Measurement of D-meson azimuthal anisotropy in Au+Au 200 GeV collisions at RHIC

0.27 MB·

by  Michael R. Lomnitz

Nuclear
Physics A
NuclearPhysicsA00(2016)1–4
www.elsevier.com/locate/procedia
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Measurement of D-meson azimuthal anisotropy in Au Au 200
GeV collisions at RHIC
Michael Lomnitz, for the STAR collaboration
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LawrenceBerkeleyNationalLaboratory,OneCyclotronRoad,MS70R0319,BerkeleyCA,94720
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KentStateUniversity,PhysicsDepartment,800E.SummitSt.,Kent,OH44240
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a
J

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Abstract

]
x Heavyquarksareproducedthroughinitialhardscatteringsandtheyareaffectedbythehotanddensemediumcreatedin
e
heavy-ioncollisionsthroughoutitswholeevolution. Duetotheirheavymass,charmquarksareexpectedtothermalize
-
p muchmoreslowlythanlightflavorquarks. Thecharmquarkflowisauniquetooltostudytheextentofthermalization
e ofthebulkmediumdominatedbylightquarksandgluons.Athighp ,Dmesonazimuthalanisotropyissensitivetothe
T
h pathlengthdependenceofcharmquarkenergylossinthemedium,whichoffersnewinsightsintoheavyquarkenergy
[ lossmechanisms-gluonradiationvs.collisionalprocesses.
√
WepresenttheSTARmeasurementofellipticflow(v )ofD0andD±mesonsinAu+Aucollisionsat s =200GeV,
1 2 NN
v forawidetransversemomentumrange.Theseresultsareobtainedfromthedatatakeninthefirstyearofphysicsrunning
3 ofthenewSTARHeavyFlavorTrackerdetector,whichgreatlyimprovesopenheavyflavorhadronmeasurementsbythe
4 topologicalreconstructionofsecondarydecayvertices. TheDmesonv2isfinitefor pT >2GeV/c2andsystematically
7 belowthemeasurementoflightparticlespeciesatthesameenergy.Comparisontoaseriesofmodelcalculationsfavors
0 scenarioswherecharmflowswiththemediumandisusedtoinferarangeforthecharmdiffusioncoefficient2πTD .
s
0
. Keywords: Quark-gluonplasma,ellipticflow,HeavyFlavorTracker
1
0
6 1. Introduction
1
: Heavy flavor quarks are suggested to be an excellent probe to study the strongly coupled quark-gluon
v
i plasma (sQGP) as they are produced early in heavy-ion collisions through hard scattering processes and
X
experience the full evolution of the system, while their large masses are mostly unaffected by the QCD
r medium. Furthermore,drawingananalogytoBrownianmotiontheheavyquarkspropagatingthroughthe
a
medium are sensitive to the sQGP transport properties, for example 2πTD , which is given in term of the
s
temperatureT andthecharmdiffusioncoefficientD [1].
s
Recent measurements at RHIC and LHC show that high p charmed hadron yields in central collisions
T
are considerably suppressed suggesting strong interactions, while the elliptic flow v measured at LHC is
2
comparabletothatoflighthadrons[2,3,4]. AtRHIC,theenhancementinthenuclearmodificationfactor
atintermediate p issuggestiveofbothcharmflowandproductionviacoalescence. However,charmflow
T
inferredfrommeasurementsofsemi-leptonicdecayssufferfromlargeuncertainties.Aprecisemeasurement
2 /NuclearPhysicsA00(2016)1–4
ofv overabroadp range,andinparticularatlowmomenta,willprovideusefulinsightsintotheproperties
2 T
ofsQGPmedium.
2. Experimentalsetup
The data used in this analysis were recorded in year 2014 by the STAR experiment at the Relativistic
HeavyionCollider(RHIC)inBrookhavenNationalLaboratory,USA.TheSTARexperimentpossessesfull
azimuthalcoverageatmid-rapidityusingtheTimeProjectionChamber(TPC)toreconstructtracksinside
a uniform 0.5 T magnetic field. The entire Heavy Flavor Tracker (HFT) micro-vertexing detector was in-
cludedforthefirsttimein2014andgreatlyimprovedSTAR’strackingresolution,providingtrackpointing
resolutionoflessthan50µmforkaonswith p =750MeV/c.
√ T
About 780 million Au+Au collisions at s = 200 GeV events recorded with a Minimum Bias (MB)
NN
triggerin2014wereanalyzedtoreconstructcharmedhadrons. Acutonthereconstructedcollisionposition
alongthebeamline(|primaryvertex|<6cm)isappliedtoensuregooddetectoracceptance.
D-mesonsarereconstructedinthehadronicchannels:
D0(D0)→ K∓π±,cτ∼120µmB.R.3.9% D± → K∓2π±,cτ∼300µmB.R.9.1%
