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The STAR Experiment

The STAR Experiment. Direct -charged hadrons measurements in STAR. High- pT physics at LHC 2009, 4-7th February, Prague, Czech Republic. Texas A&M University A. M. Hamed for the STAR collaboration. Table of Contents and Disclaimer. Table of Contents:. The Road Behind. Analysis.

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The STAR Experiment

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  1. The STAR Experiment Direct -charged hadrons measurements in STAR High-pT physics at LHC 2009, 4-7th February, Prague, Czech Republic Texas A&M University A. M. Hamed for the STAR collaboration

  2. Table of Contents and Disclaimer Table of Contents: • The Road Behind • Analysis • Results • Summary • Disclaimer: The road behind is personal view, so biases and mistakes are expected.

  3. The Road Behind: Why study high-pt particles? • High-pT particles are produced from the hard scattering processes. At mid-rapidity Factorization Hard Scattering: High-pt Particle “biased event” p+p e-6pT High momentum transfer P xP xP PDF: Extracted from data but evolution is perturbative “DGLAP" P Power law Hard scattering: Expansion in coupling constant (LO, NLO, NNLO, ..) NB: Factorization used in many context without proof Heavy-ion collisions Breakdown of factorization claimed in dijets at N3LO, Collins, Qiu ,07 • Their rates are calculable via pQCD. • Take place at early time of collisions (~0.01 fm/c). V~ 5 fm3 and  ~ 10 fm/c

  4. The Road Behind: Methods for high-pt particles “ An interesting signature may be events in which the hard collision occurs near the edge of the overlap region, with one jet escaping without absorption and the other fully absorbed” J.D.Bjorken 1982 “elastic scattering?” 1. Jet reconstruction Detection efficiency, quark- versus gluon-jet properties, Jet-mass effects. The jet modifies the medium as well as the medium quench the jet. 2. Inclusive single high-pt particle spectra- Leading particle method “RAA” dAu control experiment Glauber model uncertainties, Parton distribution functions , CGC. RAA is a measure of the deviation from the incoherent superposition of nucleon-nucleon collisions assumption. In DIS Q2 values are of orders of magnitude greater than the typical energies and momenta in nuclear physics. 3. Associated yield of high-pt particle , Fragmentation Function, dijet events “IAA”. Better to shed insight on the underlying physics , no Glauber model, modified FF, yield normalized per tigger.

  5. The Road Behind: Exp. results of high-pt particles “RAA, IAA” PRL 98 (2007)192301 • Hadron RAA is pt independent as expected by the radiative energy loss . • Direct photons follow the binary scaling. Number of binary scaling works! Equark,m=0  Equark,m>0 • Unexpected level of suppression for the heavy quarks. Egluon  Equark E CR  • No sign for the color factor effect on energy loss. • Similar near-side and strongly suppressed away-side in Au+Au relative to p+p and d+Au. • Away-side yield strongly suppressed to the level of RAA • IAA is zT independent and there is no broadening in the associated correlation peak .

  6. The Road Behind: Theoretical models “radiative energy loss” • The four major models use pQCD framework to estimate energy loss. • Different assumptions in various models lead to similar descriptions of the • light quark suppression with different model-dependent parameters. ^ ^ q q • LPM-effect based approaches: BDMPS & AMY • Opacity expansion: GLV & ASW ^ q1 GeV2/fm • radiative energy loss • Medium enhanced higher twist effects If s(T) were weak… ^ q extracted via comparison with RHIC data is larger… • Medium modified MLLA Differences • Modeling the medium evolution/structure. • Hierarchy of scales: E, Q,  Ways of extrapolation of pQCD in non-pQCD regime, just make s small ASW and GLV: Similar models different AMY and Higher twist: Different models same ^ Data doesn’t allow to distinguish between q 5 or 15 GeV2/fm There 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.

  7. The Road Behind: Time-ordered Energy loss What happens to empty space, if you keep adding heat? “Static medium, Energy dependence” dErad/dz CR s E <q2> Relative phase: form<<  Bethe-Heitler limit, E is fixed and  ^ The 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). q “We will not have done justice to the concept of weakly interacting plasma of quarks and gluons until some of the predictions are confirmed by experiment” form >> LPM limit, E and  >> 1 dErad/dz CR sln(E) <q2> ^ Where is the q ? At T >> Tc :   gT >> QCD : dynamical scale of the medium, color screening scale “mass”, 1/ color screening length F. Wilczek Medium E In particular, chiral symmetry is restored, and confinement comes completely undone. 1/ <<  Independent successive scattering centers Scattering power of the medium Lattice QCD The parton propagation is “time-ordered” and time-oredered perturbation theory is the natural framework to calculate the radiation amplitude. q ~20% F. Karsch, E. Laermann, A. Peikert, CH. Schmidt, S. Stickan q2=2/ Hep-lat/00010027v1  In DIS Q2 values are of orders of magnitude greater than the typical energies and momenta in nuclear physics but nuclear environment effect is significant and not understood yet! L IMHO • The 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.

