Overview of ALICE results

0.53 MB·English

by  Antonio Ortiz

NuclearPhysicsB
Proceedings
Supplement
NuclearPhysicsBProceedingsSupplement00(2015)1–8
Overview of ALICE results
AntonioOrtizVelasquez
onbehalfoftheALICECollaboration
InstitutodeCienciasNucleares,UniversidadNacionalAuto´nomadeMe´xico.
Circuitoexteriors/n,CiudadUniversitaria,Del.Coyoaca´n,C.P.04510,Me´xicoDF.
5
1
0
2

n
Abstract
a
J
TheALICEdetectorwasdesignedtostudythephysicsofmatterunderextremeconditionsofhighenergydensity.
2 Different results were reported by the experiment using data from the successful run I of the LHC. The goal of the
2
presentworkistopresentanoverviewofrecentALICEresults. Thiscomprisesselectedresultsfromseveralanalyses

] ofpp,p-PbandPb−PbdataattheLHCenergies.
x
e Keywords:
-
p sQGP,heavyioncollisions,protonnucleusreaction,LHC,collectivity.
e
h
[ 1. Introduction thispurpose[9]. Ithascollecteddatawhichcorrespond

1 to an integrated luminosity of ≈10µb−1 and ≈0.1nb−1
v Matterwhichsurroundsuscanbefoundinavariety during the successful runs of 2010 and 2011, respec-
4 ofphases,thechangesonitsexternalconditionsallows tively. For the hot nuclear matter effects to be studied,
9
to go from one to another phase. From lattice QCD, it analysesofppandp-Pbdatawerealsoperformed. Un-
5
is predicted that hadronic matter under extreme condi- expectedly,thesedatarevealedintriguingeffectswhich
5
0 tionsofhightemperatureanddensitychangesitsprop- are not well understood. In this paper, a review of se-
. erties,inthisstatethefundamentaldegreesoffreedom lected results of ALICE will be presented, this com-
1
0 are given by quarks and gluons [1]. The phase transi- prises several measurements in different colliding sys-
5 tionoccursatacritical(crossover)temperaturebetween tems,pp,p-PbandPb−Pb.
1 143-171MeV[2,3,4,5,6,7]. Theexperimentalaccess
:
v to explore and test the QCD phase diagram and to ad- 2. TheALICEapparatus
i dress the fundamental question of hadron confinement
X
and chiral symmetry breaking is via ultra-relativistic Particle identification (PID) is an important tool to
r
a heavyioncollisions. study the hot and dense matter created in relativistic
The hot and dense matter has been studied in the heavy ion collisions. That is why ALICE is an exper-
Relativistic Heavy Ion Collider (RHIC) at Brookhaven imentspecializedinPIDfromhundredsofMeV/cupto
√
National Lab, the collisions of Au-Au at s = 0.2 tens of GeV/c. The central barrel of ALICE is placed
NN
TeV produced a perfect fluid that is initially closer to insidealargesolenoidalmagnetwhichprovidesamag-
the ideal hydrodynamic limit, a theoretical scenario in netic field of 0.5 T. It is dedicated to detect hadrons,
which the viscosity is zero. This result has been con- electrons,andphotonsproducedatmid-pseudorapidity,
firmed by experiments at the Large Hadron Collider, |η| <0.8. It comprises an Inner Tracking System (ITS)
√
wherePb−Pbcollisionsat s =2.76TeVhavebeen ofhigh-resolutionsilicondetectors,acylindricalTime-
NN
achieved [8]. ALICE (A Large Ion Collider Experi- Projection Chamber (TPC), and particle identification
ment) is the only experiment at the LHC designed for arrays of Transition-Radiation Detectors (TRD) and of
/NuclearPhysicsBProceedingsSupplement00(2015)1–8 2
π+2+π K+2+K K0S K*2+K* φ p+2p Λ Ξ +2Ξ+ Ω +2Ω+ d Λ3H+2 Λ3H 3He
y/dN103 ALICE Preliminary
d102 Pb Pb sNN = 2.76 TeV, 0 10%
10
1
Not in fit
10 1 Extrapolated
10 2 Model T (MeV) V (fm3) χ2/NDF
THERMUS 2.3 155 ± 2 5924 ± 543 23.6/9
10 3 GSI Heidelberg 156 ± 2 5330 ± 505 17.4/9
10 4 SHARE 3 156 ± 3 4476 ± 696 14.1/9
BR = 25%
mod.0.5
mod. data)/ 0.50
(
mod. data)/σdata 02421 2 3 4 5 6 7 8 9 10 11 12 13
( 4
ALI−PREL1−744623 3 4 5 6 7 8 9 10 11 12 13
Figure1: Thermalfitsofparticleyieldsin0-10%Pb−Pbcollisions.