Daughtertracksarerequiredtohaveaminimumof20hitsintheTPCandhitsinallthreelayersofthe
HFT, p > 0.6 GeV/c and pseudorapidity |η| < 1. Particle identification is done using energy loss dE/dx
T
fromtheTPC,selectingcandidateswithin2to3standarddeviationsfromtheexpectedvalueandisfurther
enhancedbyuseoftheTimeofFlightdetector(TOF)whenavailable. 1/βisestimatedfromthemomentum
andtimingfromTOF,andisrequiredtobelessthan0.03fromtheexpectedvalue.
Once daughter candidates have been identified, the decay vertex can be reconstructed, which is displaced
fromtheprimaryvertexofcollision. Inthecaseoftwobodydecaysthedecayisreconstructedatthemid
pointontheirdistanceofclosestapproach(DCA).Forthreebodydecays,suchasD±,theaveragebetween
themidpointsofpairwiseDCA’sistakenasthedecayposition.
Thecombinatorialbackgroundcanbegreatlysuppressedbycuttingonthefollowingtopologicalvariables:
decaylength(distancebetweenprimaryanddecayvertices),DCAbetweendaughtertracks,DCAbetween
reconstructedparentandtheprimaryvertex(PV),DCAbetweendaughtertracksandthePV;andinthecase
ofD±thedistancesbetweenthemidpointsfromeachpairwisecombination.
3. Azimuthalanisotropy
OnceD-mesoncandidateshavebeenselected,thesecondorderazimuthalanisotropy,v ,isstudiedus-
2
ingtwodifferentmethods: theeventplanemethodandthetwoparticlecorrelationmethod, whichwillbe
discussedbrieflyinthefollowingparagraphs.
Intheeventplanemethodthesecondordereventplane,Ψ,isreconstructedfromTPCtracksandcorrected
forthenon-uniformdetectorefficiency[5]. Inordertoreducethenon-flowcontributionsfromothertwoor
multi-body correlations, a relative pseudorapidity gap |∆η| ≤ 0.15 around D0 candidates is excluded from
theeventplanereconstruction.TheazimuthaldistributionofDmesonswithrespecttotheeventplaneφ−Ψ
isthenobtainedandweightedby1/(cid:15)/R,theinverseoftheD0reconstructionefficiency(cid:15)andtheeventplane
resolutionR[6]foreachcentrality.
Ineachφ−Ψbinthemixed-eventbackgroundisscaledtothelike-signbackgroundandsubtractedfromthe
unlike-signinvariantmassspectrum. The D0 yieldisobtainedbyeitherthefitorsidebandmethod: atlow
p theinvariantmassspectrumisfittedwithaGaussian,representingthesignal,andafirstorderpolynomial
T
describingthecorrelatedbackground;forthelast p bin(5-10GeV/c)thefitislimitedbylowbackground
T
statistics,andtheD0yieldisobtainedbysubtractingscaledcountsintwoinvariantmassregionsaroundthe
signalregion.
Theobservedvobsisthenobtainedbyfittingtheyieldversusφ−ΨwiththefunctionalformA(1+2v cos(2(φ−
2 2
Ψ))). Finally,theobservedvobsiscorrectedfortheaverageevenplaneresolution<1/R>toobtainthetrue
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/NuclearPhysicsA00(2016)1–4 3
0.3
850 a) A3u <+ Apu <2 040 GGeeVV/,c 0-80% 0.01 b) Au+Au 200GeV, 0-80% 0.25c) AuD+0A EuP 200GeV, 0-80% Non-flow est.
d800 vo2bTs = 0.080 – 0.023 FBoarcekggrroouunndd 0.2 DD0– vE2P{2}
el750 >
Weighted yi675000 fD<cos(2) v20.01.51
0.05
600 0
0
550STAR preliminary STAR preliminary STAR preliminary
-0.05
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1 2 3 4 5 6 0 1 2 3 4 5 6 7
f -Y Transverse Momentum p (GeV/c) Transverse Momentum p (GeV/c)
T T
Fig.1: a)WeightedD0yieldvs. φ−Ψfor0-80%centralcollisionsand p ∈[3,4]GeV/c. b)<cos(2∆φ)>
T
between(un)like-signpairsandchargedparticlesasafunctionof p . c)Comparisonbetweenv fromevent
T 2
planeandtwoparticlecorrelationmethods.
valueofv . Figure1ashowstheweightedyieldasafunctionoftheanglerelativetotheeventplaneφ−Ψ
2
forD0candidateswith3< p <4GeV/c.
T
Inthetwoparticlecorrelationmethod,theaverageD0-hadroncorrelationiscalculatedVD−h ≡<cos(2φ −
2 D
2φ ) >. Assuming that the D0 and hadron have no correlation other than with the event plane, it can be
h
shownthatVD−h =vDvh. Finally,togetherwiththehadron-hadroncorrelation,Vh1−h2 =(vh)2,theD-meson
2 2 2 2 2
vD =< cos(2φ −2φ ) > /(< cos(2φ −2φ ) >)1/2 canbeobtained. D0 backgroundisestimatedbythe
2 D h h1 h2
averageofthesidebands(bothlike-signandunlike-sign)aswellastheD0like-signinvariantmassspectrum
in the signal range. Figure 1b shows V for D0 candidates and background (within the D0 invariant mass
2
range)asafunctionoftransversemomentum.
Asinthecaseoftheeventplanemethod,thecontributionfromnon-flowissuppressedbyintroducingagap
