Dijet transverse momentum imbalance, fragmentation
functions and jet–track correlations in PbPb collisions at
s[subscript NN] = 2.76 TeV with CMS
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Citation
Yilmaz, Yetkin. “Dijet Transverse Momentum Imbalance,
Fragmentation Functions and Jet–track Correlations in PbPb
Collisions at s[subscript NN] = 2.76 TeV with CMS.” Nuclear
Physics A 910–911 (August 2013): 413–416. © CERN for the
benefit of the CMS Collaboration
As Published
http://dx.doi.org/10.1016/j.nuclphysa.2012.12.111
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Elsevier
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Final published version
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Nuclear Physics A 910–911 (2013) 413–416
www.elsevier.com/locate/nuclphysa
Dijet transverse momentum imbalance, fragmentation functions and jet-track
√
correlations in PbPb collisions at sNN = 2.76 TeV with CMS
Yetkin Yilmaz on behalf of the CMS Collaboration
Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Abstract
Dijets in PbPb collisions at a nucleon-nucleon center-of-mass energy of 2.76 TeV are studied with the CMS
detector at the LHC. It is shown that the fragmentation functions of jets into charged particle tracks with transverse
momenta pT > 4 GeV/c in PbPb collisions are similar to those in pp collisions, for both the leading and subleading
jets. In addition, study of high statistics PbPb data acquired in 2011 shows that the quenching phenomenon exists up
to highest jet pT values in excess of 300 GeV/c.
Keywords: jet quenching, jet fragmentation
The energy loss phenomenon in the medium that is formed in highly energetic PbPb collisions, which was previously observed in hadron measurements at RHIC, is explored in more detail with fully reconstructed jet measurements
that are performed in LHC experiments [1, 2]. This paper discusses the recent studies from CMS [3] on jet fragmentation and the momentum dependence of the quenching.
1. Fragmentation functions
Earlier results from CMS [2], with the dataset of the 2010 LHC run with PbPb ions, have revealed various aspects
of the energy loss mechanism. Although an angular de-correlation between the jets is not observed, the average
transverse momentum imbalance is observed to be increasing with collision centrality, which is attributed to energy
loss in medium. The properties of jets are further investigated qualitatively by the study of fragmentation
functions [4]
, where tracks with pT > 4 GeV/c within a cone of ΔR < 0.3 are correlated to the jet, where ΔR = (Δη)2 + (Δφ)2
between the track and the jet. The momenta of each track is projected onto the jet axis in the reference frame where
the two jets have opposite pseudorapidity. The fragmentation functions are plotted as a function of ξ = ln(1/z) where
/pjet , ptrack
is the momentum component of the track along the jet axis, and pjet is the magnitude of the jet
z = ptrack
momentum.
The distribution of ξ is shown in Fig. 1 in bins of dijet asymmetry, A J = (pT,1 − pT,2 )/(pT,1 + pT,2 ). In any given
A J selection, fragmentation of jets display the same pattern in PbPb collisions and in pp collisions.
Email address: yetkin.yilmaz@cern.ch (Yetkin Yilmaz on behalf of the CMS Collaboration)
© 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.2012.12.111
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Y. Yilmaz / Nuclear Physics A 910–911 (2013) 413–416
CMS, PbPb, sNN = 2.76 TeV, Lint = 6.8 μb-1
3
1/N jet dN track /dξ
10
102
10
anti-kT (R = 0.3) PF Jets
pT,1>100 GeV/c, pT,2>40 GeV/c
Δφ1,2 > 2π
3
Centrality 0-30%
(a)
Tracks in cone (ΔR < 0.3)
ptrack
>4 GeV/c
T
(c)
(b)
(d)
1
10-1
10-2
10-3
0<A J<0.13
0.13<AJ<0.24
0.24<AJ<0.35
A J>0.35
(g)
(f)
(e)
3
Leading Jet
(h)
Subleading Jet
2.5
PbPb/pp
Leading jet
pp reference
Subleading jet
pp reference
2
1.5
1
0.5
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
ξ = ln(1/z)
ξ = ln(1/z)
ξ = ln(1/z)
ξ = ln(1/z)
Figure 1: The top row shows the fragmentation functions in PbPb data and pp-based reference for various dijet asymmetry selections. The bottom
row shows the ratio of each fragmentation function to its pp-based reference. The error bars represent the statistical uncertainty and the boxes
represent the systematic uncertainty.
