First Results from RHIC

1. First Results from RHIC W.A. ZajcColumbia University W.A. Zajc

2. Is There An Ultimate Temperature? No.7Are there new states of matter at ultrahigh temperatures and densities? From the National Research Council Committee on The Physics of the Universe report W.A. Zajc

3. ~1970: An Ultimate Temperature? • The very rapid increase of hadron levels with mass ~ equivalent to an exponential level density • and would thus imply a “limiting temperature”TH ~ 170 MeV W.A. Zajc

4. 0.66 TC T =0 0.90 TC 1.06 TC ~2000: TH  TC That is: The ‘Hagedorn temperature’ TH is now understood as a precursor of TC TC = Phase transition temperature of QCD Study confining potentialin Lattice QCD at various temperatures Current estimates from lattice calculations:TC ~ 150-170 MeVL ~ 0.70.3 GeV / fm3(latent heat) F. Karsch, hep-ph/0103314

5. early universe 250 RHIC quark-gluon plasma 200 Lattice QCD SPS 150 AGS deconfinement chiral restauration Chemical Temperature Tch [MeV] thermal freeze-out 100 SIS hadron gas 50 neutron stars atomic nuclei 0 0 200 400 600 800 1000 1200 Baryon Potential B [MeV] The Landscape of QCD W.A. Zajc

6. RHIC Specifications • 3.83 km circumference • Two independent rings • 120 bunches/ring • 106 ns crossing time • Capable of colliding ~any nuclear species on ~any other species • Energy: • 500 GeV for p-p • 200 GeV for Au-Au(per N-N collision) • Luminosity • Au-Au: 2 x 1026 cm-2 s-1 • p-p : 2 x 1032 cm-2 s-1(polarized) W.A. Zajc

7. STAR RHIC’s Experiments W.A. Zajc

8. How is RHIC Different? • Different from p-p, l-p colliders Atomic number A introduces new scale Q2 ~ A1/3 Q02 • Different from previous (fixed target) heavy ion facilities • ECM increased by order-of-magnitude • Accessible x (parton momentum fraction)decreases by ~ same factor • Access to perturbative phenomena • Jets • Non-linear dE/dx • Its detectors are comprehensive • ~All final state species measured with a suite of detectors that nonetheless have significant overlap for comparisons W.A. Zajc

9. ~All results presented here are from the RHIC Run-1 data set ~All results presented here are those Run-1 data in the refereed literature: 22 publications to date (19 PRL’s) PHENIX during last 10 days: 24 (mb)-1/week Lave(week) = 0.4  1026 cm-2 s-1 Lave(week)/Lave(store) = 27 % FY2001 – 02 100 GeV/amu FY2000 (66 GeV/amu) RHIC Running W.A. Zajc

10. 1 RHIC Event Data Taken June 25, 2000. Pictures from STAR Level 3 online display. Q. How to take the measure of such complexity?? (Is it possible?) A. (Yes.) Begin with single-particle momentum spectrum of identified hadrons W.A. Zajc

11. Results on Particle Composition PHOBOS and BRAHMS:ratios of p - / p + K- /K+p / p mid-rapidity PHENIX, STAR: spectra of p- , p0 , p+ , K- , Ks0 , K+ , p , p , f ,L , L , … W.A. Zajc

12. STAR preliminary Systematic errors ~10-20% 130 GeV RHIC : STAR / PHENIX / PHOBOS /BRAHMS 17.4 GeV SPS : NA44, WA97 Central K+/K- BRAHMS PHENIX PHOBOS STAR X+/X- p-/p+ p/p L/L Ratio (chemical fit) K+/p+ K-/p- K+/h- p/p+ p/p- K-/h- K0s/h- K*0/h- L/h- L/h- f/h- X-/h- X+/h- Model:N.Xu and M.Kaneta, nucl-ex/0104021 Ratio (data) Is there a ‘Temperature’? • Apparently: • Assume distributions described by one temperature T and one ( baryon) chemical potential m: • One ratio (e.g., p / p ) determines m / T : • A second ratio (e.g., K / p ) provides T m • Then predict all other hadronic yields and ratios:

13. STAR preliminary Systematic errors ~10-20% 130 GeV RHIC : STAR / PHENIX / PHOBOS /BRAHMS 17.4 GeV SPS : NA44, WA97 Locating RHIC on Phase Diagram • Previous figure  RHIC has net baryon density ~ 0: • TCH = 179 ± 4 MeV, B = 51 ± 4 MeV (M. Kaneta and N. Xu, nucl-ex/0104021) • RHIC is as close as we’ll get to the early universe for some time Previous Heavy Ion Experiments (CERN SPS) W.A. Zajc

