Energy dependence of acceptance-corrected dielectron excess mass spectrum at mid-rapidity in Au+Au collisions at √sNN=19.6 and 200 GeV

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Energy

dependence

of

acceptance-corrected

dielectron

excess

mass

spectrum

at

mid-rapidity

in

Au

+

Au collisions

at

√

s

_NN

=

19

.

6 and

200 GeV

STAR

Collaboration

L. Adamczyk

a

,

J.K. Adkins

u

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G. Agakishiev

s

,

M.M. Aggarwal

af

,

Z. Ahammed

aw

,

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q

,

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t

,

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s

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aw

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as

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a_{AGHUniversityofScienceandTechnology,Cracow30-059,Poland} b_{ArgonneNationalLaboratory,Argonne,IL 60439,USA}

c_{BrookhavenNationalLaboratory,Upton,NY 11973,USA} d_{UniversityofCalifornia,Berkeley,CA 94720,USA} e_{UniversityofCalifornia,Davis,CA 95616,USA} f_{UniversityofCalifornia,LosAngeles,CA 90095,USA} g_{UniversidadeEstadualdeCampinas,SaoPaulo13131,Brazil} h_{CentralChinaNormalUniversity(HZNU),Wuhan430079,China} i_{UniversityofIllinoisatChicago,Chicago,IL 60607,USA} j_{CreightonUniversity,Omaha,NE 68178,USA}

k_{CzechTechnicalUniversityinPrague,FNSPE,Prague,11519,CzechRepublic} l_{NuclearPhysicsInstituteASCR,25068ˇRež/Prague,CzechRepublic} m_{FrankfurtInstituteforAdvancedStudiesFIAS,Frankfurt60438,Germany} n_{InstituteofPhysics,Bhubaneswar751005,India}

o_{IndianInstituteofTechnology,Mumbai400076,India} p_{IndianaUniversity,Bloomington,IN 47408,USA}

q_{AlikhanovInstituteforTheoreticalandExperimentalPhysics,Moscow117218,Russia} r_{UniversityofJammu,Jammu180001,India}

s_{JointInstituteforNuclearResearch,Dubna,141980,Russia} t_{KentStateUniversity,Kent,OH 44242,USA}

u_{UniversityofKentucky,Lexington,KY 40506-0055,USA}

v_{KoreaInstituteofScienceandTechnologyInformation,Daejeon305-701,RepublicofKorea} w_{InstituteofModernPhysics,Lanzhou730000,China}

x_{LawrenceBerkeleyNationalLaboratory,Berkeley,CA 94720,USA} y_{MassachusettsInstituteofTechnology,Cambridge,MA 02139-4307,USA} z_{Max-Planck-InstitutfurPhysik,Munich80805,Germany}

aa

MichiganStateUniversity,EastLansing,MI 48824,USA ab_{MoscowEngineeringPhysicsInstitute,Moscow115409,Russia}

ac_{NationalInstituteofScienceEducationandResearch,Bhubaneswar751005,India} ad_{OhioStateUniversity,Columbus,OH 43210,USA}

ae_{InstituteofNuclearPhysicsPAN,Cracow31-342,Poland} af_{PanjabUniversity,Chandigarh160014,India}

ag_{PennsylvaniaStateUniversity,UniversityPark,PA 16802,USA} ah_{InstituteofHighEnergyPhysics,Protvino142281,Russia} ai_{PurdueUniversity,WestLafayette,IN 47907,USA} aj_{PusanNationalUniversity,Pusan609735,RepublicofKorea} ak_{UniversityofRajasthan,Jaipur302004,India}

al_{RiceUniversity,Houston,TX 77251,USA}

am_{UniversityofScienceandTechnologyofChina,Hefei230026,China} an_{ShandongUniversity,Jinan,Shandong250100,China}

ao_{ShanghaiInstituteofAppliedPhysics,Shanghai201800,China} ap_{TempleUniversity,Philadelphia,PA 19122,USA}

aq_{TexasA&MUniversity,CollegeStation,TX 77843,USA} ar_{UniversityofTexas,Austin,TX 78712,USA} as_{UniversityofHouston,Houston,TX 77204,USA} at_{TsinghuaUniversity,Beijing100084,China}

az_{WorldLaboratoryforCosmologyandParticlePhysics(WLCAPP),Cairo11571,Egypt} ba_{YaleUniversity,NewHaven,CT 06520,USA}

bb_{UniversityofZagreb,Zagreb,HR-10002,Croatia}

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Articlehistory:

Received23January2015

Receivedinrevisedform25June2015 Accepted18August2015

Availableonline20August2015 Editor:H.Weerts

Theacceptance-correcteddielectronexcessmassspectra,wheretheknownhadronicsourceshavebeen subtracted fromthe inclusive dielectronmass spectra, are reported for the firsttime atmid-rapidity

|yee|<1 inminimum-biasAu+Au collisionsat√sNN=19.6 and200 GeV.Theexcessmassspectraare consistentlydescribedbyamodelcalculationwithabroadened

