Recent advances from the STAR Experiment

1. Recent advances from the STAR Experiment Highlights from Inclusive hadron spectra & Azimuthal correlations Manuel Calderón de la Barca

2. Outline • Heavy Ion Physics and QCD • STAR experiment at RHIC • Measurement highlights of interest to High Energy • Case I : Inclusive charged hadron spectra • Case II: Azimuthal anisotropy • Case III: Two-particle correlations • Conclusions

3. Heavy Ions: How does nuclear matter look at high temperature? High Density QCD Matter in Laboratory Determine its properties QCD Prediction: Phase Transitions Deconfinement to Q-G Plasma Chiral symmetry restoration Relevance to other research areas? Quark-hadron phase transition in early Universe Cores of dense stars High density QCD e ~ 1-3 GeV/fm3

4. BRAHMS PHOBOS RHIC PHENIX STAR Ions: A = 1 ~ 200, pp, pA, AA, AB The Relativistic Heavy Ion Collider Two Superconducting Rings Design PerformanceAu + Aup + p Max snn 200 GeV 500 GeV L [cm-2 s -1 ] 2 x 10261.4 x 1031 Interaction rates 1.4 x 103 s -1 6 x 105 s -1

5. The STAR Experiment

6. Time Projection Chamber Magnet Silicon Strip Detector Silicon Vertex Tracker Forward Time Projection Chambers Photon Multiplicity Detector Vertex Position Detectors Endcap Calorimeter Barrel EM Calorimeter + TOF patch Detector components in STAR 1st year detectors (2000) 2nd year detectors3rd year detectors Coils TPC Endcap & MWPC Zero Degree Calorimeter Central Trigger Barrel RICH

7. Focus on high pt • We know very little about early time • Au+Au collisions to study strongly interacting matter under extreme conditions • Large momentum transfers  early time scales • Use high pt jet phenomena as probe of medium • Hard scattering has been done… but not in hot medium • Measurement of fragmentation products  insight into gluon density1 [1] R. Baier, D. Schiff, and B. G. Zakharov, Annu. Rev Part. Sci. 50, 37 (2000).

8. spectators Preliminary sNN = 200 GeV participants Uncorrected Centrality and Participants in HI Npart (Wounded Nucleons) ~ soft production Nbin ~ hard processes peripheral (grazing shot) Centrality classes based on mid-rapidity multiplicity central (head-on) collision

9. Case I : Leading hadron suppression Wang and Gyulassy: DE  softening of fragmentation  suppression of leading hadron yield Ivan Vitev, QM02

10. STAR High pT hadrons in Au+Au Preliminary (nucl-ex/0206011, PRL in press)

11. Preliminary Inclusive charged hadron suppression 130 and 200 GeV, Central/peripheral 130 GeV normalized to NN centrality dependence Clear evidence for high pT hadron suppression in central collisions significant nuclear interactions to very high pT Now seen by all 4 RHIC collaborations (BRAHMS, PHENIX, PHOBOS, STAR)

12. Asymmetry + interactions creates final state azimuthal correlations: elliptic flow lab-plane Geometry: asymmetric initial state STAR Preliminary 130GeV Case II: Azimuthal Anisotropy, or “Elliptic Flow” Fourier analysis  1+2v2cos2(lab-plane)

13. Asymmetry + interactions creates final state azimuthal correlations: elliptic flow lab-plane Geometry: asymmetric initial state STAR Preliminary 130GeV Case II: Azimuthal Anisotropy, or “Elliptic Flow” Finite v2 at high pt pT > 2GeV: v2 constant

14. Method I: Direct Jet Identification • jet-jet correlations in p+p? • jet-jet correlations in Au+Au? • Comparison statistical method

15. py trigger px associated 2 GeV 4 GeV Method II: High pT Correlations • Statistical leading particle analysis • Histogram in 2-d • N:  vs.  • project Ntrigger: Total number of trigger particles: (4<pT<6)

16. Mid-Central Au+Au Result: Au+Au Distribution • Harmonic structure • Peaks at 0, || • Non-zero mean value • How do we extract jet signal from background?

17. di-jets Flow Combinatorial background Resonance decays jets All  Small  Background Subtraction Subtract large  correlations Isolate intra-jet correlations Removes di-jet signal

18. 0-10% Most Central First Results: STAR 130 GeV Significant peak remains after subtraction Jets?!

19. Near angle persists after large  subtractions Jets at 200 GeV

20. Near angle persists after large  subtractions Jets at 200 GeV • Shape • Clear near & away side signal • Same sign correlation • Unlikely due to resonance decays

21. Near angle persists after large  subtractions Jets at 200 GeV • Shape • Clear near & away side signal • Same sign correlation • Unlikely due to resonance decays di-jets in Au+Au?