|∆η|>0.2inthemeasurementofD−hcorrelations,andparticlesinoppositesidesoftheTPC(h ∈η<0
1
andh ∈η>0)areusedforthehadron-hadroncorrelation.
2
Due to limited statistics, D± yield is obtained in and out-of plane and the v is obtained following the
2
eventplanemethod. Figure1cshowstheazimuthalanisotropyparameterforboth D0 and D± aswellasa
comparisonbetweenbothmethods. Theeventplaneandtwoparticlecorrelationresultsshowgoodoverall
agreementwithinsystematicuncertainties,andtheD± v isconsistent,withinlargeuncertainties,withthat
2
ofD0.
Theremainingcontributionofnon-floweffectstothemeasuredv isestimatedbyscalingthenon-flowin
2
p+pcollisionstoAu+Au[7].Inthiscasethenon-flowcontributioncanbewrittenas(cid:104)(cid:80) cos(2(φ −φ ))(cid:105)/Mv ,
i D0 h 2
wherethesumiisdoneovernearsidechargedhadronsinp+p, M andv aretheaveragemultiplicityand
2
chargedhadronv inAu+Au.
2
Figure 2a shows the v for D-mesons compared with other particle species. The D0 azimuthal anisotropy
2
is significantly different from zero for p > 2 GeV/c and is systematically lower than that of the lighter
T
hadrons[8]intherangefor1 < p < 4GeV/c,buttheextentofheavyflavorthermalizationisstillunder
T
investigation. In figure 2b the results for D0 are compared to four theoretical models. The calculation by
theSUBATECHgroup[9]employspQCDwithHardThermalLoopapproximationforsoftcollisionsand
agrees with the measurement over the whole p range. The TAMU model [10] uses a non-perturbative
T
T-matrixapproachassumingthetwo-bodyinteractionscanbedescribedbyapotentialasafunctionofthe
transfered4-momentum. TwocurvesfromTAMUareshown: thescenarioincludingcharmdiffusion(blue)
agreeswiththedatawhilethepredictionswithoutcharmdiffusion(magenta)areconsistentlylower. Inthe
model developed by the Duke university group [11] the diffusion coefficient 2πTD is a free parameter
s
which, in the case of the red curve shown, has been constrained using the R measured at LHC with a
AA
value of roughly 7, and under-predicts the v observed in our data. Figure 2c shows the value obtained
2
forthediffusioncoefficientfromdifferentmodelcalculationscomparedtotheyellowbandonthefarright
showingtherangeofinferredvaluesthatarecompatiblewiththemeasurementfromSTAR.
4 /NuclearPhysicsA00(2016)1–4
0.3 0.3
0.02.52(a) ADKfus0+ EAPu 200GeV, 0-80% Non-flow est. 0.02.52(b) ADSDTTuAAU0u+ kMMEBAePAuUU T 2wwE0 /C0c0G Hdcei fdVf.if,f .0-80% Non-flow est. D ×T2π3400 (c)LLaattttiiccee QQCCDD:: BDainnge erjet ea l.et al. pQCD LO 3400 STAR In 3400
v20.15 v20.15 T-Matrix F-pot. ferred
0.1 0.1 20 20 20
0.05 0.05 10 HRG T-Matrix U-pot.10 10
0 0
STAR preliminary STAR preliminary pQCD+HTL
-0.050 1 2 3 4 5 6 7 -0.050 1 2 3 4 5 6 7 0 0 0
0.5 1 1.5
Transverse Momentum p (GeV/c) Transverse Momentum p (GeV/c)
T T T/Tc
Fig. 2: a) Measured v for D0 compared to that of light hadrons. b) Comparison for measured D0 v and
2 2
modelcalculations. c)DiffusioncoefficientfrommodelcalculationsandinferredrangefromSTARresults.
4. Summary
STAR has carried out the first heavy flavor measurements in heavy ion collisions using the newly in-
stalled,stateoftheartvertexingdetector,theHFT.Themeasuredcharmedmesonv inAu+Aucollisionsis
2
foundtobefinite,thoughsystematicallybelowv oflighthadrons. Comparisontoaseriesofmodelsshows
2
thattheyareabletodescribethedata,favoringthescenariowherecharmquarksflowwiththemedium. The
modelsinferarangeofcompatiblevaluesforthecharmdiffusioncoefficient2πTD between2or10.
s
AspartofSTAR’scontinuingheavyflavorprogram,2billionAu+Auminimumbiaseventswillberecorded
in 2016 with full aluminum cables in the innermost silicon layer of the HFT. An expected 2-3 factor im-
provementintheD0 significanceat p = 1GeV/c2 willallowthestudyofthecentralitydependenceofv
T 2
inheavyioncollisions.
5. Acknowledgements
ThismaterialisbaseduponworksupportedbytheU.S.DepartmentofEnergy, OfficeofScience, Of-
fice of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research
(SCGSR)program. TheSCGSRprogramisadministeredbytheOakRidgeInstituteforScienceandEdu-
cationfortheDOEundercontractnumberDE-AC05-06OR23100.
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