Jet measurements in pp, p--Pb and Pb--Pb collisions with ALICE at the LHC

0.59 MB·

by  S. K. Prasad

Jet measurements in pp, p–Pb and Pb–Pb collisions
with ALICE at the LHC
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1 S. K. Prasad∗†
0
CentreforAstroparticlePhysicsandSpaceScience,BoseInstitute,Kolkata,700091(INDIA)
2
E-mail: [email protected]
n
a
J
We present a systematic study of jet measurements in pp, p–Pb and Pb–Pb collisions using the

8 √
ALICE detector at the LHC. Jet production cross sections are measured in pp collisions at s
1
√ √
= 2.76 and 7 TeV, in p–Pb collisions at s = 5.02 TeV and in Pb–Pb collisions at s =
NN NN
]
x 2.76 TeV. Jet shape observables and fragmentation distributions are measured in pp collisions
e
at 7 TeV. Jets are reconstructed at midrapidity in a wide range of transverse momentum using
-
p
sequentialrecombinationjetfindingalgorithms(k ,anti-k ,andSISCone)withseveralvaluesof
e T T
h jetresolutionparameterRintherange0.2–0.6. MeasurementsarecomparedtoNext-to-Leading
[
Order(NLO)perturbativeQuantumChromodynamics(pQCD)calculationsandpredictionsfrom

1
Monte Carlo (MC) event generators such as PYTHIA, PHOJET and HERWIG. Jet production
v √
2 crosssectionsarewellreproducedbyNLOpQCDcalculationsinppcollisionsat s=2.76TeV.
√
6 MC models could not explain the jet cross sections in pp collisions at s = 7 TeV, whereas
4
4 jet shapes and fragmentation distributions are rather well reproduced by these models. The jet
0 nuclearmodificationfactorR inp–Pbcollisionsisfoundtobeconsistentwithunityindicating
pPb
.
1 theabsenceoflargemodificationsoftheinitialpartondistributionorstrongfinalstateeffectson
0
6 jetproduction,whereasalargejetsuppressionisobservedinPb–Pbcentraleventswithrespectto
1 peripheraleventsindicatingformationofadensemediumincentralPb–Pbevents.
:
v
i
X
r
a
∗Speaker.
†OnbehalfoftheALICECollaboration.
Jetsinpp,p–Pb,andPb–PbusingALICE S.K.Prasad
1. Introduction
In high energy hadronic or nuclear collisions, hard (large momentum transfer Q2) scattered
partons (quarks and gluons) fragment and hadronize, resulting in a collimated shower of particles
known as a jet [1]. Jet measurements in pp collisions provide a test of perturbative and non-
perturbative aspects of jet production and fragmentation as implemented in the MC models, and
formabaselineforsimilarmeasurementsinnucleus–nucleus(A–A)andproton–nucleus(p–A)col-
lisions. In A–A collisions an energetic parton while passing through the produced medium loses
energyviainducedgluonradiationandelasticscattering. JetstudiesinA–Acollisionsincompar-
isontoppallowabetterunderstandingofthemediuminducedmodificationsinthefragmentation
ofhardscatteredpartonsandenergylossmechanisms[2,3],whereassimilarstudiesinp–Acolli-
sionspotentiallyrevealtheeffectsof(cold)nuclearmatter(CNM).Inthispaperwepresentresults
√
of jetmeasurements obtained using ALICEdetector in ppcollisions at s= 2.76, 7TeV, in p–Pb
√ √
collisionsat s =5.02TeVandinPb–Pbcollisionsat s =2.76TeV.
