Suppression of high p T hadrons from Au+Au at =130 GeV/c

1. Suppression of high pT hadrons from Au+Au at =130 GeV/c North Carolina State University Saskia Mioduszewski 14 January 2002

2. Outline • The early universe and the strong force • RHIC: Why collide heavy ions at high energies? • What we measure? • Why is high transverse momentum (pT) interesting? • The PHENIX Experiment at RHIC • Data Results: • Spectra of h±, 0 • Comparison with predictions/expectations • Why it is a novel result • Summary

3. ~ 10 ms after Big Bang Hadron Synthesis strong force binds quarks and gluons in massive objects: protons, neutrons mass ~ 1 GeV Theory of Strong Force: QCD: Quantum Chromo Dynamics two fundamental puzzles occur at the hadron synthesis confinement of quarks  quarks form hadrons broken chiral symmetry  hadrons become massive ~ 100 s after Big Bang Nucleon Synthesis protons and neutrons bind in nuclei

4. • Quarks are held together by exchanging colored gluons V~1/r at short distance V~kr at long distances • We say that quarks and gluons are confinedin hadrons – mesons & baryons The Strong Force pion proton proton • Hadrons are made of confined quarks and gluons with net zero color • Hadrons interact by exchanging other hadrons

5. Au+Au Collision VNI Simulations: Geiger, Longacre, Srivastava, nucl-th/9806102 • Nuclei collide • Matter is very hot and dense • What happens to the nuclear matter? • Matter expands and cools

6. Goals of Relativistic Heavy Ion Physics • Probe hadronic matter under extreme conditions • QCD predicts phase transition from hadronic matter to QGP in high energy nucleus-nucleus collisions • confirm this prediction • study QGP state of matter

7. QCD calculations • perturbative QCD calculations applicable only for large momentum transfer  small coupling • for small momentum transfer  large coupling only solution numerical QCD calculations on lattice QCDSP results from lattice QCD establish the QCD phase transition deconfinement and chiral symmetry at same temperature Goals of RHIC experiments: •exploit the QCD phase diagram •verify existence of QGP • study QCD confinement • study how hadrons get their masses

8. r, w, f etc.. jet p K p m Time g e Space (z) Au Au Space-time Evolution in Relativistic Heavy Ion Collisions Hadronization (Freeze-out) + Expansion Mixed phase QGP phase Thermalization Pre-equilibrium

9. The Products of a Heavy Ion Collision- What is in our detectors! • hadrons p, K, p • frequent, produced “late” when particles stop to interact • • energy density • • thermal equilibrium and collective behavior • electro-magnetic radiation g, e+e-, m+m- • rare, emitted “any time”; reach detector unperturbed by strong final state interaction • • black body radiation initial temperature • • in-medium properties of mesons  chiral symmetry restoration • “hard” probes J/y, U (->e+e-, m+m-) and jets • very rare, created “early” before QGP formation, penetrate hot and dense matter, sensitive to deconfinement • • color screeningin partonic phase  J/y suppression • • energy loss in dense colored matter jet quenching

10. hard-scattered parton from e.g. p+p cone of hadrons “jet” p p Why high pT in Au+Au Effect of nuclear collision medium on hadron pT spectra hard-scattered parton during Au+Au hadron distribution softened, broadened? increased gluon-radiation within plasma?

11. An experiment you might like to do: Modifications? Energy loss? Induced radiation? Quark probe of known energy QGP? An experiment you can actually do: View along beam direction of transverse slice in lab frame: Just after collision, hard scattered partons appear; t < 0.1fm/c Excited medium formed, t ~ 0.3fm/c; partons within medium Nuclei just before collision Partons traverse medium while it cools and expands

12. Jets in In single particle pT spectra, jets are apparent in high pT tail Low pT – exponential High pT – power law

13. SppS Collisions UA1, 900 GeV proton anti-proton s = 200, 546, 900 GeV 10’s of particles

14. RHIC Collisions sNN = 130, 200 GeV Gold Gold (center-of-mass energy per nucleon-nucleon collision) 1000’s of particles

15. Effect of collision medium on hadron pT spectra: • Parton scattering with large momentum transfer •  Hard-scattered partons (jets) present in early stages of collisions • Hot and dense medium •  Hard-scattered partons sensitive to hot/dense medium • Theory predicts radiative energy loss of parton in QGP • Emission of hadrons •  High pT hadrons (jet fragments) • Dense medium (QGP) would cause depletion in spectrum of leading hadron at high pT - “jet quenching”

16. X-N. Wang, Phys. Rev. C58 (1998) 2321 Motivation Investigate hadron pT spectra for evidence of parton energy loss (“jet quenching”) induced by dense medium It has been predicted that jet production will be affected by medium effects due to the production of hot dense matter in high energy relativistic heavy ion collisions