ρ

spectralfunctionforMee<1.1 GeV/c2. The integrated dielectron excessyield at√sNN=19.6 GeV for 0.4<Mee<0.75 GeV/c2,normalized to thecharged particlemultiplicityatmid-rapidity,hasavaluesimilar tothat inIn₊In collisionsat

√_s

NN=17.3 GeV.For√sNN=200 GeV,thenormalizedexcessyieldincentralcollisionsishigherthan thatat√sNN=17.3 GeV andincreasesfromperipheraltocentralcollisions.Thesemeasurementsindicate thatthelifetimeofthehot,densemediumcreatedincentralAu₊Au collisionsat√sNN=200 GeV is longerthanthoseinperipheralcollisionsandatlowerenergies.

©2015TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

Dileptonsarecrucialprobes forstudyingthepropertiesofthe stronglyinteracting,hot anddense matterwhichiscreatedin ul-trarelativisticheavy-ioncollisionsattheRelativisticHeavy-Ion Col-lider(RHIC)[1,2].Theyareproducedduringthewholeevolutionof thecreatedmatter,andarenotsubjecttostronginteractionswith themedium.Dielectron pairsare sensitiveprobes ofthemedium propertiesthroughoutthespacetimeevolutionofthemedium

[3,4]

becausetheyareproducedthroughavarietyofmechanismsandin severaldifferentkinematicregimes.

Inthelow invariant massregion, Mll

<

1

.

1 GeV

/

c2 (LMR),the

dilepton production is dominated by in-medium decay of vector mesons(

ρ

,

ω

and

φ

) inthehadronicgasphase.In-medium mod-ificationstothemassandwidthofthevectormesonsare consid-eredas alinktochiralsymmetryrestoration[3,4].Inthevacuum, chiral symmetry is spontaneously broken, which results in mass differencesbetween chiralpartners [e.g.

ρ

anda1

(

1260

)

]. Inthe

hot,densemedium,chiralsymmetryisexpectedtorestoreandthe massdistributionsof

ρ

anda1

(

1260

)

areexpectedtochangeand

degenerate.Sinceitisextremelychallengingtomeasureaspectral functionforthea1

(

1260

)

meson,one cannotdirectlyobservethe

disappearanceof themass splittingbetweenthe

ρ

anda1

(

1260

)

experimentally.Instead,effortsaredevotedtostudyingthe modifi-cationofvectormesonspectralfunction.Twoschematicscenarios areusedtodescribethein-medium

ρ

spectrumfunction:a broad-enedandadropping-mass

ρ

.The broadened

ρ

scenario incorpo-ratesfinitetemperatureeffectsintoself-energycorrectionsthrough mediuminteractionsand

π π

annihilations[5].Thedroppingmass scenariousesthequark meanfieldfromahigh temperature/den-sityregimewhereinconstituentquarksaretherelevantdegreesof freedom,andthenextrapolatesdowntoalowtemperature/density regimewhereinhadronsareappropriatedegreesoffreedom[6].

TheCERES experimentattheCERN-SPS reportedan excess di-electronyield withrespect totheknown hadronicsources inthe LMR in Pb

+

Au collisions at

√

sNN

=

17

.

2 GeV, which indicates

thatthevectormesonsaremodifiedinmedium[7].Morerecently,

NA60 published a precise measurement of the dimuon invariant

mass spectra in In

+

In collisions at

√

sNN

=

17

.

3 GeV [8]. The

results show a significant excess in the LMR above the hadronic sources.Inbothcases,theexcessisconsistentwithabroadened

ρ

*

Correspondingauthor.

E-mailaddress:bingchu@uic.edu(B. Huang).

spectralfunction[5],butnotwitha

ρ

dropping-massscenario[6], wherebothmodelshavebeenevaluatedforthesamefireball evo-lution.Inthemodelcalculation,thecouplingtothebaryonsinthe mediumplaysadominantroleinthebroadeningofthe

ρ

spectral function[5,7,8].

At RHIC, a significant enhancement in the dielectron

contin-uum, compared withthe known hadronic sources, has been

ob-served inthe LMR by both the PHENIX and STAR Collaborations

in Au

+

Au collisionsat

√

sNN

=

200 GeV[9,10].Results fromthe

STAR Collaboration show that the excess dielectron yield in the mass region 0

.

3–0

.