Time-Of-Flight(TOF)counters.Additionalcentralsub- Thesystemexpandsandcoolsdown,whentheinelas-
systems, not-covering full azimuth, are a ring-imaging tic interactions cease the yields of particles are fixed.
CherenkovdetectorforHigh-MomentumParticleIDen- This is the stage of the so-called chemical freeze-out
tification(HMPID),andtwoelectromagneticcalorime- whichisstudiedusingtheyieldsofidentifiedhadrons.
ters: a high-resolution PHOton Spectrometer (PHOS) AsillustratedinFig.1,within20%hadrons(exceptK∗)
and a larger-acceptance ElectroMagnetic Calorimeter aredescribedbythermalmodelswithacommonchem-
(EMCal). Themuonarmdetectsmuonsemittedwithin ical freeze-out temperature T ≈156 MeV. However,
ch
2.5 < η < 4andconsistsofacomplexarrangementof larger deviations are observed for protons and K∗, for
absorbers,adipolemagnet,fivepairsoftrackingcham- the latter, this is not a surprise since its mean lifetime
bers,andtwotriggerstations. Severalsmallerdetectors is comparable to that for the fireball (≈10 fm/c) [13].
(VZERO, TZERO, FMD, ZDC, and PMD) for trigger- As discussed here [14], the statistical model is an ef-
ing, multiplicity measurements and centrality determi- fectivemodelandthesmalldeviationsfromtheequilib-
nationareinstalledintheforwardregion. rium picture may simply indicate that the precision of
thedatahasbecomesufficienttorevealitslimitations.
Thetransversemomentumdistributionsofidentified
3. Latestprogressonthestudyofthehotanddense
hadrons contain valuable information about the collec-
matter
tiveexpansionofthesystem(p (cid:46) 2GeV/c),thepres-
T
ence of new hadronization mechanisms like quark re-
Incentralheavyioncollisionsatultrarelativisticen-
combination (2 (cid:46) p (cid:46) 8 GeV/c) [15] and, at larger
ergies it is well established that a strongly interacting T
transverse momenta, the possible modification of the
mediumofquarksandgluonsiscreated. Tolearnabout
theearlystateofthesystem,low p (< 2.2GeV/c)di- fragmentationduetothemedium[16,17]. ALICEhas
T
reported the transverse momentum spectra, as a func-
rect photons are used. A temperature of 304 MeV has
been measured for the 0-40% Pb−Pb collisions [10]. tion of the collision centrality, of charged pions, kaons
and (anti)protons from low (hundreds of MeV/c) [18]
Hence, the system at the LHC is hotter than that pro-
tohigh(20GeV/c)[12] p . ThisisillustratedinFig.2,
ducedatRHIC,whereanearlytemperatureof221MeV T
whereresultsforpp, themostcentralandthemostpe-
wasmeasuredforthe0-20%Au-Aucollisions.Thesys-
ripheralPb−Pbcollisionsareshown.
