1. X Mexican School of Particles and Fields Playa del Cármen, México November 2, 2002 Manuel Calderón de la Barca Recent advances from the STAR Experiment Highlights from Inclusive hadron spectra & Azimuthal correlations

2. Manuel Calderón de la Barca X MSPF 2/Nov/2002 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. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Heavy Ions: How does nuclear matter look at high temperature?  ~ 1-3 GeV/fm 3 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

4. Manuel Calderón de la Barca X MSPF 2/Nov/2002 The Relativistic Heavy Ion Collider STAR PHENIX PHOBOS BRAHMS RHIC Design PerformanceAu + Aup + p Max  s nn 200 GeV500 GeV L [cm -2 s -1 ]2 x 10 26 1.4 x 10 31 Interaction rates1.4 x 10 3 s -1 6 x 10 5 s -1 Two Superconducting Rings Ions: A = 1 ~ 200, pp, pA, AA, AB

5. Manuel Calderón de la Barca X MSPF 2/Nov/2002 The STAR Experiment

6. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Detector components in STAR 1 st year detectors (2000) 2 nd year detectors 3 rd year detectors Central Trigger Barrel Time Projection Chamber Magnet Coils TPC Endcap & MWPC RICH Barrel EM Calorimeter Endcap Calorimeter Silicon Strip Detector Photon Multiplicity Detector Zero Degree Calorimeter Silicon Vertex Tracker Forward Time Projection Chambers Vertex Position Detectors + TOF patch

7. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Focus on high p t  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 p t jet phenomena as probe of medium  Hard scattering has been done… but not in hot medium  Measurement of fragmentation products  insight into gluon density 1 [1] R. Baier, D. Schiff, and B. G. Zakharov, Annu. Rev Part. Sci. 50, 37 (2000).

8. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Centrality and Participants in HI Centrality classes based on mid-rapidity multiplicity Preliminary  s NN = 200 GeV Uncorrected peripheral (grazing shot) central (head-on) collision spectators participants N part (Wounded Nucleons) ~ soft production N bin ~ hard processes

9. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Case I : Leading hadron suppression - Wang and Gyulassy:  E  softening of fragmentation  suppression of leading hadron yield Ivan Vitev, QM02

10. Manuel Calderón de la Barca X MSPF 2/Nov/2002 High p T hadrons in Au+Au Preliminary (nucl-ex/0206011, PRL in press) STAR

11. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Inclusive charged hadron suppression 130 GeV normalized to NN centrality dependence Clear evidence for high p T hadron suppression in central collisions  significant nuclear interactions to very high p T Now seen by all 4 RHIC collaborations (BRAHMS, PHENIX, PHOBOS, STAR) Preliminary 130 and 200 GeV, Central/peripheral

12. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Case II: Azimuthal Anisotropy, or “Elliptic Flow” STAR Preliminary 130GeV Geometry: asymmetric initial state Fourier analysis  1+2v 2 cos2(  lab -  plane ) Asymmetry + interactions creates final state azimuthal correlations: elliptic flow  lab -  plane

13. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Case II: Azimuthal Anisotropy, or “Elliptic Flow” STAR Preliminary 130GeV Geometry: asymmetric initial state Asymmetry + interactions creates final state azimuthal correlations: elliptic flow  lab -  plane Finite v 2 at high pt p T > 2GeV: v 2 constant

14. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Method I: Direct Jet Identification  jet-jet correlations in p+p?  jet-jet correlations in Au+Au?  Comparison  statistical method

15. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Method II: High p T Correlations  Statistical leading particle analysis  Histogram in 2-d  N:  vs.   project pypy pxpx 2 GeV 4 GeV N trigger : Total number of trigger particles: (4<p T <6)

16. Manuel Calderón de la Barca X MSPF 2/Nov/2002 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. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Background Subtraction  di-jets  Flow  Combinatorial background  Resonance decays  jets All  Small  Subtract large  correlations Isolate intra-jet correlations Removes di-jet signal

18. Manuel Calderón de la Barca X MSPF 2/Nov/2002 First Results: STAR 130 GeV 0-10% Most Central Significant peak remains after subtractionJets?!

19. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Jets at 200 GeV Near angle persists after large  subtractions

20. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Jets at 200 GeV  Shape  Clear near & away side signal  Same sign correlation  Unlikely due to resonance decays Near angle persists after large  subtractions

21. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Jets at 200 GeV  Shape  Clear near & away side signal  Same sign correlation  Unlikely due to resonance decays Near angle persists after large  subtractions di-jets in Au+Au?

22. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Jet Charge Expect opposite charge sign between leading, next-to-leading charged particles Measured by DELPHI Well described by LUND string model

23. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Jets at 200 GeV  Charge Ordering  Fragmentation well described by string model  Gaussian fit to near-side: Ratio of Gaussian Yields System opposite/same LUND String2.6 (0.7) p+p2.7 (0.9) 0-10% Au+Au2.5 (0.6)

24. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Jets at 200 GeV Gaussian Width Systemoppositesame p+p 0.17 (.04)0.16 (.05) 0-10% AuAu 0.20 (.05)0.15 (.07)  Charge Ordering  Fragmentation well described by string model  Gaussian fit to near-side:

25. Manuel Calderón de la Barca X MSPF 2/Nov/2002 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. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Reference Model  Au+Au correlations:  Jets  di-jets  elliptic flow  multiple hard- scatterings per event  Incorporate known sources of signal and dominant background STAR Preliminary 130 GeV

27. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Reference Model  Algorithm: Au+Au measurement Background term pp measurement Fit B in non-jet region Add p+p to background term

28. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Data Comparison to Ref. Model  Absolute scale  Background contribution increases with centrality  Near side well matched for all centralities  4/7 centrality bins  Other bins qualitatively, quantitatively similar

29. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Data Comparison to Ref. Model  Away-side suppression  Suppression increases with increasing centrality  Quantify with centrality:

30. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Quantify with Ratio Au+Au Measurement p+p Measurement background

31. Manuel Calderón de la Barca X MSPF 2/Nov/2002  Dissappearance of the Jets from the Far Side Centrality dependent numerator Common denominator  Sys. errors: v 2 +5/-20%  HIJING model: constant ratio=1 Away-side suppression in central Au+Au

32. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Suppression of away-side jet consistent with strong absorption in bulk, emission dominantly from surface ?

33. Manuel Calderón de la Barca X MSPF 2/Nov/2002  s dependence (200/130) at high p T  Inclusive spectra: growth with  s follows pQCD prediction (XN Wang) (systematic uncertainties are correlated – better estimates in progress) Rates change but shape does not.  v 2 : independent of  s for p T >2 GeV/c  Geometric origin of v 2 at high p T ?

34. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Away side suppression: open issues near side away side Preliminary peripheral central Why not 1 for peripheral?  evidently not due to experimental error or uncertainty  not due to mismeasured v 2 : even v 2 =0 has little effect for most peripheral and central Initial state effects: Shadowing in Au+Au? Nuclear k T : Initial state multiple scattering  Hijing estimate: Maximum 20% effect d+Au Resolution: Need to measure in d+Au

35. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Summary of STAR high p T measurements  hadrons at p T >~3 GeV/c are jet fragments  central Au+Au:  strong suppression of inclusive yield at p T >5 GeV/c  suppression factor ~ constant for 5<p T <12 GeV/c  large elliptic flow, finite for non-central to p T ~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. Manuel Calderón de la Barca X MSPF 2/Nov/2002 And back to our original question… If partons absorbed: large  E  large  gluon But have not yet proven partonic  E: 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. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Extra Slides

38. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Look for partonic energy loss in dense matter Thick plasma (Baier et al.): Linear dependence on gluon density  glue : measure  E  gluon density at early hot, dense phase High gluon density requires deconfined matter (“indirect” QGP signature) Gluon bremsstrahlung Thin plasma (Gyulassy et al.):

39. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Future Coming run: 50% of full barrel Electromagnetic Calorimeter triggers: high tower, E T, jet jets,  0,  electrons d+Au: Cronin effect/nuclear enhancement of inclusive yield suppression of back-to-back pairs gluon shadowing Long term:  -jet coincidences (“ultimate” jet energy loss experiment) heavy quark jets (dead cone effect) surprises….

40. Manuel Calderón de la Barca X MSPF 2/Nov/2002 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. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Energy loss in cold matter Wang and Wang, hep-ph/0202105 Modification of fragmentation fn in e-A: dE/dx ~ 0.5 GeV/fm for 10 GeV quark F. Arleo, hep-ph/0201066 x1 Drell-Yan production in  -A: dE/dx <0.2 GeV/fm for 50 GeV quark

42. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Inclusive hadron suppression at RHIC Phenix    peripheral and central over measured p+p STAR charged hadrons: central/peripheral

43. Manuel Calderón de la Barca X MSPF 2/Nov/2002 v 2 : 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. Manuel Calderón de la Barca X MSPF 2/Nov/2002 v 2 : centrality and p T dependence broad plateau, v 2 finite at p T ~10 GeV/c except for most central collisions  significant in-medium interactions to very high p T Shuryak (nucl-th/0112042): plateau exhausts initial spatial anisotropy Preliminar y

45. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Near-angle correlations at high p T Jet core  x  0.5 x 0.5  look at near-side correlations (  ~0) of high p T hadron pairs Complication: elliptic flow high p T hadrons that are correlated with reaction plane orientation are also correlated with each other (~v 2 2 ) but elliptic flow has long range correlation (  > 0.5) Solution: compare azimuthal correlation functions for  and 

46. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Non-flow effects? Non-flow: few particle correlations not related to reaction plane jets, resonances, momentum conservation,…  contrast v 2 from reaction plane and higher-order cumulants (Borghini et al.) Non-flow effects are significant 4 th order cumulants consistent with other non-flow estimates But large finite v 2 and saturation persist at high p T Preliminary

47. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Single Particle Selection       beam

48. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Away side suppression and nuclear k T near side away side Preliminary peripheral central  same thresholds for AuAu and pp  nuclear :  enhances near-side in Au+Au  suppress away-side in Au+Au  similar centrality dependence Stronger near-side correlation for p T trig >3 GeV/c than p T trig >4 GeV/c

49. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Full dataset: 4<pt(trig)<6 GeV/c

50. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Central Au+Au: 6<p T (trigger) <8 GeV/c Stronger signal but limited statistics in non-central bins

51. Manuel Calderón de la Barca X MSPF 2/Nov/2002 Combinatorial Background  Au+Au: many hard scatterings per event  Expect peak at 0, |  |  Flat, non-zero background  p+p: 1 hard scattering per event  Expect peak at 0, |  |  zero background