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The Quark Gluon Plasma at RHIC

The Quark Gluon Plasma at RHIC. Colloquium Caltech Barbara V. Jacak Stony Brook Oct. 27, 2005. outline. What’s a plasma? and why do we expect one from quarks and gluons? The tools to make and study quark gluon plasma What do we see at RHIC? collective flow opacity of the matter

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The Quark Gluon Plasma at RHIC

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  1. The Quark Gluon Plasma at RHIC Colloquium Caltech Barbara V. Jacak Stony Brook Oct. 27, 2005

  2. outline • What’s a plasma? • and why do we expect one from quarks and gluons? • The tools to make and study quark gluon plasma • What do we see at RHIC? • collective flow • opacity of the matter • excitations of the matter? • J/y suppression to search for deconfinement • Conclusions • what we have found • what HAVE we found?

  3. what is a plasma? • 4th state of matter (after solid, liquid and gas) • a plasma is: • ionized gas which is macroscopically neutral • exhibits collective effects • interactions among charges of multiple particles • spreads charge out into characteristic (Debye) length, lD • multiple particles inside this length • they screen each other • plasma size > lD • “normal” plasmas are electromagnetic (e + ions) • quark-gluon plasma interacts via strong interaction • color forces rather than EM • exchanged particles: g instead of g

  4. Quarks, gluons, hadrons • 6 quarks: • 2 light (u,d), 1 sort of light (s) • 2 heavy (c,b), 1very heavy (t) • flavor & color quantum numbers • Quarks are bound into hadrons • Baryons (e.g. n, p) have 3 • Mesons (e.g. p, K, f): 2 (q + anti-q) • Colored quarks interact by exchange of gluons • Quantum Chromo Dynamics (QCD) • Field theory of the strong interaction • parallels Quantum Electrodynamics (QED) • EM interactions: exchanged photons electrically uncharged • gluons carry color charge

  5. At high temperature and density: force is screened by produced color-charges expect transition to gas of free quarks and gluons + +… QCD phase transition Color charge of gluons  gluons interact among themselves • theory is non-abelian • curious properties at large distance: • confinement of quarks in hadrons

  6. Karsch, Laermann, Peikert ‘99 ~15% from ideal gas of weakly interacting quarks & gluons e/T4 T/Tc Tc ~ 170 ± 10 MeV (1012 °K) e ~ 3 GeV/fm3 non-perturbative QCD – lattice gauge theory required conditions to study quark gluon plasma

  7. to get there: collide BIG ions at v ~ c • Create high(est!) energy density matter • similar to that existing ~1 msec after the Big Bang • can study only in the lab – relics from Big Bang inaccessible • T ~ 200-400 MeV (~ 2-4 x 1012 K) • e ~ 5-15 GeV/fm3 (~ 1030 J/cm2) • R ~ 10 fm, tlife ~ 10 fm/c (~3 x 10-23 sec) • Characterize the hot, dense medium • does medium behave as a plasma? coupling weak or strong? • What’s the density, temperature, radiation rate, collision frequency, conductivity, opacity, Debye screening length? • probes: passive (radiation) and those created in the collision

  8. ideal gas or strongly coupled plasma? how does it compare to interesting EM plasmas? • Huge gluon density! • estimate G = <PE>/<KE> • using QCD coupling strength g • <PE>=g2/d d ~1/(41/3T) • <KE> ~ 3T • g2 ~ 4-6 (value runs with T) • G ~ g2 (41/3T)/ 3T so plasma parameter G ~ 3  quark gluon plasma should be a strongly coupled plasma • As in warm, dense plasma at lower (but still high) T G > 1: strongly coupled, few particles inside Debye radius

  9. RHIC at Brookhaven National Laboratory Collide Au + Au ions for maximum volume s = 200 GeV/nucleon pair, p+p and d+A to compare

  10. STAR 4 complementary experiments

  11. p-p PRL 91 (2003) 241803 Good agreement with NLO pQCD Study simple complex systems: p+p, “p”+A, A+A collisions is QCD the right theory at RHIC? Perturbative for high p transfer processes?  pp collisions: itworks! Have a handle on initial NN interactions by scattering of q, g inside N p0

  12. look at radiated & “probe” particles • as a function of transverse momentum • pT = p sin q • q with respect to beam direction • 90° is where the action is (max T, r) • midway between the two beams! • pT < 1.5 GeV/c • “thermal” particles from the bulk of the plasma • pT > 3 GeV/c • fast particles – mostly part of jets of hadrons coming from hard scattered q or g • produced very early, probe the plasma