  8. The Road Behind: Direct -jet azimuthal correlation Fast Detector “Calorimeter” Leading particle “trigger” Background P P  0 xP xP xP xP P P Associated particles “Mid-rapidity” zero near-side yield for direct photons Fast Detector “Calorimeter” Direct photon “trigger” • Due to fragmentation full jet reconstruction is required to access the initial parton energy OR get the initial parton energy with a powerful alternative method: “Direct -hadron azimuthal correlations” Heavy ion collision Direct photon is a surface bias free probe. Jet-energy is calibrated by “Direct ”

  9. The Road Behind: Direct  production mechanisms Direct photon: photons unaccompanied by additional hadrons Direct photon production provides an insight into the dynamics of hadronic constituents which is not obscured by their fragmentation. High-pt direct photons are produced at a rate comparable to that of single particles: perform high-statistics measurements with practical facilities. O(αs) LO are the dominant processes: Photon Bremsstrahlung O(αs2) takes only a fraction of the proton's momentum. contribution is suppressed by a factor of  with respect to single-0 rate. This suppression is offset somewhat: q fragmentation is flatter than q0. Annihilation Compton 10% of inclusive  at intermediate pT in p+p “PHENIX”! ~30-40% of direct  at PT > 8 GeV/c in p+p “NLO pQCDVogelsang”, Examples of Bremsstrahlung diagrams Heavy ion collisions Sources of suppression and enhancement of direct photons yield. RAA saturates “pt-independent” LPM effect RAA follows binary scaling

  10. The Road Behind: Direct -jet production mechanisms Both mechanisms yield associated photons recoiling against a gluon or quark jet depending on the value of xT . In the approximation that the colliding partons are collinear in the CMS frame: D0: NLO pQCD is unable to describe the shape of the pT dependence of the  across four Different kinematic regions simultaneously . arXiv:hep-ex/0804.1107 Effects due to intrinsic motion Enhancement of single-particle and jet cross sections due to the parton transverse momentum at moderate pT. The effect of parton kT is greatly reduced in the case of direct gamma- jet compared to single photon cross section. J. F. Owens, Phys. Rev. D 20, 221 (1979) Direct photon-hadron correlations Direct photon energy balances the outgoing parton, module negligible correction from initial state radiation. Calibrated probe of the QGP – at LO. No Surface Bias. Hard process. Possible discrimination power for q/g • Challengeable measurements! 0 is suppressed at high pT by a factor of ~5 in central AuAu collisions.

  11. Analysis: Analysis technique • Build correlation function for neutral “triggers” with “associated” charged particles • Use transverse shower profile to distinguish 2-photon “0” from single-photon showers “rich” • Comparison of 0 – triggered yields with previously measured h triggered yields. 0 rich • A method of statistical subtraction of yield • associated per direct trigger using thefactthatdirect • photon has no near side yieldandassumingall sources of • background have similar correlations • to that of symmetric decay 0.

  12. Analysis: -jet azimuthal correlation in STAR One tower out of 4800 towers (0.05 x 0.05) No track with p > 3 GeV/c points to the trigger tower ~2.2m Eγ= Eparton  2 Beam axis 0 180° Use  triggers to explore fragmentation functions in p+p and Au+Au Associated charged particles “3 <pT< 16 GeV/c” Eπ‹ Eparton 0 • Correlate photon candidate “triggers” with “associated tracks” pT,trig > 8 GeV/c Charged hadrons 3 <pT < 16 GeV/c Online trigger: Etower > 5.76 GeV, Ecluster > 7.44 GeV “cluster =2 towers out of 3x3 towers” Au+Au 506 ub-1 (p+p 19.6 pb-1) BEMC TPC Offline trigger: Etower > 6 GeV, Ecluster > 8GeV, Esmd > 0.5 GeV, Cluster is away from the tower edge BEMC: Barrel Electro-Magnetic Calorimeter Track quality, eff. TPC: Time Projection Chamber Event general QA Full azimuthal coverage |TPC| < 1.5, |TPC selected| < 1 |bemc| < 1, |bemcselected| < 0.9 How to distinguish between 0/ ?

  13. Analysis: Transverse shower shape analysis in STAR  7 RM 0 The two photons originated from 0 hit the same tower at pT>8GeV/c STAR Shower Maximum Detector is embedded at ~ 5x0 between the lead-scintillator layers “BEMC” i : strip energy The tower energy asymmetry cut to purify the rich sample in case of 0 decay across the module in  ri : distance relative to energy maxima Use the shower-shape analysis to separate the two close photons shower from one photon shower. Frag. Photons, asymmetric decay of pi0, and eta?

