RHIC Experimental Overview: What we have (not) learned

1. RHIC Experimental Overview:What we have (not) learned Thomas S. Ullrich (BNL/Yale) Colliders to Cosmic Rays 2007 Granlibakken, Tahoe 25 Feb - 1 Mar 2007

2. as(Q2) ~ 1 / log(Q2/L2) Theory of Strong Interactions: QCD • “Emergent” Phenomena not evident from Lagrangian • Asymptotic Freedom • Confinement • QCD vacuum has non-perturbative structure driving • Color confinement • Chiral symmetry breaking • Glue: Responsible for > 98% of the visible mass in universe QuantumChromoDynamics QCD leaves ample room for fundamental investigation via experiment

3. QCD Lattice Calculations TC ~ 175 MeV  eC ~ 1 GeV/fm3 • Recently extended to mB> 0 • mB> mcritical transition of 1st or 2nd order • mB < mcritical likely crossover Critical energy density: The Landscape of QCD Plasma ≡ ionized gas which is macroscopically neutral & exhibits collective effects Usually plasmas are e.m., here color forces • T >> LQCD: • weak coupling as(Q2, T)  deconfined phase

4. The Phase Transition in the Laboratory Chemical freezeout (Tch  Tc): inelastic scattering ceases Kinetic freeze-out (Tfo Tch): elastic scattering ceases

5. Long Island Relativistic Heavy Ion Collider - RHIC • 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) • Large √s  • Access to reliable pQCD probes • Clear separation of valence baryon number and glue • Routine operation at 2-4 x design luminosity (Au+Au) • Previous runs: • Species: Au+Au, d+Au, Cu+Cu, p+p • Energies √s : • 22 GeV (Au+Au, Cu+Cu, p) • 56 GeV (Au+Au) • 62 GeV (Au+Au,Cu+Cu, p+p) • 130 GeV (Au+Au), • 200 GeV (Au+Au, Cu+Cu, d+Au, p+p) • 410 GeV & 500 GeV (p) BRAHMS PHOBOS PHENIX STAR

6. Experiments at RHIC STAR Solenoidal Field Large- Tracking TPC’s, Si-Vertex Tracking RICH, EM Cal, TOF ~500 Collaborators PHENIX Axial Field High Resolution & Rates 2 Central Arms, 2 Forward Arms TEC, RICH, EM Cal, Si, TOF, -ID ~500 Collaborators Silicon Vertex             Tracker Coils Magnet E-M Calorimeter Time Projection           Chamber Time of    Flight Electronics Platforms Forward Time Projection Chamber • Leptons, Photons, and Hadrons in Selected • Solid Angles • Simultaneous Detection of Various Phase • Transition Phenomena • Measurements of Hadronic Observables • using a Large Acceptance • Event-by-Event Analyses of Hadrons • Jets

7. What Did We Find at RHIC and How? • Will present sample of results from various points of the collision process: • Final State • Yields of produced particles Thermalization, Hadrochemistry 3. Probes of dense matter Tomography: jets traversing the hot and dense matter 2. Early State Hydrodynamic flow from initial spatial asymmetries

8. “Peripheral” “Central” Reaction Plane Final State • Does the huge abundance of final state particles reflect athermaldistribution? Central: 200 GeV Au+Au: ~4800 charged particles in final state • In these complicated events, we have (a posteriori ) control over the event geometry: • Degree of overlap • Orientation with respect to overlap

9. A. Adronic et al., NPA772:167 Origin of the (Hadronic) Species • Ansatz: • Assume all 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 ratios and yields  Temperature of hadron gas at chemical freeze-out T ~ 160 MeV, mb ~ 20 MeV NOTE: Truly thermal implies No memory (!)

10. out-of-plane y number of particles  1 + 2v2cos(2) + 2v4cos(4) + … Au nucleus in-plane x Au nucleus z Non-central Collisions Elliptic Flow – Indicator for Early Thermalization Use a Fourier expansion to describe the angular dependence of the particle density • v2 provides information about the interactions while the system was still oblong Au+Au at b=7 fm • shape washes out as it expands • v2 sensitive to early interactions and pressure gradients

11. The “Flow” is ~Perfect • Huge asymmetry found at RHIC • massive effect in azimuthal distribution w.r.t reaction plane • The “fine structure” v2(pT) for different mass particles shows good agreement with ideal (zero viscosity) hydrodynamics  “perfect liquid” • Hydro favors soft equation of state liquid is not a hadron gas

12. baryons mesons The Constituents “Flow” • Scaling flow parameters by quark content nq (baryons=3, mesons=2) resolves meson-baryon separation of final state hadrons  liquid of constituents (partons)

13. Simplest way to establish the properties of a system Calibrated probe (electrons, X-Rays) Calibrated interaction (beam of known energy and direction) Suppression pattern tells about density profile Probes of Dense Matter – Jet Tomography Tomography: • Simplest way to establish the properties of a system • Calibrated probe (jets -> leading hadrons – pT distributions) • Calibrated interaction (energy loss in the hot, dense medium) • Suppression pattern tells about density profile Au+Au Collision p+p Collision