415
Y. Yilmaz / Nuclear Physics A 910–911 (2013) 413–416
2. Dijet imbalance as a function of leading jet pT
With the availability of a large dataset from the 2011 LHC run with PbPb ions, the dijet imbalance is investigated
through a more differential approach [5].
Dijets with Δφ1,2 > 2π/3 are selected. The contamination from fake jets, due to background fluctuations, is subtracted as estimated from dijet events with Δφ1,2 < π/3.
It is observed that when the leading jet of the event has a pT higher than 180 GeV/c, it is more than 95% of the
time accompanied by a recoiled partner in the opposite direction in azimuth. The fraction of such correlated events
after background subtraction and the fraction of the estimated background are shown in Fig. 2 as a function of leading
jet pT and event centrality.
CMS
Ndijet/Nleading jet
1
1
0.95
0.95
0.9
0.9
∫
0.85
PbPb sNN = 2.76 TeV, Ldt = 150 μb
∫
0.8
0.85
-1
0.8
-1
pp s = 2.76 TeV, Ldt = 231nb
0.75
0.75
PYTHIA+HYDJET
Nbackground/Nleading jet
0.7
0.7
0.08
0.08
0.06
0.04
Centrality 0-20%
pT,1 > 120 GeV/c
p
p
> 30 GeV/c
T,2
Δφ
12
> 2π
3
T,2
Δφ
0.06
> 30 GeV/c
12
> 2π
3
0.04
0.02
0.02
0
0
150
200
250
300
350 0
pT,1 (GeV/c)
100
200
300
Npart
Figure 2: Fraction of events with a genuine subleading jet with Δφ1,2 > 2π/3, as a function of leading jet pT,1 (left) and Npart (right). The
background due to underlying event fluctuations is estimated from Δφ1,2 < π/3 events and subtracted from the number of dijets. The fraction of
the estimated background is shown in the bottom panels. The error bars represent the statistical uncertainties.
In Fig. 3, the average ratio of subleading jet pT to the leading jet pT , pT,2 /pT,1 , is shown as a function of leading
jet pT in different bins of centrality. In the central events, a significant shift of the pT,2 /pT,1 with respect to the
MC and pp results is observed. This shift, while changing monotonically with centrality, does not show a significant
dependence on the leading jet pT . Since both data and MC include an intrinsic imbalance from hard gluon radiation
and detector resolution, the implications on the absolute amount of energy loss should be extracted via realistic models
of quenching which take into account these effects.
References
[1] Atlas Collaboration, Phys. Rev. Lett. 105, 252303 (2010) [arXiv:1011.6182].
[2] CMS Collaboration, Phys. Rev. C 84, 024906 (2011) [arXiv:1102.1957].
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Y. Yilmaz / Nuclear Physics A 910–911 (2013) 413–416
CMS
0.7
T,2
T,1
⟨p /p ⟩
0.8
0.6
pp
∫
-1
-1
s = 2.76 TeV, Ldt = 231 nb
0.5
PYTHIA+HYDJET
PbPb - MC
∫
PbPb sNN = 2.76 TeV, Ldt = 150 μb
50-100%
pT,2 > 30 GeV/c
Δφ > 2π
12
3
20-50%
0-20%
0
-0.1
-0.2
150
200
250
300
350
150
200
250
300
350
150
200
250
300
350
pT,1 (GeV/c)
Figure 3: Average dijet momentum ratio pT,2 /pT,1 as a function of leading jet pT for three bins of collision centrality, from peripheral to central
collisions, corresponding to selections of 50–100%, 30–50% and 0–20% of the total inelastic cross section. Results for PbPb data are shown as
points with vertical bars and brackets indicating the statistical and systematic uncertainties, respectively. Results for pythia+hydjet are shown as
squares. In the 50–100% centrality bin, results are also compared with pp data, which is shown as the open circles. The difference between the
PbPb measurement and the pythia+hydjet expectations is shown in the bottom panels.
[3] CMS Collaboration, JINST 3 S08004 (2008)
[4] CMS Collaboration, Submitted to JHEP [arXiv:1205.5872].
[5] CMS Collaboration, Phys. Lett. B 712, 176 (2012) [arXiv:1202.5022].