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
Publisher
Elsevier
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Final published version
Citable link
http://hdl.handle.net/1721.1/90293
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Dijet transverse momentum imbalance, fragmentation functions and jet-track
correlations in PbPb collisions at
√
s
NN= 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 pTvalues 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 previ-ously 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 fragmen-tation 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 ofξ = ln(1/z) where
z= ptrack
/pjet, ptrack is the momentum component of the track along the jet axis, and pjetis the magnitude of the jet
momentum.
The distribution ofξ is shown in Fig. 1 in bins of dijet asymmetry, AJ= (pT,1− pT,2)/(pT,1+ pT,2). In any given
AJselection, 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)
0375-9474/ © 2013 CERN Published by Elsevier B.V. All rights reserved.
www.elsevier.com/locate/nuclphysa
http://dx.doi.org/10.1016/j.nuclphysa.2012.12.111
ξ
/d
trackdN
jet1/N
-3 10 -2 10 -1 10 1 10 2 10 3 10 <0.13 J 0<A (R = 0.3) PF Jets T anti-k >40 GeV/c T,2 >100 GeV/c, p T,1 p π 3 2 > 1,2 φ Δ Centrality 0-30% (a) <0.13 J 0<A (R = 0.3) PF Jets T anti-k >40 GeV/c T,2 >100 GeV/c, p T,1 p π 3 2 > 1,2 φ Δ Centrality 0-30% (a)= ln(1/z)
ξ
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5PbPb/pp
0 0.5 1 1.5 2 2.5 3 (e)(e) <0.24 J 0.13<A R < 0.3) Δ Tracks in cone ( >4 GeV/c track T p (b) <0.24 J 0.13<A R < 0.3) Δ Tracks in cone ( >4 GeV/c track T p (b)= ln(1/z)
ξ
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 (f) (f) <0.35 J 0.24<A (c) <0.35 J 0.24<A (c)= ln(1/z)
ξ
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 (g) (g) >0.35 J A Leading jet pp reference Subleading jet pp reference (d) >0.35 J A Leading jet pp reference Subleading jet pp reference (d)= ln(1/z)
ξ
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Leading Jet Subleading Jet (h) Leading Jet Subleading Jet (h) -1 b μ = 6.8 int = 2.76 TeV, L NN s CMS, PbPb, = 6.8 μb-1 int = 2.76 TeV, L NN s CMS, PbPb,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.
Y. Yilmaz / Nuclear Physics A 910–911 (2013) 413–416 414
through a more differential approach [5].
Dijets withΔφ1,2> 2π/3 are selected. The contamination from fake jets, due to background fluctuations, is sub-tracted as estimated from dijet events withΔφ1,2< π/3.
It is observed that when the leading jet of the event has a pThigher 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 pTand event centrality.
leading jet
/N
dijetN
0.7 0.75 0.8 0.85 0.9 0.95 1 -1 b μ Ldt = 150∫
= 2.76 TeV, NN s PbPb -1 Ldt = 231nb∫
= 2.76 TeV, s pp PYTHIA+HYDJET(GeV/c)
T,1p
150 200 250 300 350 leading jet/N
backgroundN
0 0.02 0.04 0.06 0.08 Centrality 0-20% > 30 GeV/c T,2 p π 3 2 > 12 φ Δ 0.7 0.75 0.8 0.85 0.9 0.95 1 CMS partN
0 100 200 300 0 0.02 0.04 0.06 0.08 > 120 GeV/c T,1 p > 30 GeV/c T,2 p π 3 2 > 12 φ ΔFigure 2: Fraction of events with a genuine subleading jet withΔφ1,2 > 2π/3, as a function of leading jet pT,1(left) and Npart(right). The
background due to underlying event fluctuations is estimated fromΔφ1,2< 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 pTto the leading jet pT,pT,2/pT,1, is shown as a function of leading
jet pTin different bins of centrality. In the central events, a significant shift of thepT,2/pT,1with 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].
〉 T, 1 /p T, 2 p 〈 0.5 0.6 0.7 0.8 -1 b μ Ldt = 150
∫
= 2.76 TeV, NN s PbPb -1 Ldt = 231 nb∫
= 2.76 TeV, s pp PYTHIA+HYDJET 50-100% CMS 150 200 250 300 350 PbPb - M C -0.2 -0.1 0 20-50% > 30 GeV/c T,2 p π 3 2 > 12 φ Δ (GeV/c) T,1 p 150 200 250 300 350 0-20% 150 200 250 300 350Figure 3: Average dijet momentum ratio pT,2/pT,1as a function of leading jet pTfor 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].
Y. Yilmaz / Nuclear Physics A 910–911 (2013) 413–416 416