temcreatedattheLHCisalsodenser,theaveragemul-
tiplicitypernumberofparticipantistwotimesthatmea- FromtheBlast-Waveanalysis[19]appliedtothelow
suredatRHIC[11]. Alsointerestingisthefactthatthe p partofthespectra,theradialflow,(cid:104)β (cid:105),inthemost
T T
samecentralitydependenceisfoundatRHICandLHC. central collisions is found to be ≈ 10% higher than
/NuclearPhysicsBProceedingsSupplement00(2015)1–8 3
104 4 4
-2]) 103 3 ALICE sNN=2.76 TeV 3
c 0-5% Pb-Pb
V/
Ge 10 10 60-80% Pb-Pb 10
( pp (Scaled, N )
) [ coll
y 10-1 -1 -1
d
T
p
dT10-3 -3 -3
p
π
2v 10-5 -5 -5
e
N
/(N 10-7 π+ + π- -7 K+ + K- -7 p + p
2
d
10-9 -9 -9
0 2 4 6 8 1012141618 0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18
p (GeV/c)
ALI-PUB-84484
T
Figure2: Solidmarkersshowtheinvariantyieldsofidentifiedparticlesincentral(circles)andperipheral(squares)Pb−Pbcollisions. Open
pointsshowtheppreferenceyieldsscaledbytheaveragenumberofbinarycollisionsfor0-5%(circles)and60-80%(squares).Thestatisticaland
systematicuncertaintiesareshownasverticalerrorbarsandboxes,respectively.FigurereproducedfromRef.[12].
at RHIC, while the kinetic freeze-out temperature was For p below4GeV/ctheratiobecomeflatwiththede-
T
found to be comparable to that extracted from data at creasingoftheimpactparameter. Thissuggeststhatthe
RHIC, T = 95 MeV [18]. The spectra are well de- mass, and not the number of quark constituents, deter-
kin
scribedbyhydrodynamicmodels,exceptthelow p (< minesthespectralshapeincentralPb−Pbcollisions,this
T
1 GeV/c) proton yield [20, 21, 22, 23]. Models which isinagoodagreementwiththehydrointerpretation.
bestdescribethedataincludehadronicrescatteringwith From the behavior of the particle ratios in both col-
non-negligibleantibaryonannihilation[22,23]. Forin- lidingsystemsonecanestablishthatthesuppressionof
termediatetohigh p (> 3GeV/c),thespectradevelop thethreeparticlespeciesathigh p (>10GeV/c)isthe
T T
thepowerlawtailwhichcharacterizesthehardpartonic samewithinthestatisticalandsystematicuncertainties.
processes. Thissuggeststhatthechemicalcompositionofleading
The particle ratios as a function of p are shown in particles from jets in the medium is similar to that of
T
Fig.3forppandthemostcentralPb−Pbcollisions.The vacuum jets. Another result concerning jet-quenching
proton-to-pionratioincreasesfrom≈ 0.38to≈ 0.8go- is the first indication for the mass dependence of the
ingfromperipheral(60-80%)tocentral(0-5%)Pb−Pb partonenergy-loss,thisbycomparingthenuclearmod-
collisionsat p ≈ 3GeV/c,thendecreasestothevalue ificationfactorofthepromptDmesonstothatfornon-
T
promptJ/ψmeasuredbyCMS.
measuredforvacuumfragmentation(ppcollisions)for
p >10GeV/c.Theresultobtainedforthemostcentral Theellipticflowforidentifiedhadronshasbeenalso
T
collisionsissimilartothatmeasuredatRHIC[24,25]. measured [26] over a broad pT range. Figure 5 shows
The kaon-to-pion ratio also exhibits a bump around v21 as a function of pT for central (10-20%) and semi-
p = 3 GeV/c, this effect is not predicted by quark re- peripheral(40-50%)Pb−Pbcollisions.Goingfromcen-
T
combinationsuggestingthattheactualenhancementof tral to semi-peripheral Pb−Pb collisions v2 increases
thebaryon-to-mesonratioisnotanomalousandinstead as expected due to the eccentricity increase. For pT
it is most likely driven by hydrodynamical flow. This below 2 GeV/c a mass ordering is observed indicat-
pictureistestedbycomparingtheshapesofthe p dis- ing the interplay between elliptic and radial flow. For
T
tributions of φ-meson and protons. The results shown
in Fig. 4 indicate that for central Pb−Pb collisions the
shapesoftheratiostopionsarethesame. Alsoshown 1TheellipticflowcoefficientswereobtainedwiththeScalarProd-
uct method, a two-particle correlation technique, using a pseudo-
istheevolutionwithcollisioncentralityoftheφ-meson rapiditygapof|∆η| >0.9betweentheidentifiedhadronunderstudy
yieldnormalizedtothatforprotonsasafunctionof pT. andthereferenceparticles[26].