NN NN
2. Datasample,eventselection,trackselection,andjetreconstruction
The data used in this analysis were collected during the LHC run in 2010 for pp collisions
at 7 TeV, in 2011 for pp collisions at 2.76 TeV, in fall of 2010 for Pb–Pb collisions, and in the
beginning of 2013 for p–Pb collisions using the ALICE detector [4, 5]. Minimum bias events
are selected based on information from Silicon Pixel Detector (SPD) [6] and V0 [7] detectors
(V0A, V0C) [8, 9, 10, 11]. The Electromagnetic Calorimeter (EMCal) [12] is used in addition
to select EMCal triggered events [8]. Information from the Time Projection Chamber (TPC) [13]
and Inner Tracking System (ITS) [6] are used to select charged tracks using a hybrid approach as
discussed in [8]. The reconstruction of neutral particles is performed using the Electromagnetic
Calorimeter (EMCal) [8]. Charged tracks with transverse momentum ptrack > 0.15 GeV/c at
T
midrapidity(|ηtrack| <0.9)areusedasinputtojetreconstruction. Inaddition,forjetsincluding
neutral particles, EMCal clusters with energy greater than 0.3 GeV/c are considered. Jets are
reconstructedusingtheinfraredcollinearsafesequentialrecombinationalgorithmanti-k [14]. In
T
p–Pb and Pb–Pb collisions, the k [15, 16] algorithm is used for the estimation of background.
T
JetsarereconstructedwithseveralvaluesoftheresolutionparameterRintherange0.2–0.6. Jets
reconstructed using charged particles only as input are referred to as ‘charged jets’ whereas jets
reconstructedusingbothchargedandneutralparticlesasinputareknownas‘fulljets’hereafter.
3. Correctionfordetectoreffectsandbackground
Measureddistributionsarecorrectedfortheinstrumentaleffectsandpresentedatparticlelevel.
Corrections for the instrumental effects, such as limited track reconstruction efficiency and finite
momentumresolution,areperformedusingtheunfoldingtechniques[17,18]forjetcrosssections
whereasjetshapeandfragmentationobservablesarecorrectedusingabin-by-bintechnique. Afull
detectorsimulationisperformedusingthePYTHIA6.425[19]eventgeneratorandGEANT3[20]
particle transport package. All observables are also corrected for the contamination from the sec-
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Jetsinpp,p–Pb,andPb–PbusingALICE S.K.Prasad
ondary particles 1 and background 2. The method for estimation and correction of background is
however different in pp to that in p–Pb and Pb–Pb collisions (see [8, 9, 10, 11] for details). In
caseofp–PbandPb–Pbevents,regiontoregionfluctuationsoftheestimatedaveragebackground
density arising due to fluctuations in the particle multiplicity and momentum, elliptic flow etc.,
areconsiderable. Backgroundfluctuationsarecorrectedforonanstatisticalbasisusingunfolding
techniques (see Sec. 2 of [11]). The corrected results are compared to that obtained from various
MCeventgeneratorse.g. PYTHIA(tunePerugia-0,Perugia-2011,AMBT1),HERWIG,PHOJET
andNLOpQCDcalculations.
4. Results
4.1 Jetmeasurementsinppcollisions
c/GeV)10 3 anti kT, R A=L 0IC.2E, |ηp|p< 0.s5 = 2.76 TeV: Lint = 13.6 nb 1 c/GeV)10 3 anti kT, R A=L 0IC.4E, |ηp|p< 0.s5 = 2.76 TeV: Lint = 13.6 nb 1
mb 10 4 Systematic uncertainty mb 10 4 Systematic uncertainty
(dη10 5 (dη10 5
pT pT
/dσ10 6 NLO (N. Armesto) /dσ10 6 NLO (N. Armesto)
2d NLO (G. Soyez) 2d NLO (G. Soyez)
10 7 NLO + Hadronization (G. Soyez) 10 7 NLO + Hadronization (G. Soyez)
data1.52 data1.52
O/ 1 O/ 1
NL0.5 NL0.5
data1.5220 40 60 80 100 120 data1.5220 40 60 80 100 120
O/ 1 O/ 1
NL0.5 NL0.5
20 40 60 80 100 120 20 40 60 80 100 120
p (GeV/c) p (GeV/c)
ALI−PUB−46771 T,jet ALI−PUB−46767 T,jet
Figure1: (Coloronline)Fulljetproductioncrosssectionsasafunctionofjet p comparedtoNLOpQCD
√ T
calculationsinppcollisionsat s=2.76TeVforjetsreconstructedwithR=0.2(left)and0.4(right).
