jet p > 100GeV/c, |η| < 2
T
Track p > 1 GeV/c, r < 0.3
Systematic uncertainty
PbPb
pp reference
LInt = 129 μ b-1
T
1
10-1
50% - 100%
30% - 50%
10% - 30%
0% - 10%
10-2
3
PbPb/pp
2.5
2
1.5
1
0.5
0
0
1
2
3
4
5
0
ξ = ln(1/z)
1
2
3
ξ = ln(1/z)
4
5
0
1
2
3
ξ = ln(1/z)
4
5
0
1
2
3
4
5
ξ = ln(1/z)
Figure 1: Top row shows fragmentation function in PbPb in bins of increasing centrality overlaid with pp. Jets have pT
above 100 GeV/c, and tracks have pT above 1 GeV/c. The PbPb data is shown in the top row in four increasing centrality
bins from left to right. The bottom row shows the ratio of each PbPb fragmentation function to its pp reference. Error
bars are statistical, and yellow boxes are the systematic uncertainty.
One can investigate in which track pT ranges the fragmentation functions exhibit an excess
by re-plotting the jet fragmentation for the same tracks in the jet cone as a function of track pT .
These distributions are obtained with the same background subtraction described above. Fig. 2
shows the spectra of tracks in the jet cone, background subtracted and compared to pp reference.
The bottom panels show the difference of the two distributions, pp subtracted from PbPb, in order
to quantify the excess of tracks at a given pT . The excess that is observed at the high-ξ region of
the fragmentation functions is localized at low-pT tracks below ∼ 3 GeV/c.
In the previous published result [9] for particle pT > 4 GeV/c on a smaller data, no modification in that kinematic region within the experimental uncertainties was observed. The new
fragmentation ratio was directly compared to the previous one and they are in good agreement.
The newly observed deficient of fragmentation products compared to pp at intermediate pT range
starts to become significant due to the decreased systematic uncertainty.
4. Summary and Conclusion
In summary, the first detailed measurements of the fragmentation function for inclusive jets
with pT, jet > 100 GeV/c in PbPb have been presented. The results were compared to measurements with the same selection in pp collisions at the same collision energy. The jet fragmentation
function ratios for central PbPb/pp collisions indicate a transport of energy from intermediate to
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F. Ma / Nuclear Physics A 904–905 (2013) 740c–743c
CMS Preliminary
LInt = 129 μ b-1
1/N jet dN track /dpT
10
Jet p > 100 GeV/c, |η| < 2
Systematic uncertainty
PbPb
T
Track p > 1 GeV/c, r < 0.3
pp reference
T
1
10-1
PbPb - pp (1/(GeV/c))
10-2
50-100%
30-50%
10-30%
0-10%
1.5
1
0.5
0
-0.5
1
10
p track (GeV/c)
T
1
10
p track (GeV/c)
T
1
10
p track (GeV/c)
T
1
10
p track (GeV/c)
T
Figure 2: The spectrum of tracks inside the jet cone, as a function of track pT , for PbPb and pp. Both the PbPb and pp
results are background subtracted, in the same manner as the fragmentation functions. Bottom panel shows the difference
of PbPb and pp spectra, which shows that there is an excess of low-pT tracks in the PbPb events.
low pT below ∼3 GeV/c. In the most central 0–10% collisions and for the lowest charged particle
momenta studied, the PbPb/pp fragmentation function ratio rises to 2.2 ± 0.3(stat.) ± 0.7(syst.).
References
[1] J. D. Bjorken. Energy loss of energetic partons in QGP: possible extinction of high pT jets in hadron-hadron
collisions. FERMILAB-PUB-82-059-THY, 1982.
√
[2] S. Chatrchyan et al. Study of high-pT charged particle suppression in PbPb compared to pp collisions at sNN =
2.76 TeV. Eur. Phys. J. C, 72:1945, 2012.
√
[3] S. Chatrchyan et al. Observation and studies of jet quenching in PbPb collisions at sNN =2.76 TeV. Phys. Rev. C,
84:024906, 2011.
√
[4] S. Chatrchyan et al. Jet momentum dependence of jet quenching in PbPb collisions at sNN =2.76 TeV. 2012.
[5] S. Chatrchyan et al. Studies of jet quenching using isolated-photon+jet correlations in PbPb and pp collisions at
√
sNN = 2.76 TeV. 2012.
[6] M. Gyulassy, I. Vitev, X.N. Wang, and B.W. Zhang. Jet quenching and radiative energy loss in dense nuclear
matter. 2003.
[7] M. Gyulassy and M. Plumer. JET QUENCHING IN DENSE MATTER. Phys.Lett., B243:432–438, 1990.
[8] X.N. Wang and M. Gyulassy. Gluon shadowing and jet quenching in relativistic heavy ion collisions. Nucl.Phys.,
A544:559–564, 1992.
[9] S. Chatrchyan et al. Measurement of jet fragmentation into charged particles in pp and pbpb collisions at 2.76 tev.
Journal of High Energy Physics, 2012:1–30, 2012. 10.1007/JHEP10(2012)087.
[10] The CMS Experiment at the CERN LHC. JINST, 3:S08004, 2008.
[11] Particle-flow event reconstruction in cms and performance for jets, taus, and emiss
T . CMS Physics Analysis Summary,
CMS-PAS-PFT-09-001, 2009.
[12] O. Kodolova, I. Vardanian, A. Nikitenko, and A. Oulianov. The performance of the jet identification and reconstruction in heavy ions collisions with CMS detector. Eur. Phys. J. C, 50:117, 2007.
[13] Tracking and vertexing results from first collisions. CMS Physics Analysis Summary, CMS-PAS-TRK-10-001,
2010.
√
[14] F. Abe et al. Jet-fragmentation properties in p p̄ collisions at s = 1.8 tev. Phys. Rev. Lett., 65:968, 1990.