76 GeV

/

c2 _{follows an N}1.54±0.18

part dependence,

where Npart is the number of participant nucleons in a

colli-sion [10].However, thePHENIXCollaborationreported significant higher excess dielectron yields in central collisions [9]. Theoreti-cal calculations[11–14],whichdescribetheSPSdileptondata,fail to consistentlydescribe thelow-mass enhancementatlow

trans-verse momentum (pT) observed by PHENIX in both 0–10% and

10–20%centralAu

+

Au collisions[9].Thesamecalculations, how-ever,correctlydescribetheSTARmeasurementofthelow-pT and

low-mass enhancement from peripheralto central Au

+

Au colli-sions [10]. While the discrepancy between STAR and PHENIX in central Au

+

Au collisions at

√

sNN

=

200 GeV isstill under

inves-tigation, it is important to have dilepton measurements at RHIC atlowerbeamenergieswiththesamelargeacceptanceasforthe 200 GeVdata.Sincethetotalbaryondensitydoesnotchange sig-nificantly from

√

sNN

=

17

.

3 GeV to

√

sNN

=

200 GeV [15], it is

essentialtoconfirmthatthebroadened

ρ

spectralfunction,which describes theresults at17.3 GeV andthe 200 GeV STAR data,is consistentwiththe19.6 GeVresults.

Intheintermediatemassregion,1

.

1

<

Mll

<

3

.

0 GeV

/

c2 (IMR),

dilepton production is expected to be directly related to

ther-mal radiation of the partonic phase, which is considered to

be the prime signature of deconfinement [11,12]. An enhanced

yield in this region was first observed by HELIOS/3 [16] and

NA38/NA50 [17]. More recently, theNA60 Collaboration reported anenhancementintheIMRwhichcannot beconnectedtodecays ofD mesons,butmaybetheresultofthermalradiation[8]. How-ever, it is experimentally challenging to extract the signal in the presenceofsignificantbackgroundsourcesfromopenheavy-flavor semi-leptonicdecays,suchascc

¯

→

l+l−X orbb

¯

→

l+l−X .

In this letter, we report the first dielectron measurements at mid-rapidity in minimum-bias Au

+

Au collisions at

√

sNN

=

withmeasurementsfromNA60andtheoreticalmodelcalculations. Theinvariantexcessdielectronspectraatdifferentcentralitiesand energies allow for a first systematic studyof the lifetime of the hot,dense medium using electromagneticprobes atRHIC. It was pointedoutthattheexcessdielectronyieldatlowmassis propor-tionalto the total lifetimeof the hot, dense medium at

√

sNN

=

6–200 GeV[19].

2. Experimentanddataanalysis

Inthisanalysis,33millionminimum-bias(MB)Au

+

Au (0–80%) eventsat

√

sNN

=

19

.

6 GeV, recordedby the STARexperimentin

the year 2011, were used. The results at

√

sNN

=

200 GeV are

derived from the same data analysis reported in Ref. [10]. The MB trigger at

√

sNN

=

19

.

6 GeV was defined asa coincidence of

thetwo Beam Counterscovering the pseudorapidity range 3

.

3

<

|

η

|

<

5

.

0 [20]. Charged tracks were reconstructed by the Time ProjectionChamber(TPC)[21],whichhasfullazimuthal coverage at

|

η

|

<

1. The absolute distance between collision vertices and theTPCcenter along thebeamdirection was requiredto be less than70 cm.Thetransverse momentumresolutionismeasuredto be



pT

/

pT

=

0

.

01

× [

1

+

pT

/(

2 GeV

/

c

)

]

for pT

<

5 GeV

/

c. The

Time-Of-Flight(TOF)[22]detector,whichcoversthe pseudorapid-ityrange

|

η

|

<

0

.

9,providesthearrivaltimeofchargedtracksfrom the collision vertex. Slow hadrons can be rejected by a velocity cut

|

1

/β

−

1

/β

exp

|

<

0

.

025 in the range of 0

.

2

<

pT

<

3 GeV

/

c,

where

β

is the measured velocity and

β

exp is the expected

ve-locitycalculatedusing thetracklength andmomentum withthe assumption ofthe electron mass. After the velocity cut, electron identificationisachievedbycutting onthenormalized ionization energy loss (n

σ

e

=

log

(

dE_dx

/

Ie

)/

Re) measured by the TPC, where

dE

/

dx is the energy loss, Ie is the expected dE

/

dx for an

elec-tronandRe isthedE

/

dx resolutionofanelectron,whichisbetter

than8% [23].Then

σ

e cutismomentumdependentandresultsin

ahighelectronpurityof

>

93% andanefficiencyof

>

65% on av-erage

[10,24]

.

Theelectronandpositroncandidatesarepairedbyoppositeand samesign charges,called unlike-sign andlike-sign pairs, respec-tively. The like-sign pairs are used to statisticallyreproduce the combinatorialandcorrelated pairbackgrounds.Thecombinatorial

background comes from two random tracks without correlation.

