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The STAR ExperimentTexas A&M University A. M. Hamed for the STAR collaborationDirect -charged hadrons Measurements1
Hot Quarks 2008, 18-23th August, Estes Park ColoradoOne more ingredient for energy loss quantification
Table of Contents:The Road Behind ResultsAnalysis SummaryTable of Contents and DisclaimerDisclaimer: The road behind is personal view, so biases and mistakes are expected. D. dEnterria and adapted from T. Schafer2The multiple facets of QCD3The Road BehindJ.D.Bjorken 1982
4The Road BehindHigh-pT: Nuclear modification factor RAA of light quarks, heavy quarks and gluons at mid rapidity!V~5 fm3 and ~10 fm/cRAA is a measure of the deviation from the incoherent superposition of nucleon-nucleon collisions assumption. Nuclear modification factorHigh-pT particles are produced from the hard scattering processes.Hard processes take place at early time of collisions (0.1 fm/c). pQCD
CTEQ6MProton Parton distribution functions.RHICRHIC0.20.01x2Pt/sAt mid rapidity at RHIC
The ratio of quark structure functions RHICSoftHard5The Road BehindHigh-pT: Nuclear modification factor RAA of light quarks, heavy quarks and gluons at mid rapidity!
RAA of light quarks is pt independent as expected by the radiative energy loss. PRL. 96, 202301 (2006) Direct photons follow the binary scaling.6The Road BehindHigh-pT: Nuclear modification factor RAA of light quarks, heavy quarks and gluons at mid rapidity!
Unexpected level of suppression for the heavy quarks. PRL 98 (2007)192301
According to QCD at zero temperatureEquark,m=0 Equark,m>0
Vacuum and medium radiation is suppressed due to quark massDokshitzer, kharzeev. PLB 519 (2001) 199
Dead cone effect7The Road BehindHigh-pT: Nuclear modification factor RAA of light quarks, heavy quarks and gluons at mid rapidity!
STAR QM08No sign for the color factor effect on energy loss. According to QCD at zero temperatureEgluon EquarkCasimir factor (CF=4/3 quarks , CA=3 gluons ), i.e 2.25Gluon should show stronger coupling to the medium.E CR 8The Road BehindHigh-pT: Nuclear modification factor RAA of light quarks, heavy quarks and gluons at mid rapidity!
PRL 98 (2007)192301
STAR QM08PRL. 96, 202301 (2006) But nature cannot realize contradictions. Paradoxes focus our attention, and we think harder F. Wilczek Nobel Lecture 2004The fundamental theoretical result regarding the asymptotic high temperature phase is that it becomes quasi-free. That is, one can describe major features of this phase quantitatively by modeling it as a plasma of weakly interacting quarks and gluons. In this sense the fundamental degrees of freedom of the microscopic Lagrangian, ordinarily only indirectly and very fleetingly visible, become manifest (or at least, somewhat less fleetingly visible).What happens to empty space, if you keep adding heat?The Road Behind9F. Wilczek hep-ph/0003183v1In particular, chiral symmetry is restored, and confinement comes completely undone.Weakly coupled or Strongly coupled medium!The Road Behind10
Hep-lat/00010027v1F. Karsch, E. Laermann, A. Peikert, CH. Schmidt, S. StickanLattice QCD ~20%pT (GeV/c)
v2Romatschke & Romatschke, arXiv:0706.1522v2 of hadrons at RHIC data are in agreement with the ideal relativistic fluid dynamics predictions /s=0-0.8/s ~ 1 pQCD calculations of a weakly coupled quark gluon plasma./s ~ 0.08 is reached in strongly coupled supersymmetric gauge theories.Weakly coupled or Strongly coupled medium!The Road Behind11We will not have done justice to the concept of weakly interacting plasma of quarks and gluons until some of the predictions are confirmed by experimentF. WilczekThe applicability of pQCD in describing the parton-matter interaction has been increasingly challenged by the speculated strongly coupled nature of the produced matter at RHIC.IMHOWeakly coupled or Strongly coupled medium!The Road BehindThe four major models use pQCD framework to estimate energy loss.Modeling the medium evolution/structure.Hierarchy of scales.12DifferencesOn the jet quenching parameter q ^Different assumptions in various models lead to similar descriptions of the light quark suppression with different model-dependent parameters.Medium