ALI-PUB-90009
Figure2: (Coloronline)Chargedjetcrosssectionsasafunctionofjet p comparedtoMCmodelsinpp
√ T
collisionsat s=7TeVforjetsreconstructedwithR=0.2(left),0.4(middle)and0.6(right).
1Particlesproducedbyweakdecaysandinteractionsofprimaryparticleswithdetectormaterialandbeampipe.
2Particlesinaneventwhicharenotproduceddirectlybyhardscatteringofpartons.
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Jetsinpp,p–Pb,andPb–PbusingALICE S.K.Prasad
The full jet production cross sections as a function of jet p compared to NLO pQCD calcu-
√ T
lations[21]areshowninFig.1[8]forppcollisionsat s=2.76TeVforjetsreconstructedwithR
= 0.2 (left) and 0.4 (right). The NLO calculations reproduce the full jet cross sections reasonably
wellwhenhadronizationeffectsareincluded. ThechargedjetcrosssectionscomparedtoMCpre-
√
dictions, are shown in Fig. 2 for pp collisions at s = 7 TeV for R = 0.2 (left), 0.4 (middle) and
0.6 (right). None of the models can explain the data in the entire p range, the discrepancy being
T
largerforlargerR. Thejetshapeobservablesasdefinedbytheradialtransversemomentumdensity
distributionsaboutthejetaxisasafunctionofdistance‘r’,jetconstituentsmultiplicityandaverage
radiuscontaining80%ofjet p asafunctionofleadingjet p ,andthefragmentationdistributions
T T
are,however,ingeneralreasonablywellreproducedbythesemodels(figuresnotshown,see[9]).
4.2 Resultsfromp–PbandPb–Pbcollisions
ch jetspPb1.82 ALICE p Pb sNN = 5.02 TeV
R Charged jets, anti k , |η | < 0.5
1.6 T lab
Reference: Scaled pp jets 7 TeV
1.4
Global normalization uncertainty
1.2
1
0.8
0.6
0.4
0.2 Resolution parameter R = 0.2 Resolution parameter R = 0.4
0
20 40 60 80 100 120 20 40 60 80 100 120
p (GeV/c) p (GeV/c)
T, ch jet T, ch jet
√
Figure3: (Coloronline)Jetnuclearmodificationfactors(R )inp–Pbcollisionsat s =5.02TeVfor
pPb NN
chargedjetsreconstructedwithR=0.2(left)and0.4(right).
√
The jet nuclear modification factor R 3 is shown in Fig. 3 for p–Pb collisions at s =
pPb NN
5.02 TeV for charged jets reconstructed with R = 0.2 (left) and 0.4 (right). It is found to be con-
sistent with unity in the measured p range indicating the absence of large modifications of the
T
initial parton distribution or strong final state effects on jet production. The charged jet nuclear
modification factor, R 4 is shown in Fig. 4 as a function of jet p for three centrality bins for
CP T
√
Pb–Pbcollisionsat s =2.76TeV.Alargejetsuppressionisobservedinmostcentral(0–10%)
NN
Pb–Pb collisions indicating the formation of a dense medium in such collisions. It is found to be
centralityand p dependent. Theleft(right)panelofFig.5showsratiosofspectra(crosssections)
T
√
forjetsmeasuredwithR=0.2and0.4(0.2and0.3)inp–Pb(Pb–Pb)collisionsat s =5.02TeV
NN
(2.76 TeV) compared to that obtained in pp collisions (PYTHIA). The ratio of jet spectra is sen-
sitive to the collimation of particles around the jet axis and serves as an indirect measure of jet
3R isdefinedastheratioofp spectrainp–Pbnormalizedbythenuclearoverlapfunction(cid:104)T (cid:105)obtainedfrom
pPb T √ AA
Glaubermodelandppcrosssectionextrapolatedto s =5.02TeV.
NN
4R isdefinedastheratioofjet p spectraincentralandperipheralPb–Pbcollisionsnormalizedby(cid:104)T (cid:105)for
CP T AA
eachcentralityclass.