  14. Results: inclusive -charged hadrons azimuthal correlation Data Set: L=535 ub-1 of Au+Au and L=11 pb-1 of p+p STAR Preliminary STAR Preliminary Clear dijet structure is seen for inclusive  – charged hadron azimuthal correlation in STAR Background level increases with centrality as expected Both near side yield and away side increase with trigger energy as the initial parton energy increases. Near side is suppressed with centrality which might due to the increase of /0 ratio . Near and away side yields decrease with associated pt: the jet cross section falls more steeply than the Fragmentation Function does.

  15. Results: rich and 0-charged hadronazimuthal correlation Centrality Centrality Vacuum QCD Medium effect • The away-side correlation strength is suppressed compared to pp and peripheral Au+Au. • The -rich sample has lower near-side yield than 0but not zero.

  16. Results: Associated yield with 0 triggerresults The near-side associated yield of 0trigger is consistent with that of previous measurements of ch-ch correlations over different collision systems. No significant medium effect on the near-side yield. The away-side yield is continuously suppressed with centrality. The away-side yield of 0trigger is pt-independent at the same centrality and trigger. The IAA of 0 triggers and charged hadron triggers are similar.

  17. Results: Associated yield with 0 trigger vs. h trigger results Surface bias ? Central Au+Au • Away side: Yields show siginificant medium effect Completely different data set from different RHIC runs, and different detectors were involved in the analysis. This analysis PRL 97 162301 (2006). Associated yields per trigger • Near side: Yields are similar for p+p, d+Au and central Au+Au • 0-charged and charged-charged results are consistent.

  18. Results: Method of extraction away-side yield of direct   0 Background is dominated by 0symmetric decay Ydir+h = 0 NS Way to estimate systematic errors Use Pythia to estimate the contribution from other sources and propagate it in Au+Au

  19. Results: Associated yield with direct  trigger results The associated yield with direct gamma trigger: agrees with theoretical model predictions. Shows no associated pt-dependence within the current scaling uncertainty. Has a value similar to RAA of charged hadrons. Has a value similar to IAA of 0 triggers and charged hadron triggers. No sign for the color factor effect. 0 h 0h h  h h  IAA , IAA, IAA, RAA and RAA suppress to the same level.

  20. Results: Associated yield of direct  vs. 0 triggers results Differences between  and 0 triggers 0 -triggers are resulted from higher parton energy than -triggers. 0 -triggers are surface biased. Color factor effect. The associated yield with direct  trigger: Shows smaller yield compared to 0 trigger at the same centrality and zT. Supporting evidence for direct  production. The difference between the associated yields with direct  and 0indicates the absence /less dominant of color factor effect.

  21. Results: Medium effect on the associated yield of direct  Eliminate the effect of the difference in the parton initial energy. = associated yield per trigger in Au+Au 0-10% / associated yield per trigger in Au+Au 40-80% The medium effect on the associated yield with direct  agrees with the theoretical prediction. • More precision is needed for the measurements to distinguish between different color charge densities. Within the current uncertainty the medium have similar effect on the away-side of direct  and 0.

  22. Results: Future of direct -jet measurements at STAR Projection is for ET γ> 15 GeV, associated particle pT from 4-6 GeV/c. Luminosity Projections More to come soon: • Improving correlated/uncorrelated systematics. • Check for kT effect on direct -jet azimuthal correlation. • More statistics with d+Au (Run 8) results to reduce scaling uncertainties. • Out-of-plane and in-plane of direct -jet azimuthal correlation. • Study the low zT region, low pT associated . • Search for the glasma.

  23. Summary and Outlook • All results of 0’s near and away-side associated particle yields show reasonable consistency with that of charged hadron triggers within the measured pT trigger and pT associated. • Latest result of -jet azimuthal correlations, IAA (pT assoc.) and fragmentation function D(zT) in Au+Au at RHIC energy is reported. • Associated yield for direct photons is significantly suppressed • compared to that of 0 as a reflection of the difference in the parton • initial energy. This suppression level agrees with theoretical predictions. • IAA and Icp of direct  show neither pT nor zT dependence within the current • uncertainties and the measured pT trigger and pT associated. • Path length-dependence of energy loss make no significant difference between 0’s and direct -results within the current uncertainty and the measured pT trigger and pT associated. • No sign for the color factor effect within the current study. • Large luminosity at RHIC enables these measurements. Expect reduced uncertainties from further analysis and future runs, more precise study to come soon.

  24. Thank you for your attention

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