/NuclearPhysicsBProceedingsSupplement00(2015)1–8 4
p + p ALICE s =2.76 TeV K+ + K- 0-5% Pb-Pb
π+ + π- NN π+ + π-
1 1
0-5% Pb-Pb Krakow
pp Fries et al.
0.8 0.8
EPOS
o
ati0.6 0.6
R
0.4 0.4
0.2 0.2
0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18
p (GeV/c)
ALI-PUB-84513
T
Figure3: ParticleratiosasafunctionofpTmeasuredinppandthemostcentral,0-5%,Pb-Pbcollisions.Statisticalandsystematicuncertainties
aredisplayedasverticalerrorbarsandboxes,respectively.ThetheoreticalpredictionsrefertoPb−Pbcollisions.FigurereproducedfromRef.[12].
higher p hadron-v ’sseemtobegroupedintobaryons interactions (MPI) [31]. One therefore cannot rule out
T 2
andmesons,theexceptionisthev ofφ-mesons,which alternative explanations, but interestingly, it illustrates
2
for central Pb−Pb collisions follows that for baryons. thatlikelythereisastrongcouplingofthisphenomenon
This observation indicates that the behavior of v is totheunderlyingeventalsoinppcollisions.
2
driven by the hadron mass and not for the number of Theevolutionofthespectralshapeswithmultiplicity
quarkconstituents. ALICEhasalsoreportedtheviola- is studied using the the blast-wave analysis. Figure 6
tionofthescalingofv withthenumberofconstituent (left)showsthataqualitativelysimilarbehaviorforT
2 kin
quarks, such a observation is also against the scenario vs. (cid:104)β (cid:105)isobtainedforthethreesystems(pp,p-Pband
T
withquarkrecombination/coalescence. Pb−Pb)eveninppeventssimulatedwithPythia8. To
study the effect on the p spectra directly, the proton-
T
to-pion ratio was constructed, the results for p-Pb and
4. Featuresofthe“cold”matter
Pb−PbcollisionsarepresentedinFig.6(right)fortwo
Surprisingly, early results from LHC showed that p- extreme multiplicity intervals. For p below (above) 2
T
Pbcollisionsexhibitbehaviorsreminiscenttothosedue GeV/ctheratiosexhibitadepletion(enhancement)go-
to final state effects, namely, hints of collective effects ingfromlowtohighmultiplicity. Thehighest(lowest)
(radialandellipticflow,ridgestructure),butnosignof multiplicityintervalsgiveratioswhichreachmaximaat
jetquenching [27,28]. p ≈ 3 GeV/c amounting to ≈0.4 and ≈ 0.8 (≈ 0.28
T
Forp-Pbcollisionsthetransversemomentumspectra and≈0.38)inp-PbandPb−Pbcollisions,respectively.
ofchargedpions,kaonsand(anti)protonsasafunction Above 3 GeV/c, the ratios start to decrease down to
ofthemultiplicitymeasuredintheVZERO-Adetector ≈0.1at p ≈10GeV/c,whichaccordingto[12]corre-
T
havebeenreportedfromhundredsofMeV/c[28]upto spondstothevaluemeasuredforvacuumfragmentation
15GeV/c[29]. Athighmultiplicitythelow p partsof (ppcollisions).
