Nuclear Modification Factor of $D^0$ Meson in Au+Au Collisions at $\sqrt{s_{NN}}$ = 200 GeV

0.32 MB·

by  Guannan Xie

Nuclear
Physics A
NuclearPhysicsA00(2016)1–4
www.elsevier.com/locate/procedia
Nuclear Modification Factor of D0 Meson in Au+Au
√
=
Collisions at s 200 GeV
NN
Guannan Xie (for the STAR Collaboration)1
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1 LawrenceBerkeleyNationalLaboratory,Berkeley,CA94720,USA
0
UniversityofScienceandTechnologyofChina,Hefei,230026,China
2

n
a
J

4
Abstract

]
x Heavy-flavorquarksaredominantlyproducedininitialhardscatteringprocessesandexperiencethewholeevolutionof
e thesysteminheavy-ioncollisionsatRHICenergies. Thustheyaresuggestedtobeanexcellentprobetothemedium
-
l properties through their interaction with the medium. In this proceedings, we report our first measurement of D0
c
productionviatopologicalreconstructionusingSTAR’srecentlyinstalledHeavyFlavorTracker(HFT).Wealsoreport
u √
our new measurement of Nuclear Modification Factor (R ) of D0 mesons in central Au+Au collisions at s =
n AA NN
200GeVasafunctionoftransversemomentum(p ). Newresultsconfirmthestrongsuppressionathigh p witha
[ T T
muchimprovedprecision,andshowthattheR athigh p arecomparablewithlighthadrons(π)andwithDmeson
AA T
1 measurements at the LHC. Furthermore, several theoretical calculations are compared to our data, and with charm
v diffusioncoefficient2πTD ∼2-12canreproduceboththeD0R andv datainAu+AucollisionsatRHIC.
S AA 2
5
9 Keywords: Quark-gluonplasma,Nuclearmodificationfactor,HeavyFlavorTracker
6
0
1. Introduction
0
.
1 Themassofcharmquarkissignificantlylargerthanthoseoflightquarks,Λ ,andtheQGPtempera-
QCD
0 tureatRHICenergies(m >> m ,Λ ,T ). Thereforecharmquarksaredominantlyproduced
6 c u,d,s QCD QGP(RHIC)
intheearlystageofthecollisioninhardscatteringprocessesatRHIC.Theyexperiencethewholeevolution
1
: of the system and offer unique information for the study of hot and dense strongly-coupled Quark-Gluon
v
Plasma(sQGP)matter.
i
X Charmproductionhasbeensystematicallymeasuredin p+p(p)collisionsinvariousexperiments. Due
r tothelargequarkmass,charmproductionin p+ pcollisionsisexpectedtobecalculablewithagoodpre-
a cisioninperturbativeQCD.Figure1(left)showsthecharmdifferentialcross-sectionatmidrapidityversus
√
transverse momentum in p + p collisions at s = 200 GeV-7 TeV [1, 2, 3, 4]. Experimental data are
compared with Fixed-Order Next-to-Leading-Log (FONLL) pQCD calculations shown as grey bands [5].
Within uncertainties, FONLL pQCD calculations describe the data over a broad range of collision ener-
gies. The precision of the experimental data allows to constrain the theoretical uncertainty in the pQCD
calculations.
1AlistofmembersoftheSTARCollaborationandacknowledgmentscanbefoundattheendofthisissue.
2 /NuclearPhysicsA00(2016)1–4
AtRHICenergies,charmquarksareproducedmostlyviainitialhardscatterings. Thishasbeenproved
by charm total cross sections measured from different collision systems. Figure 1 right panel shows that,
charmtotalcrosssectionfollowsanumber-of-binary-collision(N )scaling[1,5,6,7,9].
bin
ThemodificationofthecharmedmesonproductionisquantifiedwithaNuclearmodificationfactorR .