qTELEnergy lossASW and GLV: Similar models different ^qAMY and Higher twist: Different models same ^qStatic medium sCRqL2^ Scattering power of the medium ^qq k2=2/^>>1
Pion gasIdeal QGPThe Road Behind13
On the jet quenching parameter q ^radiative energy lossIf s(T) were weakq1 GeV2/fm^Baier Schiff8-19 GeV2/fm3 GeV2/fm4-14 GeV2/fmPHENIX; at 2, neglecting theoretical uncertaintiesZhang Owens Wang WongDainese Loizives Palcq extracted via comparison with RHIC data is larger^q 8 GeV2/fm Armesto,Cauiari Hirono Salgado ^Strong coupling calculation of q is required !^Nonperturbative calculation is needed !The Baier plotCold nuclear matterThe Road BehindAll four major models utilize factorization:
Extracted from data, but evolution is perturbativeExpansion in the coupling constant (LO,NLO,NNLO)The entire effect of energy loss in concentrated in the modification of FFFactorization is used without proof! 14On the pQCD frameworkThe characteristics time and length scale of the parton-parton interaction is short compared to the soft interactions between the bound partons in the initial state and to those of the fragmentation process of the scattered partons in the final state.Factorization validityThere is no single commonly accepted calculation of the underlying physics to describe in-medium energy loss for different quark generations as well as for the gluon.Summary15RAA saturates!If the medium is black somewhere already, you cant see it getting even blacker.at some point, large changes in q do not map into large changes of RAA, or:^The Road BehindSingle particle spectra and di-hadron azimuthal correlationsSingle jetDijetNo Glauber calculation is required for the suppression measurements.Different geometric bias and different fragmentation bias.Di-hadron azimuthal correlations
PRL 98 (2007) 212301Model dependent calculations show that IAA is more sensitive than RAA but both have diminished sensitivity at high gluon density.IAA is a quantity that measure the medium effect on the FF on dijet analysis.16High-pT: di-hadron azimuthal correlations conservation of pAway side In the near-side p+p, d+Au, and Au+Au are similar while in the away-side back-to-back Au+Au is strongly suppressed relative to p+p and d+Au.
4 < pT,trig < 6 GeV/c2 < pT,assoc < pT,trigPRL. 91, 072304 (2003)Background is subtracted
The Road Behind
Away-side yield neither depend on zT nor broaden in .Clear jet-like peaks seen on near and away side in Au+Au at high trigger pT and high associated pT
Away-side yield strongly suppressed to level of RAA An access to the parton initial energy is required in order to better quantify the energy lostSurface bias free probe is needed1617Jet-energy calibration Direct
Mid-rapidityPP
Fast DetectorCalorimeterLeading particletrigger
xPxPAssociated particlesBackground
Due to fragmentation full jet reconstruction is required to access the initial parton energy
0ORxPxPPP
Direct photontriggerFast DetectorCalorimeterzero near-side yieldfor direct photonsget the initial parton energy with a powerful alternative method: Direct -hadron azimuthal correlationsDirect photon is not a surface bias probe.The Road Behind
Examples of higher order diagrams
Examples of Bremsstrahlung diagrams
ComptonAnnihilation18Direct photonThe Road BehindDirect photon-hadron correlationsDirect photon energy balances the outgoing parton.
Calibrated probe of the QGP at LO.No Surface BiasHard process Challengeable measurements!Photon doesnt couple to the medium. Possible candidate for quark/gluon jet discrimination.0 is suppressed at high pT by a factor of ~5 in central AuAu collisions.