14. Baryon number, found(?) at non-zero rapidities, must be measured away from 90 degrees Where do the Baryons Go? • There is a (not-quite-perfect) correspondence between longitudinal momentum (rapidity) and the angle of emission in the center-of-mass W.A. Zajc

15. Net baryon number ~2 units from central rapidity BRAHMS Results • Anti-proton/proton ratio as a function of rapidity: (Three points are reflected about y=0) Clear evidence for development of (nearly) baryon-free central region W.A. Zajc

16. Hydrodynamic Behavior • Superimposed on the thermal (~Boltzmann) distributions: • Collective velocity fields from • Momentum spectra ~ • ‘Test’ by investigating description for different mass particles: • Excellent description of particle production (P. Kolb and U. Heinz, hep-ph/0204061) W.A. Zajc

17. Late Breaking News • “Since the identified particle spectra from PHENIX and STAR come continuously, we show the current comparison of experiment vs. model in this note.” (W. Broniowski and W. Florkowski, 19-Apr-02 update to nucl-th/0112043) • 4 parameters ( T , m, velocity profile, freeze-out shape) describe ~ all identified hadrons (spectrum and yields) W.A. Zajc

18. z Hydrodynamic limit STAR: PRL86 (2001) 402 PHOBOS preliminary (scaled) spatial asymmetry y x (PHOBOS : Normalized Paddle Signal) Compilation and Figure from M. Kaneta Hydrodynamics of Elliptic Flow Parameterize azimuthal asymmetry of charged particlesas 1 + 2 v2cos (2 f) Evidence that initial spatial asymmetry is translated quickly to momentum space ( as per a hydrodynamic description) W.A. Zajc

19. RSIDE p1 ROUT Beamaxis p2 Measuring Space-Time Dimensions • Can we “image” the particle source? • No – But Hanbury-Brown—Twiss (HBT) measurements provide 3-D measure of spatial dimensions W.A. Zajc

20. Sizes • Measurements of RSIDE • ‘Large’ source • Transverse momentum dependence  strongly expanding source • In rough agreement with hydrodynamic description • But • Complete disagreement with predictions for ROUT / RSIDE • ‘Freeze-out’ time development not understood? • Problems in formalism? • An outstanding open question W.A. Zajc

21. Binary Collisions Spectators Participants Participants Spectators b (fm) Systematizing our Knowledge • To date, RHIC has run one heavy-ion species (Au  atomic number A = 197 ) • All four RHIC experiments have carefully developed techniques for determining • the number of participating nucleons NPART in each collision(and thus the impact parameter) • The number of binary nucleon-nucleon collisions NCOLL as a function of impact parameter • This effort has been essential in making the QCD connection • Soft physics ~ NPART • Hard physics ~ NCOLL W.A. Zajc

22. Making the QCD Connection • A surprising connection has emerged between softphenomena (charged multiplicity) and QCD • The measured multiplicities at RHIC are low compared to (pre-data) calculations • This is (now) understood* as a manifestation of saturation in the initial state gluon distributions • Nch ~ Nuclear Gluon Density ~ A xg (x, Q2)not ~ A xg (x, Q2)  xg (x, Q2) • ‘Understood’ in the sense that this is an area of intense theoretical activity and discussion Eskola, QM2001

23. dN/dh / .5Npart Npart Saturation in Multiplicity • Large nucleus (A) at low momentum fraction x gluon distribution saturates ~1/as(QS2) with QS2~ A1/3 • A collision* puts these gluons ‘on-shell’ r ~ A xg(x,Q2) / R2 • Parton-hadron duality maps gluons directly to charged hadrons • Each collision varies the effectiveA , i.e, the number of participants NPART • Shattering the ‘Color Glass Condensate’)

24. pp Extensions of Saturation Approach* • Use HERA data, counting rules • x G(x,Q2) ~ x-l (1-x)4 • Describe rapidity dependence: • y ~ ln(1/x)  QS2(s,y) = QS2(s,y=0)ely • Predict energy dependence: • x = QS / s QS2(s,y) = QS2(s0,y) (s/s0) l/2 • Predict10-14% increase betweens = 130 and 200 GeV • Versus 146% reported by PHOBOS * D. Kharzeev and E. Levin, nucl-th/0108006 W.A. Zajc