14. How to Measure ? Compare Au+Au with p+p Collisions  RAA Nuclear Modification Factor: Average number of NN collision in an AA collision No “Effect”: R < 1 at small momenta R = 1 at higher momenta where hard processes dominate Suppression: R < 1

15. High-pT Suppression – Matter is Opaque • Observations at RHIC: • Photons are not suppressed • Good! g don’t interact with medium • Ncoll scaling works • Hadrons are not suppressed in peripheral collisions • Good! medium not dense • Hadrons are suppressed in central collisions • Huge: factor 5 • Azimuthal correlation function shows ~complete absence of “away-side” jet • Partner in hard scatter is absorbed in the dense medium

16. Cause of Jet Quenching ? • Elastic scattering (Bjorken 1982) • Gluon Bremsstrahlung (factor ~ 10 larger) • Multiple final-state gluon radiation of the produced hard parton induced by the traversed dense colored medium • Theory: • BDMPS, GLV • DEloss ~ gluon (gluon density) • DEloss ~ DL2 (medium length) • Deduced initial density at t0 = 0.2 fm/c: • e≈ 15-25 GeV/fm3 • Other “causes” were ruled out

17. Q Dead cone effect implies lower heavy quark energy lossin matter: High-pT Heavy Quarks also Suppressed Dokshitzer and Kharzeev, PLB 519 (2001) 199. Indicatesubstantial suppression of charm in central Au+Au collisions almost on same level to that of light mesons  theory is struggling to describe observation collisional E-loss appears to play strong role ?! Wicks, Horowitz, Djordjevic and Gyulassy, nucl-th/0512076 electrons from heavy flavor c,b  e K n centrality

18. Fluid Effects on Jets ? • Mach cone? • Jets travel faster than the speed of sound in the medium. • Depositing energy via gluon radiation. • QCD “sonic boom” (?) • To be expected in a dense fluid which is strongly-coupled

19. Observation of Mach Cone? • Seen in di-hadron correlation functions in Df : Modifications to di-jet hadron pair correlations in Au+Au collisions at √sNN = 200 GeV Sensitive to • Speed of sound • Equation of state Df Df S.S. Adler et al., PRL 97:052301, 2006

20. To Summarize (so far) • At RHIC we see the • hottest • densest • matter • ever studied in the laboratory • flows • as a (nearly) perfect fluid • with systematic patterns consistent withquark degrees of freedom • and a viscosity to entropy density ratio • lower • than any other known fluid T=200-400 MeV ~ 3.5·1012 K e=30-60 enuclear matter large “elliptic” flow valence quark scaling All hints towards a strongly coupledsystem

21. How Perfect is “Perfect” ? • All “realistic” hydrodynamic calculations for RHIC fluids to date have assumed zero viscosity • h = 0 “perfect fluid” • But there is a (conjectured) quantum limit: “A Viscosity Bound Conjecture”, • Where do “ordinary” fluids sit w.r.t. this limit? • RHIC “fluid” mightbe at ~2-3 on this scale (!) T=1012 K

22. We’ve yet to understand the discrepancy between lattice results and Stefan-Boltzmann limit The success of naïve hydrodynamics requires (?) very low viscosities Both are predicted by “AdS/CFT” correspondence between “stringy gravity” and (supersymmetric) QCD cousins: Calculating In Strongly-Coupled Gauge Theories

23. RHIC, Strings, and the Maldacena Conjecture The coming year will see a number of interesting developments as the Large Hadron Collider (LHC) goes online. … Also of interest is the recent application of string theory to the physics being done at the Relativistic Heavy Ion Collider (RHIC), where string theory permits some calculations that would otherwise be intractable. The idea at RHIC is to better understand the strong force that binds together the elements of a nucleon, and 2007 may see the theoretical advances of string theory inform the experimental results from RHIC. —Lisa Randall, Harvard University seedmagazin.com, January 3, 2007 Of course, some disagree... ...but in the end the “right” approach will be validated by both qualitative concepts, and quantitative predictions New Yorker, Jan 8, 2007

24. Strongly Coupled Plasma: sQGP • “Formerly known as quark-gluon plasma?” • You can still use that label if you like, but  PARADIGM SHIFT • RIHC does not produce asymptotically “free” quarks and gluons • Contrary to expectations (and announcements ! ), we did not find evidence for “quarks (that)are liberated to roam freely” • The analogy to atomic plasmas is also strained: • Atomic plasmas: • Can vary density and temperature independently • Can be strongly-coupled or weakly coupled • “QGP” • One number (the temperature T) determines all properties • Intrinsically strongly-coupled fluid for any(?) accessible T

25. RHIC Future The fundamental matter created at RHIC compels further investigation: • How imperfect is its “perfection” ? • How does it respond to truly heavy probes? (charm, beauty) • Can even higher energy densities be achieved in U+U collisions? • Can we introduce a new parameter mB and search for the QCD critical point in lower-energy collisions? • Near-term goals: • Addressed by ongoing upgrades to STAR and PHENIX • Vertex detectors, increased coverage, improved triggering capabilities • Electron Beam Ion Source (EBIS) to extend ranges of species • Mid-term goal: • RHIC-II  x10 Luminosity increase by electron cooling • Efficient access to the rare probes that have proven so incisive in the first generation discovery measurements at RHIC.