O(s)O(s2(1/s+g))O(s2)19Analysis techniqueBuild correlation function for neutral triggers with associated charged particlesUse transverse shower profile to distinguish 2-photon from single-photon showersComparison of 0 triggered yields with previously measured charged-hadrons- triggered yields.Extract the yields associated with direct photon triggers1920Correlate photon candidate triggers with associated tracks Use triggers to explore fragmentation functions in p+p and Au+Au02E Eparton0BEMCBeam axisTPCAnalysis technique pT,trig > 8 GeV/c180E = EpartonAssociated charged particles 3 3 GeV/c points to the trigger towerOne tower out of 4800 towers (0.05 x 0.05)~2.2mCharged hadrons
2021The two photons originated from 0 hit the same tower at pT>8GeV/cAnalysis: Shower Shape Analysis i : strip energyri : distance relative to energy maxima
7 RM0Use the shower-shape analysis to separate the two close photons shower from one photon shower.
STAR Shower Maximum Detector is embedded at ~ 5x0 between the lead-scintillator layers BEMC
2122
STAR PreliminaryNear side is suppressed with centrality which might due to the increase of /0 ratio .Trigger photons-charged particles azimuthal correlations 2223Results: Effect of shower-shape cut The away-side correlation strength is suppressed compared to pp and peripheral Au+Au.Medium effectThe -rich sample has lower near-side yield than 0 but not zero.-sample is not pure direct ! How about the 0 ?Vacuum QCD
CentralityCentrality
2324Comparison of 0-triggered yields to charged-hadron triggered yields Completely different data set from different RHIC runs, different detectors were involved in the analysis too.Associated yields per trigger0-charged and charged-charged results are consistent.
Near side: Yields are similar for p+p and central Au+Au Central Au+Au?Surface bias0 sample is pure.PRL 97 162301 (2006).This analysis
Away side: Yields show big difference between p+p and central Au+Au
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0Extraction of direct away-side yieldsR=Y-rich+h/Y0+hnearnearY+h = (Y-rich+h - RY0+h )/(1-R)awayawayAssume no near-side yield for direct then the away-side yields per trigger obey Method of extract direct associated yield This procedure removes correlations due to contamination (asymmetric decay photons+fragmentation photons) with assumption that correlation is similar to 0 triggered correlation at the same pT.
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Direct 0Associated yields per triggerFragmentation function of direct triggers and 0 triggers The away-side yield per trigger of direct triggers shows smaller value compared to 0 triggers which is consistent with partons loose energy dense medium and then fragment. Differences between and 0 triggers0 -triggers are resulted from higher parton energy than -triggers.0 -triggers are surface biased.Color factor effect.What is the medium color charge density?
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Icp agrees with theoretical predictions.Results: Medium effect on fragmentation function Icp(zT) =D0-10% (zT)D40-80% (zT)
STAR Preliminary 7 < pT < 9 GeV/ctrigMore precision is needed for the measurements to distinguish between different color charge densities.
STAR PreliminaryWithin the current uncertainty in the scaling the Icp of direct and 0 are similar.If there is no medium effectIcp(zT) = 1Strong medium effectIAA(zT) =DAA (zT)Dpp (zT)8 < pT < 16 GeV/ctrigpT > 3 GeV/cassocData points2728First result of -jet azimuthal correlations and fragmentationfunction D(zT) in AuAu at RHIC energy is reported.All results of 0s near and away-side associated particle yields shows consistency with that of charged hadron triggers. Summary and OutlookLarge luminosity at RHIC enables these measurements. Expect reduced uncertainties from further analysis and future runs.
Away-side yield for direct photons is significantly suppressed in heavy ion events. Suppression level agrees with theoretical expectations.2829Thank you for your attention and thanks to all STAR Collaborators30Shower Shape Cuts:Reject most of the 0s.highly asymmetric 0 decay.But do not reject photons from:s - similar level of background as asymmetric 0fragmentation photons10% of all 0 with pT > 8 GeV/c10% of inclusive at intermediate pT in p+p~30-40% of direct at PT > 8 GeV/c.
Limitations of the shower shape cut 3031
Breakdown of factorization claimed in dijets at N3LO Collins, Qiu 07Measurement of the differential cross section for the production of an isolated photonwith associated jet in pp collisions at s =1.96 TeVarXiv:0804.1107v2