AA
R iscalculatedastheratiobetweentheD0invariantyieldinAu+Aucollisionstothe p+pdatascaledby
AA
N . OurpreviousresultshowsthattheD0R asafunctionof p issignificantlydifferentfromunity[7].
bin AA T
400
d+Au S = 200 GeV
350 (D0+e) NN
Sys. error
300
NLO err.
p+p
NNdb) (/dy|σµccy=0122505000 (D0+Dr*u)n12 Au+Au (D0)
100 run9
FONLL in p+p
STARPreliminary 50 STARPreliminary FONLL err.
0
1 10 102 103
number of binary collisions N
bin
√
Fig.1. (Left)Charmquarkpairproductioncrosssectionvs. pT atmid-rapidityin p+p(p)collisionsat s = 200GeV-7TeV.
FONLLpQCDcalculationsareshownasshadedbands[5].(Right)Charmcrosssectionatmid-rapidityfromp+ptocentralAu+Au
collisionsfromSTAR.
2. ExperimentandAnalysis
TheSTARexperimentisalarge-acceptancemulti-purposedetectorwhichcoversfullazimuthandpseu-
dorapidityof|η|<1. Inthisanalysis,thedataweretakenbytheSTARexperimentusingthenewlyinstalled
Heavy Flavor Tracker (HFT) in the year 2014 RHIC run. The HFT is a high resolution silicon detec-
tor, which consists of three subsystems: two conventional strip detectors and one pixel detector using the
state-of-the-artMonolithicActivePixelSensors(MAPS)technology[10]. TheHFTisdesignedtogreatly
improveopenheavyflavorhadronmeasurementsbythetopologicalreconstructionofsecondarydecayver-
tices.
The data sample used in this analysis is ∼780M minimum bias events taken in Au+Au collisions at
√
s = 200 GeV with the HFT. Events are required to have reconstructed collision position within 6 cm
NN
along the beam direction from the detector center. D0 and D0 are reconstructed in the hadronic K∓π±
channel, withabranchingratioof∼ 3.9%andalifetimeofcτ ∼ 123µm. Kaonsandpionsareidentified
viaacombinationoftheenergylossdE/dxmeasuredbytheTimeProjectionChamberandβmeasuredby
the Time-Of-Flight detector [8]. Secondary vertices are reconstructed as the middle point at the Distance
oftheClosestApproach(DCA)betweenthetrajectoriesoftwodaughterparticles. WiththeHFT,several
topologicalcutsareusedtogreatlyreducethecombinationalbackground. Rectangulartopologicalcutsare
optimized in each D0 p bins using the Toolkit for Multivariate Data Analysis (TMVA) package for best
T
significanceofD0signal.
Figure2showstheD0invariantmassdistributionsintwo p bins. Comparedwithourformerpublished
T
result,the D0 signalsignificancescaledtothesamenumberofeventshasbeenimprovedbyafactorof4.
The remaining combinatorial background is estimated with like-sign and mixed-event methods. The TPC
efficiency is calculated by embedding simulated tracks into real event background. The HFT related and
topologycutefficienciesarefrom D0 decaysimulationbasedontheHFTmatchingtoTPCratioandtrack
pointingresolutiondirectlyfromdata. Theresultsfor p <2GeV/carebeingfinalized, andweshowthe
T
resultsbelowforD0 p >2GeV/c.
T
/NuclearPhysicsA00(2016)1–4 3
220
30 Au+Au 200GeV, 0 80% MB Au+Au 200GeV, 0 80% MB
200
3]0 1.0 < pT < 2.0 GeV/c 180 5.0 < pT < 8.0 GeV/c
×21) [V/c 2205 uluiknnellii,kk esea,, msmaeixm eeedv t eevvtt 2)MeV/c114600 #DS0/ =S 4+1B7= 1±4 2.49 uluiknnellii,kk esea,, msmaeixm eeedv t eevvtt
unts/(10 Me 1105 #DS0/ =S 8+0B3=82 2±. 3360 Counts/(10 1168020000
Co 5STARPreliminary 40 STARPreliminary
20
10.7 1.75 1.8 1.85 1.9 1.95 2 2.05 2.1 10.7 1.75 1.8 1.85 1.9 1.95 2 2.05 2.1
MKπ (GeV/c2) MKπ (GeV/c2)
Fig.2.InvariantmassspectraofKπpairsfor1<pT <2GeV/c(Left)and5<pT <8GeV/c(Right).Theblackpointsareunlike-sign
pairswiththeD0signal.Theblueandredpointsshowthelike-signandmixed-eventbackgroundrespectively.