Thecorrelated backgroundis theresultof two electrons,each of whichcomesfromadifferentbutcorrelatedprocess ofaparticle decayorajetfragmentation.Forexample,considera

π

0

_→

_γ

_e+_e−

Dalitzdecaywherethe gammamayconverton some materialto

formanadditional e+e− pair. Thee± fromthe

π

0 _pairedwitha

e∓fromthe

γ

canproduceacorrelatedbackgroundpair.This cor-relatedbackgroundcanbereproducedbylike-signpairs.

The unlike-sign andlike-sign pairs havedifferent acceptances duetodeadareasofthedetectorandthedifferentbending curva-turesofpositivelyandnegativelychargedparticlesinthemagnetic field. The dead area fraction is 13% along the azimuthal distri-bution at

η

<

1. A mixed-event technique [9]is applied to esti-matetheacceptancedifferencesbetweentheunlike-signand like-signdistributions.

Fig. 1

(a)showstheratiobetweenmixed-event unlike-signpairsandmixed-eventlike-signpairs asa functionof dielectronmass.Azoom-inversionisshownin

Fig. 1

(b).

Thebackgroundsubtractionisbasedonthemeasuredlike-sign spectrawiththeassumptionthattheshapeandmagnitudeofthe correlated background are the same in the unlike- and like-sign spectra. We subtract the like-signbackground (corrected for the acceptancedifferenceusingthemixedeventtechniquementioned above)fromtheunlike-signdistributionstoobtaintheraw dielec-tron signals. The mixed-event background is not used for back-groundsubtraction,sincethecorrelatedbackgroundcontributionis

Fig. 1. (Color online.)(a): Ratioof mixed-eventunlike-sign pair tomixed-event like-signpair dielectronmassdistributions. (b): A zoom-inversionofPanel (a). (c): Reconstructeddielectronunlike-signpairs(invertedtriangles),like-signpairs (opencircles)andsignal(filledcircles)distributions.(d):Thesignaltobackground ratio(S/B).Allpanelsarepresentedasafunctionofdielectroninvariantmassin Au+Au collisionsat√sNN=19.6 GeV.

difficulttoaddresswithlimitedstatisticsatMee

>

1

.

5 GeV

/

c2 for

√

sNN

=

19

.

6 GeV.

Fig. 1

(c)showstheinvariantmassdistributions

ofunlike-signpairs,like-signpairsandbackground-subtracted sig-nals.Thesignaltobackgroundratioisshownin

Fig. 1

(d). Dielec-tron pairs fromphoton conversions in the detectormaterials are suppressedbyselectingtrackswithadistanceofclosestapproach to the collision vertex that is less than 1 cm, and a minimum openinganglecutbetweenthetwoelectroncandidates[9,10].The minimum opening angle is 0.84 rad at Mee

<

0

.

03 GeV

/

c2 and

Fig. 2. (Coloronline.)TheTsallisBlastWave(TBW)functionfit[26,27]totheNA49

pT spectraofpions,kaonsandprotonsinPb+Pb at√sNN=17.3 GeV[28].The

datapointsofπ+completelyoverlapwiththatofπ−onthefigure.Othermeson

pTspectraarepredictedbytheTBWfunction.For J/ψ,thepTshapeisdetermined

byanindependentTBWfunctionfittothe J/ψ pTspectrameasuredbyNA50[29].

Moredetailsareinthetext.

A

/

[

B

+

exp

(

C

/

Mee

)

]

, inwhich A, B, andC are input parameters.

ForMee

>

0

.

1 GeV

/

c2,theminimumopeningangleiszero.

The raw dielectron signal is corrected for the electron recon-struction efficiency. The single electron reconstruction efficiency includes TPC tracking, electron identification and TOF matching efficiencies. TheTPC trackingefficiencyis determined by embed-ding MonteCarlo(MC) tracksinto realraw dataevents,

process-ing the track reconstruction with a GEANT model of the STAR

detector [25], and determining the fraction of those embedded MC tracks whichare reconstructed asgoodtracks. The efficiency correction includes the effect of dead areas in the detector. The TOF matching and electron identification efficiencies are repro-ducedfromreal data.Detailedprocedures toobtain the TPCand TOF efficiencies are explained in Ref. [24]. The energy loss and bremsstrahlung radiation effects for electrons are reproduced by theGEANTsimulation.Thesingleelectronefficiencyisconvoluted intothe pairefficiency withthedecay kinematicsin the simula-tion.