17. What is PHENIX? • Pioneering High Energy Nuclear Interaction eXperiment • Goals: • Broadest possible study of A-A, p-A, p-p collisions to • Study nuclear matter under extreme conditions • Using a wide variety of probes sensitive to all timescales • Study systematic variations with species and energy

18. Where is PHENIX ?

19. Where is RHIC ? RelativisticHeavyIonCollider located at BrookhavenNationalLaboratory Long Island, New York

21. PHENIX at RHIC • 2 central spectrometers • 2 forward spectrometers • 3 global detectors

22. PHENIX Detector at Collision Point East Carriage Being Moved in Place Ring Imaging Cerenkov Drift Chamber Beam-Beam Counter Central Magnet West Carriage

23. PHENIX Detector

24. Collision Geometry -- “Centrality” Spectators Participants For a given b, Glauber model predicts Npart (No. participants) and Nbinary (No. binary collisions) 15 fm b 0 fm 0 N_part 394

25. Zero Degree Calorimeters (ZDC) Sensitive to spectator neutrons Using a combination of the ZDC’s and BBC’s we can define Centrality Classes “Spectators” “Participants” “Spectators” Impact Parameter Beam-Beam Counter (BBC) . n n n p Zero Degree Calorimeter (ZDC) p p Measuring Centrality Cannot directly measure the impact parameter! peripheral central

26. How to Detect Phase Transition Look at transverse momentum spectrum of leading 0 for evidence of parton energy loss (“jet quenching”) induced by dense medium Compare spectra for most violent collisions to those for least violent collisions

27. High pT Measurements in PHENIX 2 different measurements and 3 different analyses with very different systematics • Charged Hadron Measurement • Neutral Pion Measurement • PbSc Detector • PbGl Detector

28. Left panel: (h++h-)/2 Right panel: 0 Central: 0-10% Peripheral: 60-80%  “Binary-scaled” ratio of central to peripheral ? Spectra – Charged and Neutral

29. Scaling from pp to AA (in the Absence of Nuclear Effects) • For hard-scattering processes, expect point-like scaling. For inclusive cross sections : • For semi-inclusive yields, expect :

30. Ratios We measure yield = • Define Ratios:  Effect of nuclear medium on yields

31. Expectations in nuclear collisions • In the absence of nuclear effects, these ratios are expected to be 1 at high pT • Departures from 1, measure nuclear effects • Previously observed effects: • Shadowing • “Cronin effect” (pT broadening) • Possible new effect: • Parton energy loss in dense medium

32. Central to Peripheral Ratio Charged hadrons Neutral pions peripheral: <Nbinary> = 20 ± 6 central: <Nbinary> = 905 ± 96 Ratio <= unity

33. Data as Reference QM Proc. 2001, A. Drees • Data available for large range of s, but not for 130 GeV • Power law fit to: A (p0 + pT ) -n • Interpolate pT to s=130 GeV  reference for p+p(h++h-)/2 • reference for p+p 0 = (h++h-)/2 * (0.63±0.06) Systematic errors: 20-35%

34. Left panel:(h++h-)/2 Right panel:0 Lines show comparison to binary-scaled N+N reference peripheral: <Nbinary> = 20 ± 6 central: <Nbinary> = 905 ± 96 Suppression seen in central events Spectra compared to binary scaled N+N

35. Nuclear effects observed at CERN Binary-scaled ratio to pp: RAA at SPS is greater than 1 • Ratio exhibits “Cronin effect”:

36. Charged hadrons Neutral pions a+a Data (ISR) Band of uncertainty surrounding SPS Pb+Pb(Au) Data  PHENIX data < unity (unlike previously observed Cronin effect) Ratio of per-collision yield to pp yield - RAA

37. E. Wang and X-N. Wang, nucl-th/0104031 (April 2001) Theoretical Prediction of RAA RAA = 1 for point-like scaling In the high pT region, Cronin effect dominates in the absence of energy loss

38. 0Data: 60-80%, <Nbinary> = 20 (± 6) Theory: X.N. Wang, Phys. Rev. C61, 064910 (2000). Black : no nuclear effects Red andGreen predictions include nuclear effects, but not significantly different from point-like scaling in peripheral collisions Comparison to theory - peripheral collisions Red: “Cronin” + shadowing Green: dE/dx = 0.25 GeV/fm

39. Comparison to theory – central collisions 0Data: 10% central, <Nbinary> = 905 (± 96) Inconsistent withpoint-like scaling+ shadowing + “Cronin” Not inconsistent with parton energy loss, dE/dx=0.25 GeV/fm

40. Summary • RHIC is a machine where nuclear matter can be studied under extreme conditions • Goal of colliding heavy ions at high energies is to detect and study the properties of a QCD phase transition (QGP) • One possible signature of the QGP is energy loss of “hard-scattered” partons in the dense medium • PHENIX measured charged particle pT spectra up to 5 GeV/c and neutral pions up to 4 GeV/c • Spectra exhibit significant suppression in yield at high pT in central collisions relative to peripheral collisions • Not inconsistent with parton energy loss in dense medium