25. RHIC and HERA (?) • The gluon densities at low x and their Q2 evolution are the same as those used in saturation models applied at HERA: • A.M. Stasto, K. Golec-Biernat, J. Kwiecinski, Phys. Rev. Lett. 86, 596 (2001) • J. Bartels, K. Golec-Biernat, H. Kowalski hep-ph/0203258 • More detailed understanding of • the precise connection • implications for other RHIC observables an area of intense investigation W.A. Zajc

26. Rare Processes • Particle production via rare processes should scale with Ncoll , the number of underlying binary nucleon-nucleon collisions • Roughly: Small s no shadowing  per nucleon luminosity is relevant quantity • Take scaling with Ncoll as our null hypothesisfor hard processes W.A. Zajc

27. g conversion         Open Charm as a Rare Process • Via analysis of single-electron spectrum: • Measure electron pT spectrum • Quantify (or bound) all other sources of e’s • Remaining excess: from semi-leptonic decays of D’s W.A. Zajc

28. NLO PQCD calculation PHENIX Pythia Open Charm Yields • Charm cross-sections in Au+Au at RHIC, extracted assumingNcollscaling,in good agreement with world’s data, NLO pQCD PYTHIA W.A. Zajc

29. Jet Axis R ‘Jets’ at RHIC • Tremendous interest in hard scattering (and subsequent energy loss in QGP) at RHIC • Production rate calculable in pQCD • But strong reduction predicted due to dE/dx ~ path-length (due to non-Abelian nature of medium) • However: • “Traditional” jet methodology fails at RHIC • Dominated by the soft background • Investigate by (systematics of) high-pT single particles W.A. Zajc

30. Rare processes at RHIC Both PHENIX and STAR have measured charged particle spectra out to “small” cross sections W.A. Zajc

31. ‘High’ pT Data PHENIX: Has published both • Charged hadrons and • Identified p0’s to pT ~ 5 GeV/c (Run-1 Luminosity limit) for peripheral and central events Compared to expectations from binary scaling W.A. Zajc

32. A*B scaling at RHIC? Deficit opposite sign of enhancements previously seen in nuclear collisions (Cronin effect) • NO! • Compare per collision yield RAA of p0’s for • Pb+Pb CERN SPS (17 GeV) • exceed unity at pT ~ 2 GeV/c • > A*B scaling • RAA > 1 ‘Cronin effect’ • Au+Au RHIC (130 GeV) • Never reach unity • < A*B scaling(as distinct from charm yields) • Clear deficit even at the highest pT W.A. Zajc

33. Probing the suppression Is it (predicted) enhanced energy loss in hot nuclear matter?: To be determined!: • Study particle composition(next slide) • Extend to much higher pT(Run-2 data) • Study in proton-nucleus collisions(to be scheduled) W.A. Zajc

34. How Strong is the Suppression? • Strong enough to nearly extinguish pions at high pT : • At pT ~ 3 GeV/c • Protons ~ 2 x p+ • Anti-protons ~ p- • Qualitatively different from all previous data • Again– requires investigation at higher pT (Run-2) W.A. Zajc

35. Run-2 and Beyond • Run-2 (Aug-01 to Jan-02) • Au-Au at full energy (200 GeV) • RHIC reaches design luminosity • Data sets increased by ~ order-of-magnitude over Run-1 • Detectors significantly upgraded from their initial Run-1 configurations • p-p comparison running (200 GeV) • RHIC commissions p-p collisions • RHIC become first polarized hadron collider • Experiments measure vital baseline data for comparison • Experiments start on spin physics • Run-3 (Nov-02 start) • To be determined (Program Advisory Committee 26-27 Aug-02 ) W.A. Zajc

36. RHIC Dynamics From (just) the first run: • Thermodynamics • Established • Hydrodynamics • Validated • Chromodynamics • In progress W.A. Zajc

37. RHIC Open Questions • Is the quark-gluon plasma being formed in RHIC collisions? To be determined: • Does charmonium show the expected suppression from (color) Debye screening? • Is there direct (photon) radiation from the plasma? • Do the suppression effects extend to the highest pT’s? • What is the suppression pattern in cold nuclear matter? (proton-nucleus collisions) • What are the gluon and sea-quark contributions to the proton spin? (polarized proton running) W.A. Zajc

38. Closed Questions  • Has the accelerator worked? • Have the experiments worked? • Are the data analyzable? • Are they being analyzed? • Do the data validate the premise of RHIC? • Collective, ~thermal behavior • Contact with basic QCD phenomenology • Are there new phenomena? • Are there prospects for a long and fruitful experimental program?        