The hadronic sources of dielectron pairs include: Dalitz de-cays

π

0

_→

_γ

_e+_e−_,

_η

_→

_γ

_e+_e− _and

_η



_→

_γ

_e+_e−_{; vector}

me-son decays:

ω

→

π

0_e+_e−_,

_ω

_→

_e+_e−_,

_ρ

0

_→

_e+_e−_,

_φ

_→

_η

_e+_e−_,

φ

→

e+e− and J

/ψ

→

e+e−; heavy-flavor hadron semi-leptonic decays:cc

¯

→

e+e−X ;Drell–Yan. The

ρ

mesoncontributionisnot evaluatedinthesimulation,butincludedinthemodelcalculation (as described in Section 3). The bb

¯

→

e+e−X process is not in-cludedasit hasnegligiblecontributiontothecocktail inAu

+

Au collisionsat

√

sNN

=

19

.

6 GeV.

The input hadron spectra to the cocktail are derived from a TsallisBlastWave(TBW)functionfit[26,27]totheNA49 pT

spec-tra of pions, kaons andprotons in Pb

+

Pb at

√

sNN

=

17

.

3 GeV [28], asshown in Fig. 2. Other meson pT spectra are predicted

by theTBW function usingthe samefreeze-out parameters from

pT fit ofpions, kaons andprotons. The extra uncertainty caused

bytheinput pT spectraisfoundtobelessthan10%andhasbeen

Table 1

Themesonyields,dN/dy,atmid-rapidityusedinthehadroniccocktailfor0–80% Au+Au collisionsat√sNN=19.6 GeV.Theuncertaintyincludescontributionsfrom

theTBWfitandthemeson-to-pionratio.

Meson yield dN/dy Uncertainty (%)

π0 _{49.6 8} η 4.22 14 ω 3.42 16 φ 0.89 13 η 0.39 17 J/ψ 2.18×10−4 ₃₂

propagatedtothefinalcocktailuncertainty.For J

/ψ

,thepT shape

isdeterminedbyanindependentTBWfunctionfittothe J

/ψ

pT

spectrameasuredbyNA50[29].

The

π

0 _{contribution is obtained by matching the dielectron}

mass distribution from simulated

π

0

_→

_γ

_e+_e− _and

_η

_→

_γ

_e+_e−

decays to the efficiency-corrected dielectron mass spectrum for

Mee

<

0

.

1 GeV

/

c2. We also match the J

/ψ

→

e+e− distribution

fromsimulationtothemeasureddielectronproductioninthe cor-respondingmassregion.Themesonyieldsofothermesonsare de-rivedbythemeson-to-pion ratios[7]andthepionyields.

Table 1

lists the integrated yields used inthe simulation at mid-rapidity forAu

+

Au collisionsat

√

sNN

=

19

.

6 GeV.Thebranchingratiosof

mesonstodielectronsandtheiruncertaintiesarefromRef.[30]. The e+e− massdistribution fromopenheavy-flavor sources is generated usingPYTHIA 6.416 [31]. Previouscharm cross section

measurementsfromtheSPS,FNAL,STARandPHENIXexperiments

[33] arewell describedby theupperlimitofaFixed-Order Next-to-LeadingLogarithm(FONLL)calculation[34].Thereforeweobtain the charmtotalcrosssection in p

+

p at

√

s

=

19

.

6 GeV by

scal-ing the FONLL upper limit to the previous measurements using

the minimum

χ

2 _{method.Thistotalcross section8}

_.

₂

_±

₀

_.

_{5 μb is}

usedtonormalizethedielectronyieldfromthePYTHIAsimulation, whichisadditionallyscaledbythenumberofbinarycollisionsfor Au

+

Au at

√

sNN

=

19

.

6 GeV tobecomparedwiththedata.

Forthe efficiency-corrected dielectroninvariant mass distribu-tion,the systematicerrorsaredominatedby uncertainties onthe TPC tracking efficiency (14% in the dielectron yields), the TOF matching efficiency (10% in the dielectron yields), hadron con-tamination(0–20%),andelectronidentification(2%).Thetotal sys-tematic uncertainty on the pair reconstruction efficiency is esti-mated to be 18%. The systematic uncertainties on the like-sign backgroundsubtractionweremainlyfromtheuncertaintiesonthe acceptance difference factors between the unlike-sign and like-sign pairs. The acceptance difference factors were derived using mixed-eventtechnique.Inthemixed-eventtechnique,tracksfrom different eventswere used toform unlike-signor like-sign pairs. Theeventsweredividedintodifferentcategoriesaccordingtothe collision vertex, eventplane, azimuthal angle, andcentrality. The binsizesofcollisionvertex,eventplane,azimuthalangle,and cen-tralitywere chosentobe smallenoughandthetwoeventstobe

mixedmust come fromthesame eventcategory to ensure

simi-lardetectorgeometricacceptance,azimuthal anisotropy,andtrack multiplicities. The uncertainties in the acceptance difference fac-tors were found to be 0.003% andresult in 1% uncertainties for the dielectron signals. Forthe cocktail simulation,the systematic uncertainties come from the uncertainties of particle yields, de-cay branching ratios and form factors. Table 2 lists all the con-tributions tothe systematicuncertainties onthe dielectron mass spectrum andcocktail simulation within the STAR acceptance at

√

sNN

=

19

.