T
thespectraarebetterdescribedbymodelswhichincor- Figure7showstheaveragep forK∗0,protonsandφ-
T
poratehydro[28].Alsointriguingisthefactthatasimi- mesonsasafunctionofthesystemsize. Themeasure-
lareffecthasbeenobservedinppcollisions,inthatcase mentswereperformedforpp,p-PbandPb−Pbcollisions
√
the multiplicity was determined at mid-rapidity. The at s =7,5.02and2.76TeV,respectively. The(cid:104)p (cid:105)
NN T
featureisalsopresentinPythia8tune4C[30],thisbe- for MB pp collisions fits well with the behavior of the
haviorisaconsequenceoftheinteractionsamongfinal p-Pbresults. Actually,forinclusivechargedparticlesa
partonscomingfromindependentsemi-hardscatterings similareffectisseen,namely,theaverage p forppand
T
whichincreaseswithincreasingnumberofmulti-parton p-Pbdataagreewhentheeventmultiplicity(|η|<0.3)is
/NuclearPhysicsBProceedingsSupplement00(2015)1–8 5
)/pφ10 ALICE 0 10% ALICE 10 20% Pb Pb sNN = 2.76 TeV
+ 20 40%
p 0.4 π± K±
( 8 60 80% K0s p+p
6 80 90% 0.9} 0.3 Ξφ +Ξ+ ΛΩ+ +ΛΩ+
>
4 | η 0.2
∆
P,|
2 Pb Pb s = 2.76 TeV v{S2 0.1
NN
0 0
0 1 2 3 4 5
p (GeV/c) 0 2 4 6
ALI−PUB−67447 T p (GeV/c)
o π 1 ALICE T
s t ALI−PUB−82653
atio ALICE 40 50% Pb Pb sNN = 2.76 TeV
R 0.4 π± K±
10 1 Centrality 0 10% K0s p+p
pp//ππ (rebinned) 0.9} 0.3 Ξφ +Ξ+ ΛΩ+ +ΛΩ+
>
φ/π ×4.8 | η 0.2
∆
10 2 Pb Pb sNN = 2.76 TeV SP,| 0.1
0 1 2 3 4 5 v{2
p (GeV/c)
ALI−PUB−67452 T 0
0 2 4 6
Figure 4: Ratio p/φ as a function of pT for Pb−Pb collisions at p (GeV/c)
2.76 TeV for four centrality intervals. The statistical uncertainties T
are shown as bars and the total systematic uncertainties (including ALI−PUB−82660
pT-uncorrelatedand pT-correlatedcomponents)areshownasboxes
(top).Ratiosofprotonandφ-mesonyieldstochargedpionyieldasa Figure5:Ellipticflowcoefficient(v2)ofidentifiedhadronsasafunc-
functionof pT forcentralPb−Pbcollisionsat2.76TeV.Inorderto tionofpTmeasuredforcentral(top)andperipheral(bottom)Pb−Pb
showthesimilarityoftheshapesofthetworatiosforpT <3GeV/c, collisions.
theφ/πratiohasbeenscaledsothattheφ-mesonandprotonintegrated
yieldsareidentical(bottom).
liptic flow coefficient, right panel of Fig.8 shows that
the p differential v exhibits a mass ordering and a
T 2
below ∼ 20, and for higher multiplicities the strongest crossing for p around 1.5-2 GeV/c, again this feature
T
rise of (cid:104)pT(cid:105) is observed for pp data[32]. For Pb−Pb iswellknowfromheavyioncollisions. Theresultsare
collisionstheriseof(cid:104)pT(cid:105)withtheeventmultiplicityis reportedinthispaper[27]. Tounderstandthepotential
weakerthanforsmallsystems. Thiscouldindicatethat roleofMPIinp-Pbcollisionstheminijetproductionas
toreachthehighmultiplicityinsmallsystems,thepar- a function of the event multiplicity was studied. In pp
tonicprocessesneedtobeharder. datathisanalysiswassuccessfullyapplied.Throughthe
Another interesting result is shown in Fig. 8, where measurement of the so-called number of uncorrelated
themultiplicitydependenceoftheproton-to-φratioasa seeds,whichinPythiaaredirectlyrelatedwiththenum-
functionof p isshown. Thep-Pbresultsarecompared berofMPIs,itwasobservedadeviationofthisquantity
T
totheratiosmeasuredinperipheralandcentralPb−Pb withrespecttothelinearbehavior,thiswasinterpreted
collisions. The ratio for high multiplicity p-Pb events asanindicationofthelimitationofthenumberofMPIs
exhibitsaflatteningfor p below1.5GeV/c, whilefor toproducehighmultiplicityevents[33]. Contrary,inp-
T
higher p the ratio decreases in the same amount as Pbdataaclearlinearbehaviorathighmultiplicitywas
T
those measured for low multiplicity p-Pb and the most observed, this could indicate the presence of MPIs in
peripheralPb−Pbcollisions. Thebehavioroftheratio thiscolliding-systems[34].