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Jetsinpp,p–Pb,andPb–PbusingALICE S.K.Prasad
CP 11..42 CPeerniptrhael:r 0a l:1 500% 80% PCbh aPrgbe ds JNeNt=s2.76 TeV
R 1 Anti kT R = 0.3
ALICE ptrack > 0.15 GeV/c
0.8 LeTading track p > 5 GeV/c
0.6 T
0.4
0.2 shape uncertainty
correlated uncertainty
CP 11..40220 CPeern3ipt0rhael:r 1a0l: 543000 %80%50 60 70 80 90
R
1
0.8
0.6
0.4
0.2 shape uncertainty
correlated uncertainty
CP11..40220 CPeern3ipt0rhael:r 3a0l: 545000 %80%50 60 70 80 90
R
1
0.8
0.6
0.4
0.2 shape uncertainty
correlated uncertainty
020 30 40 50 60 70 80 90
p (GeV/c)
ALI−PUB−64273 T,ch jet
Figure 4: (Color online) Charged jet nuclear modification factors (R ) in Pb–Pb collisions at
√ CP
s =2.76TeVfor0–10%(top),10–30%(middle)and30–50%(bottom)centralityclasses.
NN
) 2
4) 1 .31.8 ALICE Pb Pb 0 10%
ℛ(0.2,0.000...789 Charged jets, anti-kT σR=0(111...462 PApLetTbnraa tciPd kk bi>nT g0 s.t1rNa5Nc= Gk2 .ep7VT6 / >cT 5e VGeV/c PPbY TPHbI A50 80%
0.6 )/ 1
2
0.5 .0.8
0
0.4 ALICE p-Pb sNN = 5.02 TeV, |ηlab| < 0.5 =0.6
0.3 ALICE pp s = 7 TeV, |ηlab| < 0.3 R0.4
NLO pQCD calculations 5.02 TeV (boosted) ( correlated uncertainty
0.2 POWHEG+PYTHIA8 with CTEQ6.6+EPS09 σ 0.2 shape uncertainty
0.1 PYTHIA6 Perugia 2011 5.02 TeV 0
0 20 40 60 80 100
20 30 40 50 60 70 80 90 100 110 120 p (GeV/c)
p (GeV/c)
T, ch jet ALI−PUB−64303 T,ch jet
Figure5: (Coloronline)TheratiosofjetspectrameasuredwithR=0.2tothatobtainedwithlargerR(0.4
forp–Pband0.3forPb–Pb)asafunctionofjet p inp–Pb(left)andPb–Pb(right)collisionsat5.02and
T
2.76TeVrespectively.
structure. In minimum bias p–Pb collisions the ratio of jet spectra is found to be compatible with
thatinppcollisions,PYTHIAandNLOpQCDcalculations,andthecrosssectionratiosinPb–Pb
is found to be similar for most central and peripheral collisions and compatible with PYTHIA in-
dicatingthatthecoreofthejetwithinthemeasuredR,remainsunmodifiedinminimumbiasp–Pb,
peripheralPb–PbandeveninmostcentralPb–Pbcollisions.
5. Summaryandconclusions
We reported jet measurements for pp, p–Pb and Pb–Pb collisions at various centre-of-mass
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Jetsinpp,p–Pb,andPb–PbusingALICE S.K.Prasad
energies using the ALICE detector. Jets are measured at midrapidity using the anti-k jet finding
T
algorithm with several values of the jet resolution parameter (R in the range 0.2 to 0.6). Full jet
√
cross sections are well reproduced by NLO pQCD calculations in pp collisions at s = 2.76 TeV.
√
NoneoftheMCmodelsunderstudycanexplainthechargedjetcrosssectionsinppcollisionsat s
=7TeV,howeverjetshapeobservablesandfragmentationdistributionsareratherwellreproduced
bythesemodels. Thejetnuclearmodificationfactorforminimumbiasp–Pbcollisionsisfoundto
beconsistentwithunitywhereasalargejetsuppressionisobservedforcentralPb–Pbeventswith
respecttoperipheraleventsindicatingthepresenceofadensemediuminthesecollisions. Thejet
spectra(orcrosssection)ratiosindicatethatthecoreofthejetremainsunmodifiedeveninthemost
centralPb–Pbcollisions.
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