6 GeV.

After efficiency correction, the dielectron excess mass spec-trum is corrected for the detector acceptance. The acceptance

in-Table 2

Summaryofsystematicuncertaintiesforthemeasureddielectronmass spectrumandsimulatedcocktailwithinSTARdetectoracceptanceinAu+ Au at√sNN=19.6 GeV.Theuncertaintyonhadroncontaminationleads

toamass-dependentuncertaintyforthemeasureddielectroncontinuum. The uncertaintiesofparticle yields,branching ratios,andform factors resultinmass-dependentuncertaintiesforthesimulatedcocktail.

Syst. error (%) Tracking efficiency 14

TOF matching 10

Electron selection 2 Hadron contamination 0–20 Sum of data uncertainties 17–26 Particle yield 8–24 Branching ratio and form factors 1–10 Sum of simulation uncertainties 11–27

Fig. 3. (Coloronline.)Theacceptanceofvirtualphotondecayeddielectronsinthe STARdetectorinAu+Au collisionsat√sNN=19.6 GeV.

putsofvirtualphotonyieldspectra,phasespacedistributions and

decay kinematics. The method is similar to the approach used

by NA60 [35], in which one assumes that the excess yields are frommedium emission.The acceptanceiscalculatedby theyield ratioofreconstructed dielectrons inthe STARdetectorto the in-put dielectrons. Fig. 3 shows the two-dimensional acceptance of the virtual photons with a Gaussian-like rapidity distribution in Au

+

Au at

√

sNN

=

19

.

6 GeV at STAR. The

σ

value of the

dis-tribution is 1.5 [35]. The same approach was used in Au

+

Au at

√

sNN

=

200 GeV except that we used a flat rapidity

distri-bution as our default case. The acceptance correction factor at

√

sNN

=

200 GeV differs from that at

√

sNN

=

19

.

6 GeV by 5%

mainlyduetotheinput pT spectraofvirtualphotons.

Forthedielectronexcessmassspectrum,additionalsystematic uncertainties comefromthesubtraction ofthecocktail contribu-tionandtheacceptancecorrection.InAu

+

Au at

√

sNN

=

200 GeV,

thecocktailsimulationisdetailedinRef. [36].Forthecharm cor-relationcontribution,we studiedthefollowingcases: a)keep the directPYTHIAcorrelationbetweenc and

¯

c whichwasusedinour defaultcocktailcalculations;b)breaktheazimuthalangular corre-lationbetweencharmdecayedelectrons completelybutkeep the

pT,

η

,and

φ

distributionsfromPYTHIA;c)randomlysample two

electronswiththesingle electron pT,

η

,and

φ

distributions from

PYTHIA;andd)based onc), butsample the pT ofeach electron

accordingtothemodified pT distributionfromthemeasurements

ofnon-photonic electronnuclear modification factors in Au

+

Au collisions.The maximal differencebetweencasea)andthe other threeistakenasthesystematicuncertainties onthecharm corre-lationcontribution.

Theuncertaintyfromacceptancecorrectioncontains uncertain-ties from the rapidity distribution and input dielectron sources. A uniformrapiditydistributioniscomparedwiththeGaussian-like case, and the resultinguncertainty is 2% in the LMRin Au

+

Au at

√

sNN

=

19

.

6 GeV.For200 GeV,we usedapionrapidity

distri-bution to compare tothe default caseandquoted the difference betweenthem assystematic uncertainty, whichis about2%. The uncertainty from the input pT spectrum is at the same level as

therapiditydistributionuncertainty.

We also obtain the acceptanceof the excess dielectrons from modelcalculations[32].Thedifferencebetweenthesimulationand theoreticalcalculationisabout20%forMee

<

0

.

4 GeV

/

c2 andless

than 10%for Mee

>

0

.

4 GeV

/

c2.Itisincluded intheexcess yield

uncertainties.

3. Resultsanddiscussion

The dielectroninvariant massdistribution after efficiency cor-rectionisshownintheupperpanelof

Fig. 4

forAu

+

Au collisions at

√

sNN

=

19

.

6 GeV.Itiscomparedwithahadroniccocktail

sim-ulation, whichconsists ofall the dielectronhadronic sources ex-cept the

ρ

0_{.An enhancement ofthe dielectronyield isobserved}

in the mass region Mee

<

1 GeV

/

c2. A model calculation witha

broadened

ρ

spectral function[12] isaddedtothehadronic cock-tail andcompared with the data,as shownin the bottom panel of

Fig. 4

.The dielectronyields in themodel calculationwere fil-teredby theSTARacceptance(pe

T

>

0

.