inhighmultiplicityp-Pbdatacouldbeinterpretedasa To close this section it is important to mention the
hintoftheonsetofcollectivebehavior. first measurements which were published at the LHC
Another feature of the p-Pb data is the non-zero el- for high multiplicity pp data. The CMS Collaboration
/NuclearPhysicsBProceedingsSupplement00(2015)1–8 6
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V) 0.2
T (Gekin 000...111468 π ) 0.81 PAreLlimICinEarypPpTL P B<b 7063,2 0.058 s %8,G N20eN5% V =3/ c85 .(02201 T4e)V 0.81 PPLbB P760b30 56, 8%, 1s0N9%N6 =2 027.7 (62 0T1e4V)
0.12 π+ + 0.6 0.6
0.00.81 AVL0IAC ME,u plt ipPlbic,i tys CNNla =s s5e.0s2 ( PTebV side) )/( p+ 0.4 V0A Multiplicity Classes (Pb side) 0.4
ALICE, Pb Pb, s = 2.76 TeV p
0.06 ALICE, pp, s = 7N TNeV ( 0.2 0.2
0.04 PYTHIA8, s = 7 TeV (with Color Reconnection)
PYTHIA8, s = 7 TeV (without Color Reconnection)
0.02
0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 2 4 6 8 10 12 14 2 4 6 8 10 12 14
〈β〉 p (GeV/c)
ALI−DER−58133 T ALI−DER−87391 T
Figure6: Comparisonoftheresultsfromtheblast-waveanalysisappliedtoallavailablesystems: pp,p-PbandPb−Pbcollisions. Thespectral
shapeanalysiswasalsoappliedtoPythia8events. Charged-particlemultiplicityincreasesfromlefttoright. (Right.) Proton-to-pionratioasa
√
functionofpTmeasuredinp-PbandPb−Pbcollisionsat sNN=5.02and2.76TeV,respectively.Thesystematicandstatisticaluncertaintiesare
plottedasboxesanderrorbars,respectively.
2 2 2
ALICE, pp 7 TeV
1.8 1.8 1.8
ALICE, Pb Pb 2.76 TeV
ALICE preliminary, p Pb 5.02 TeV
1.6 V0A Multiplicity Event Classes (Pb sid1e.)6 1.6
)
c
V/ 1.4 1.4 1.4
e
G
(
〉 1.2 1.2 1.2
T
p
〈
1 1 1
0.8 K*0 0.8 p 0.8 φ
0.6 0.6 0.6
1 10 1 10 1 10
〈dN /dη 〉1/3 〈dN /dη 〉1/3 〈dN /dη 〉1/3
ALI−PREL−84361ch lab|ηlab| < 0.5 ch lab|ηlab| < 0.5 ch lab|ηlab| < 0.5
Figure7: Meantransversemomentumofresonancescomparedtothatofprotoninthethreedifferentcollisionsystems,asafunctionofthesystem
size.
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1124 ApV0 0LP5AI%bC MEs uPNltNripe =llii cm5it.yi0n 2Ea vTryeenVt, C 0la.5s s<e sy (<P b0 Side) PC, sub} 00.2.25 p( 0P 2b0 %sA)N L N I(=C6 0E5 .10020 T%e)V |∆η| > 0.8 (Near side only)
60 80% 2
10 ParbX Pivb:1 4s0N4N. 0=4 925.76 TeV, |y| < 0.5 {v2 h π
)/pφ 8 08 01 09%0% 0.15 K p
+
p
( 6 0.1
4
0.05
2
Uncertainties: stat. (bars), sys. (boxes)
0 0
0 1 2 3 4 5 6 0.5 1 1.5 2 2.5 3 3.5 4
p (GeV/c) p (GeV/c)
ALI−PREL−71209 T ALI−PUB−52116 T
Figure8: (Left). Ratiosofprotonyieldstoφ-mesonyieldsvs pT inp-Pbcollisionat5.02TeVandcomparisonwithPb−Pb. (Right). The
Fouriercoefficientv2 forhadrons(blacksquares),pions(redtriangles),kaons(greenstars)andprotons(bluecircles)asafunctionof pT from
thecorrelationinthe0-20%multiplicityclassaftersubtractionofthecorrelationfromthe60-100%multiplicityclass. Errorbarsshowstatistical
uncertaintieswhileshadedareasdenotesystematicuncertainties.
√
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