3. PhysicsResultsandDiscussion
Although the total charm cross section follows N scaling, the p spectrum is significantly modified
bin T
in Au+Au collisions [7]. D0 invariant yield (left) and nuclear modification factor R (right) in the most
√ AA
centralAu+Aucollisionsat s =200GeVareshowninFigure3. ThenewD0invariantyieldsfromthe
NN
HFT(blue)areconsistentwithpublisheddata(red)withsignificantlyimprovedprecision[7].
π 22]dy) [(GeV/c)dppN/(2dTT1111100000−−−−−54321 STAR D0 @ Au+Au 200 GeV Au+ADDu00 22000 111004%/ 1[/12 0[/]20] RAA 1.521 Au+Au → D0 @ 200 GeV 0 10% pπDDD+ 00 0p0 22 1u0012n110%c04%e/ 1 rSAt1.TLAICRE
10−6
p+p D0+D* 0.5
10−7 STApR+p LPevry esclailmed biyn <aNbriny> STARPreliminary
10−80 1 2 3 4 5 6 7 8 00 2 4 6 8
pT(GeV/c) pT (GeV/c)
√
Fig.3.D0mesoninvariantyields(Left)andnuclearmodificationfacto√rRAA(Right)inmostcentralAu+Aucollisionsat sNN=200
GeV.AlsoshownintheleftpanelareD0yieldsinp+pcollisionsat sNN=200GeVfittedwithaLevyfunction.Intherightpanel,
verticalbarsondatapointsindicatestatisticaluncertainties,whilethebracketsaresystematicuncertaintiesinAu+Aucollisions.The
greybandsareuncertaintiesfromp+pbaseline.RAAofDmesonsfromLHCandπfromRHICarealsoshown.Therearetwovertical
boxesaroundunityfromlefttorightrelatedtoAu+AuNcollandp+pnormalizationuncertainties.
Figure3(rightpanel)showstheR resultsfromthemostcentral(0-10%)Au+Aucollisions. Thenew
AA
resultsfromtheHFTareconsistentwiththepublishedonesinthemeasured p range. Furthermore, they
T
have highly improved precision in Au+Au collisions. The grey bands show uncertainties from the p+ p
baselinefromourpreviousmeasurementbeforetheHFTinstallation[1],whichisexpectedtobeimproved
aswellwiththeHFTdatatakenintheyear2015RHICrun. TheR showsastrongsuppressionathigh p
AA T
indicatingstrongcharmmediuminteractionsatthiskinematicregion. Attheintermediate p range(∼0.7-2
T
GeV/c),thedatashowsanenhancementwhichcanbedescribedbymodelsincludingcoalescenceofcharm
andlightquarks.
WecompareourD0 R resultswiththoseofpionsatRHIC(redsquares)andD-mesonsatLHC(blue
AA
triangles)[1,7,11,12].AsshowninFigure3,athighp R (D0)isclosetoR (π),whichcanbeexplained
T AA AA
bytakingintoaccountthefactthatcharmenergylossisaninterplayofelasticandradiativeenergyloss[14].
The D-meson R at p > 2 GeV/c is also comparable between RHIC and LHC despite of a factor of 14
AA T
differenceincollisionenergy.
The left panel of Fig. 4 shows the R results from the most central (0-10%) Au+Au collisions com-
AA
paredwithvariousmodelcalculations. TheDukemodelusesaLangevinsimulationwithaninputdiffusion
coefficientparameter-2πTD = 7,where D isheavyquarkspacialdiffusioncoefficientandTismedium
S S
4 /NuclearPhysicsA00(2016)1–4
1.8 Au+Au 200GeV, 0-10% 0.3
D0 2014 Au+Au 200GeV, 0-80% Non-flow est.