2 GeV

/

c and

|

η

e

|

<

1).The

model calculation involves a realistic space–time evolution, and includescontributionsfromquark–gluon-plasma(QGP),4-pion an-nihilation andin-medium vector meson contributions.The initial temperaturefromthemodelis224MeVandthestarting time

τ

0

is 0

.

8 fm

/

c [32]. The comparisonof the model withdata shows that a broadened

ρ

-spectra scenario isconsistent withthe STAR datawithinuncertainties.Thesameconclusionhasbeendrawnin Au

+

Au collisionsat

√

sNN

=

200 GeV[10].Usingthebroadened

ρ

spectral function, QCD andWeinbergsumrules, andinputsfrom Lattice QCD,theorists have demonstrated that when the temper-aturereaches170 MeV, the deriveda1

(

1260

)

spectral function is

thesameasthein-medium

ρ

spectralfunction,asignatureof chi-ralsymmetryrestoration[37].

Toquantifytheyield,theknownhadroniccocktail,cc

¯

→

e+e−X

and Drell–Yancontributions were subtracted from the dielectron mass spectrum at

√

sNN

=

19

.

6 GeV. At

√

sNN

=

200 GeV, the

known hadronic sources, cc

¯

→

e+e−X , bb

¯

→

e+e−X , and Drell– Yan contributions were subtracted. The excess dielectron mass spectra,correctedfordetectoracceptance,areshownin

Fig. 5

for Au

+

Au MB collisions at

√

sNN

=

19

.

6 and 200 GeV. The

spec-tra are normalized to mid-rapidity dNch

/

dy in absolute terms

to cancel out the volume effect, and compared with the excess dimuon yieldsfromthe NA60measurements in In

+

In collisions at

√

sNN

=

17

.

3 GeV. The model calculation [11,32] including a

broadened

ρ

spectralfunction andQGPthermalradiationis con-sistent withtheacceptance-correctedexcessinAu

+

Au collisions at

√

sNN

=

19

.

6 GeV.Theexcessat

√

sNN

=

200 GeV ishigherthan

thatat

√

sNN

=

17

.

3 GeV in theLMRandIMR,butwithin 2

σ

un-certainty.Furthermeasurementswithbetterprecision areneeded toobtain theaveragetemperatureofthehot,densemedium cre-ated.

Fig. 5 shows that the excess dielectron yield in the LMR at

√

sNN

=

19

.

6 GeV has a magnitudesimilar to the excess dimuon

yieldat

√

sNN

=

17

.

3 GeV.Toquantitativelycomparetheexcessin

the LMR, the integratedexcess yields of dielectrons in the mass region 0

.

4

<

Mll

<

0

.

75 GeV

/

c2 are shown in Fig. 6 for 0–80%

Au

+

Au collisions at

√

sNN

=

19

.

6 and 200 GeV. The results in

Fig. 4. (Coloronline.)DielectroninvariantmassspectrumintheSTARacceptance(|yee|<1,0.2<peT<3 GeV/c,|ηe|<1)afterefficiencycorrection,comparedwiththe

hadroniccocktailconsistingofthedecaysoflighthadronsandcorrelateddecaysofcharminAu+Au collisionsat√sNN=19.6 GeV.Thedatatococktailratioisshown

inthebottompanel.Theoreticalcalculations[11,32]ofabroadenedρspectralfunctionareshownupto1.5 GeV/c2_{forcomparison.Systematicuncertaintiesforthedata}

pointsareshownasgreenboxes,andthegray bandrepresentstheuncertaintiesforthecocktailsimulation.

Fig. 5. (Coloronline.)Theacceptance-correctedexcessdielectronmassspectra, nor-malizedtothe chargedparticle multiplicityatmid-rapiditydNch/dy,inAu+Au

collisionsat √sNN=19.6 (solidcircles) and 200 GeV(diamonds).The dNch/dy

valuesin Au+Au collisions at √sNN=19.6 and200 GeV are fromRefs. [38]

and[39], respectively.ComparisontotheNA60data[8,40]for In+In collisions at √sNN=17.3 GeV (opencircles)is alsoshown. Barsarestatistical

uncertain-ties,and systematicuncertaintiesareshown asgray boxes. Amodelcalculation (solidcurve)[11,32] with a broadenedρ spectralfunction inhadron gas (HG) andQGPthermalradiationiscomparedwiththeexcessinAu+Au collisionsat

√

sNN=19.6 GeV.ThenormalizationuncertaintyfromtheSTARmeasureddN/dy is

about10%,whichisnotshowninthefigure.

√

sNN

=

200 GeV collisions. The excess yield hasa centrality

de-pendence and increases from peripheral to central collisions at

√

sNN

=

200 GeV.ComparingtotheresultsfromIn

+

In collisionsat

√

sNN

=

17

.