22. Jet Charge Measured by DELPHI Well described by LUND string model Expect opposite charge sign between leading, next-to-leading charged particles

23. Charge Ordering • Fragmentation well described by string model • Gaussian fit to near-side: Jets at 200 GeV

24. Charge Ordering • Fragmentation well described by string model • Gaussian fit to near-side: Jets at 200 GeV

25. What Have we Shown? • First direct evidence of jets at RHIC • What about di-jets at RHIC? • Study away side in Au+Au • But… large  subtraction removes away side • Need different method to deal with background

26. STAR Preliminary 130 GeV Reference Model • Au+Au correlations: • Jets • di-jets • elliptic flow • multiple hard-scatterings per event • Incorporate known sources of signal and dominant background

27. pp measurement Fit B in non-jet region Add p+p to background term Reference Model • Algorithm: Au+Au measurement Background term

28. Data Comparison to Ref. Model • Absolute scale • Background contribution increases with centrality • 4/7 centrality bins • Other bins qualitatively, quantitatively similar • Near side well matched for all centralities

29. Data Comparison to Ref. Model • Away-side suppression • Suppression increases with increasing centrality • Quantify with centrality:

30. Au+Au Measurement background p+p Measurement Quantify with Ratio

31.   Dissappearance of the Jets from the Far Side Centrality dependent numerator Common denominator • Sys. errors: v2 +5/-20% Away-side suppression in central Au+Au • HIJING model: constant ratio=1

32. ? Suppression of away-side jet consistent with strong absorption in bulk, emission dominantly from surface

33. s dependence (200/130) at high pT • Inclusive spectra: growth with s follows pQCD prediction (XN Wang) • (systematic uncertainties are correlated – better estimates in progress) • v2: independent of s for pT>2 GeV/c • Geometric origin of v2 at high pT? Rates change but shape does not.

34. Preliminary near side away side peripheral central Away side suppression: open issues • Why not 1 for peripheral? • evidently not due to experimental error or uncertainty • not due to mismeasured v2: even v2=0 has little effect for most peripheral and central • Initial state effects: • Shadowing in Au+Au? • Nuclear kT: Initial state multiple scattering  Hijing estimate: Maximum 20% effect Resolution: Need to measure ind+Au

35. Summary of STAR high pT measurements • hadrons at pT>~3 GeV/c are jet fragments • central Au+Au: • strong suppression of inclusive yield at pT>5 GeV/c • suppression factor ~ constant for 5<pT<12 GeV/c • large elliptic flow, finite for non-central to pT~6 GeV/c • strong suppression of back-to-back hadron pairs • Possible interpretation: • Hard scattered partons (or their fragments) interact strongly with medium • Observed fragments are emitted from the surface of the hot & dense zone created in the collision ?

36. And back to our original question… • If partons absorbed: large DE  large gluon • But have not yet proven partonicDE: where does absorption occur? • Is it an initial state, partonic effect, or late hadronic effect? • theory input: what are experimental handles to distinguish hadronic from partonic absorption? (e.g. correlation function widths) JETS JETS ?

37. Extra Slides

38. Look forpartonic energy loss in dense matter Thick plasma (Baier et al.): Gluon bremsstrahlung Thin plasma (Gyulassy et al.): • Linear dependence on gluon density glue: • measure DE  gluon density at early hot, dense phase • High gluon density requires deconfined matter (“indirect” QGP signature)

39. Future • Coming run: 50% of full barrel Electromagnetic Calorimeter • triggers: high tower, ET, jet • jets, p0, g, electrons • d+Au: • Cronin effect/nuclear <kT> • enhancement of inclusive yield • suppression of back-to-back pairs • gluon shadowing • Long term: • g-jet coincidences (“ultimate” jet energy loss experiment) • heavy quark jets (dead cone effect) • surprises….

40. Soft Physics • Chemical Freezeout ~ 170 MeV • Lattice 160 - 180 MeV • Collective motion • Large “Elliptic flow” • Large pressure gradients in the system • System seems to approach thermodynamic equilibrium • Kinetic freezeout ~ 110 MeV • Freezeout seems to be very fast, almost explosive

41. Energy loss in cold matter Wang and Wang, hep-ph/0202105 F. Arleo, hep-ph/0201066 Modification of fragmentation fn in e-A: dE/dx ~ 0.5 GeV/fm for 10 GeV quark x1 Drell-Yan production in p-A: dE/dx <0.2 GeV/fm for 50 GeV quark

42. Inclusive hadron suppression at RHIC Phenix p0: peripheral and central over measured p+p STAR charged hadrons: central/peripheral

43. v2: comparison to parton cascade Parton cascade (D. Molnar) • Detailed agreement if: • 5x minijet multiplicity from HIJING or • 13x pQCD gggg cross section  extreme initial densities or very large cross sections

44. Preliminary v2: centrality and pT dependence • broad plateau, v2 finite at pT~10 GeV/c except for most central collisions • significant in-medium interactions to very high pT Shuryak (nucl-th/0112042): plateau exhausts initial spatial anisotropy

45. Near-angle correlations at high pT • Jet core:Df x Dh ~ 0.5 x 0.5 • look at near-side correlations (Df~0) of high pT hadronpairs • Complication: elliptic flow • high pT hadrons that are correlated with reaction plane orientation are also correlated with each other (~v22) • but elliptic flow has long range correlation (Dh > 0.5) • Solution: compare azimuthal correlation functions for Dh<0.5 and Dh>0.5

46. Preliminary Non-flow effects? • Non-flow: few particle correlations not related to reaction plane • jets, resonances, momentum conservation,… •  contrast v2 from reaction plane and higher-order cumulants (Borghini et al.) • Non-flow effects are significant • 4th order cumulants consistent with other non-flow estimates • But large finite v2 and saturation persist at high pT

47. beam Single Particle Selection

48. Preliminary near side away side peripheral central Away side suppression and nuclear kT • same thresholds for AuAu and pp • nuclear <kT>: • enhances near-side in Au+Au • suppress away-side in Au+Au • similar centrality dependence Stronger near-side correlation for pTtrig>3 GeV/c than pTtrig>4 GeV/c

49. Full dataset: 4<pt(trig)<6 GeV/c

50. Central Au+Au: 6<pT(trigger)<8 GeV/c Stronger signal but limited statistics in non-central bins