1.6 D02010/11 0.25 D0
TAMU TAMU
11..24 SDUukBeATECH 0.2 SDUukBeATECH
RAA 1 p+puncert. v20.15
0.8 0.1
0.6
0.05
0.4
0.2 0
STAR Preliminary STAR Preliminary
00 1 2 3 4 5 6 7 8 0.050 1 2 3 4 5 6 7
Transverse Momentum p (GeV/c) Transverse Momentum p (GeV/c)
T T
√
Fig.4.D0mesonRAA(left)andv2(right)vs.pTinAu+Aucollisionsat sNN=200GeVcomparingwithvariousmodelcalculations.
temperature,whichwastunedtotheLHCD-mesonR data[13,14]. TheTAMUcalculationusesanon-
AA
perturbativeapproachandthefullT-matrixcalculationwithinternalenergypotential,whichpredicts2πTD
S
tobe∼2-10[14]. TheSUBATECHgroupusesthepQCDcalculationwithHardThermalLooptechnique
whichindicatesthe2πTD ∼2-4[14].ThesethreemodelscandescribeourR datapointsreasonablewell.
S AA
Inthemeantime,wealsocompareourfirstmeasurementof D0 v tomodelcalculations[10]. TheTAMU
2
andSUBATECHcalculationscandescribethemeasuredD0v aswell,whilethespecificDUKEcalculation
2
with2πTD =7seemstounderestimatetheD0v .Ourdatafavormodelswithfinitecharmflow.Tofurther
S 2
constrainourunderstandingofthemediumdiffusioncoefficient,itwillbebeneficialtosystematicallystudy
eachingredientindifferentmodelcalculations.
4. SummaryandOutlook
√
Wereportthefirstmeasurementof D0 R inthemostcentralAu+Aucollisionsat s =200GeV
AA NN
usingtheSTARnewlyinstalledHFTdetector. Newresultsconfirmthestrongsuppressionathigh p witha
T
muchimprovedprecision.Theoreticalmodelswithcharmdiffusioncoefficient2πTD ∼2-12canreproduce
S
simultaneouslyboththeD0R andv datainAu+AucollisionsatRHIC.
AA 2
Inyear2015,STARhascollectedhighstatisticsdatasetswiththeHFTin p+pand p+Aucollisionsat
√
s =200GeV.Analysesofthesedatasetswillimproveourp+pbaselinefortheR estimationandhelp
NN AA
understand cold nuclear matter effects. In addition, STAR has requested to collect 2 billion MB Au+Au
eventsintheyear2016runwhichwillallowmoreprecisedeterminationofthesQGPtransportproperties.
Acknowledgement
WeexpressgratitudetoRNCgroupatLBNLandHEPGatUSTCfortheirsupport. FromUSTC,the
authorissupportedinpartbytheNSFCunderGrantNo. 11375172andMOSTunderNo. 2014CB845400.
References
[1] L.Adamczyketal.(STARCollaboration),Phys.Rev.D86,072013(2012).
[2] Z.Ye,QM14(STARCollaboration),Nucl.Rhys.A931,520(2014).
[3] D.Acostaetal.(CDFCollaboration),Phys.Rev.L91,241804(2003).
[4] B.Abelevetal.(ALICECollaboration),Jour.ofHighEnergyPhys.01,128(2012).
[5] M.Cacciarietal.,Phys.Rev.L95,122001(2005).
[6] L.Adamczyketal.(STARCollaboration),Phys.Rev.L94,62301(2005).
[7] L.Adamczyketal.(STARCollaboration),Phys.Rev.L113,142301(2014).
[8] Ackermannetal.Nucl.Instrum.Meth.A499624-632(2003).
[9] R.Vogt,Eur.Phys.J.ST155,213(2008).
[10] G.Contin,M.Lomnitz,QM15(STARCollaboration)
[11] L.Adamczyketal.(STARCollaboration),Phys.Lett.B655,104-113(2007)
[12] B.Abelevetal.(ALICECollaboration),arXiv:1509.06888[nucl-ex]
[13] S.Caoetal.,Phys.Rev.C92,024907(2015)
[14] A.Andronicetal.,arXiv:1506.03981[nucl-ex]