3 GeV,theexcess yieldat

√

sNN

=

19

.

6 GeV is

consis-tentwithintheuncertaintieswhiletheexcessat

√

sNN

=

200 GeV

is higher in central collisions, but within 2

σ

uncertainty. This might indicate that the lifetime of the medium created in cen-tral collisions at

√

sNN

=

200 GeV is longer than those in

pe-ripheral collisions and at

√

sNN

=

17

.

3 GeV, which enhancesthe

dileptonproductionfromthermalradiation.Thesame model

cal-Fig. 6. (Coloronline.)Integratedyieldsofthenormalizeddileptonexcessesfor0.4<

Mll<0.75 GeV/c2asafunctionofdNch/dy.Thesolidcircleanddiamondrepresent

theresultsin0–80%Au+Au collisionsat√sNN=19.6 and200 GeV,respectively.

Thesquares arethe resultsfor 40–80%,10–40%,and0–10%Au+Au at√sNN=

200 GeV.TheopencirclerepresentsthedimuonresultfromtheNA60measurement withdNch/dη>30.Barsarestatisticaluncertainties,andsystematicuncertainties

areshownasgray boxes.Thetheoreticallifetimesfor√sNN=200 GeV Au+Au as

afunctionofdNch/dy inthemodelcalculations[19]areshownasadashedcurve.

Thelifetimesfor√sNN=17.3 GeV In+In and√sNN=19.6 GeV Au+Au inthe

samemodelcalculations[19]areshownasthetwohorizontalbars.ThedNch/dy

valuesforthehorizontalbarsareshiftedforclarity.

culations [11,32] that consistently describe the dilepton excesses inthe

√

sNN

=

17

.

3

,

19

.

6,and200 GeVA

+

Adatagivelifetimesof

6

.

8

±

1

.

0 fm

/

c,7

.

7

±

1

.

5 fm

/

c,and10

.

5

±

2

.

1 fm

/

c forthe17.3 GeV In

+

In,19.6 GeVAu

+

Au,and200 GeVAu

+

Au dataasshownin

Fig. 6 [19].Inaddition,thelifetimehasastrongcentrality depen-dencein

√

sNN

=

200 GeV Au

+

Au collisionsinthecalculations,as

indicatedbythedashedcurvein

Fig. 6

.Withthetotalbaryon den-sitynearly aconstantandthedileptonemissionratedominantin thecriticaltemperatureregionat

√

sNN

=

17

.

3–200 GeV, the

measurements are proportional to the calculated lifetimes ofthe medium [19]. We note that the lifetimemight be model depen-dent. It is important to have the calculated lifetimes from other modelstoverifythisproportionality.

4. Summary

In summary, the dielectron mass spectrum is measured in

Au

+

Au collisionsat

√

sNN

=

19

.

6 GeV by the STARexperiment

atRHIC.Comparedwithknownhadronicsources,asignificant ex-cessisobserved,whichcan beconsistentlydescribed inallbeam energiesby amodel calculationinwhich abroadened

ρ

spectral functionscenarioatlowtemperatureandchiralsymmetry restora-tion are included. Furthermore,the excess dielectron mass spec-tra, corrected for the STAR detector acceptance,are reported for thefirsttime inAu

+

Au collisionsat

√

sNN

=

19

.

6 and 200 GeV.

Inthe LMR, the excess yield at

√

sNN

=

19

.

6 GeV, normalized to

the charged particle multiplicity dNch

/

dy, is comparable to that

in In

+

In collisions at

√

sNN

=

17

.

3 GeV. For

√

sNN

=

200 GeV,

the normalized excess yield is higher in central collisions than that at

√

sNN

=

17

.

3 GeV and increases from peripheral to

cen-tral collisions. These measurements indicate that the hot, dense mediumcreated incentral Au

+

Au collisionsattop RHICenergy has a longer lifetime than those in peripheral collisions and at

√

sNN

=

17

.

3 GeV.

Acknowledgements

We thank the RHIC Operations Group and RCF at BNL, the

NERSC Center atLBNL, the KISTI Center in Korea, and the Open ScienceGridconsortiumforprovidingresourcesandsupport.This

workwas supportedinpartby theOfficesofNPandHEP within

theU.S. DOE Officeof Science,the U.S. NSF, CNRS/IN2P3,FAPESP CNPqofBrazil, theMinistryofEducation andScience ofthe Rus-sianFederation, NNSFC,CAS,MoSTandMoEofChina,the Korean ResearchFoundation,GAandMSMToftheCzechRepublic,FIASof Germany,DAE,DST,andCSIRofIndia,theNationalScienceCentre of Poland, National Research Foundation (NRF-2012004024), the MinistryofScience,EducationandSportsoftheRepublicof Croa-tia,andRosAtomofRussia.

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