  13. z y x Almond shape overlap region in coordinate space search for collectivity (plasma feature) momentum space dN/df ~ 1 + 2 v2(pT) cos (2f) + … “elliptic flow”

  14. Hydro. Calculations Huovinen, P. Kolb, U. Heinz Kolb, et al Hydrodynamics can reproduce magnitude of elliptic flow for p, p. BUT mass dependence → softer than hadronic EOS!! NB: these calculations have viscosity = 0 medium behaves as perfect liquid! v2 reproduced by hydrodynamics • see large pressure buildup! • anisotropy  happens fast • early equilibration STAR PRL 86 (2001) 402 central

  15. 0 1 2 pT/n (GeV/c) • v2 scales ~ with # of quarks! • quarks are the particles when the pressure is built up Elliptic flow scales as number of quarks

  16. even charm quarks flow! measure D→e± + hadrons Mcharm = 1.3 GeV D’s flow to ~2 GeV/c then expect e from B decays (Mb~4 GeV → shouldn’t flow) collective flows tell us: RHIC creates matter – not just collection of particles the matter catches even the heavy c quarks!

  17. How to actively probe the deep interior? measure “hard scatterings”of q,g at large pT“transverse momenta” p pT q

  18. Direct Photon Spectra in Au+Au • q + g → q + g • Should not interact with the color charges • data and theory agree → calibrated probe • pQCD works in the complex environment of two nuclei (Au+Au ) colliding at high energies

  19. peripheral Ncoll = 12.3  4.0 central Ncoll = 975  94 strongly interacting probe: a different story!

  20. suppression persists to 20 GeV/c! nuclear modification factor ratio of data on previous slide

  21. so we see: photons shine, pions don’t • Direct photons are not inhibited by hot/dense medium • Pions (all hadrons) are inhibited by hot/dense medium

  22. Medium is opaque! look for the jet on the other side STAR PRL 90, 082302 (2003) Peripheral Au + Au Central Au + Au

  23. Pedestal&flow subtracted Are back-to-back jets there in d+Au? Yes!

  24. QGP properties, so far • Extract from models, constrain by data Equation of state? Early degrees of freedom and their s? Deconfinement? Thermalization mechanism? Conductivity?

  25. Lattice QCD shows qq resonant states at T > Tc, also implying high interaction cross sections How to get fast equilibration & large v2 ? parton cascade using free q,g scattering cross sections doesn’t work! need s x50 in medium

  26. What is going on? • The objects colliding inside the plasma are not baryons and mesons • The objects colliding also do not seem to be quarks and gluons totally free of the influence of their neighbors • Quarks and gluons are interacting, but are not locally (color) neutral like the baryons & mesons. Neutrality scale likely larger, as expected for a plasma.

  27. strongly coupled gas of atoms • M. Gehm, S. Granade, S. Hemmer, K, O’Hara, J. ThomasScience 298 2179 (2002) strongly coupled ↓ elliptic flow! weakly coupled

  28. what about the heavier quarks? e± in Au+Au vs <Ncoll>*p+p peripheral collisions central collisions c quark suppression is nearly as large as for pions!

  29. where does the lost energy go? • transferred to the plasma? • does the medium respond? • look at “away side” jet’s particles near thermal pT

  30. STAR Preliminary (1/Ntrig)dN/d(Df) M.Miller, QM04 PHENIX dN/d(Df) 0 p/2 p p/2 p Df =+/-1.23=1.91,4.37 → cs ~ 0.33 (√0.33 in QGP, 0.2 in hadron gas) a sonic boom? jury is still out… g radiates energy kick particles in the plasma accelerate them along the jet

  31. RHIC How about the screening length? • J/Y • Test confinement: • do bound c + c survive? • or does QGP screening kill them?

  32. AtCERN (√s = 17 GeV): • NA50 and NA60 show suppression in Pb+Pb & In+In • suppression follows system size • Normal nuclear absorption from p+A data:  = 4.18±0.35 mb

  33. At RHIC: J/ymm muon arm 1.2 < |y| < 2.2 measured/expected dAu mm 200 GeV/c AuAu mm 200 GeV/c CuCu mm 200 GeV/c

  34. At RHIC: J/ymm muon arm 1.2 < |y| < 2.2 J/yee Central arm -0.35 < y < 0.35 dAu mm 200 GeV/c AuAu mm 200 GeV/c CuCu mm 200 GeV/c AuAu ee 200 GeV/c CuCu ee 200 GeV/c

  35. At RHIC: J/ymm muon arm 1.2 < |y| < 2.2 J/yee Central arm -0.35 < y < 0.35 ! Factor ~3 suppression in central events dAu mm 200 GeV/c AuAu mm 200 GeV/c CuCu mm 200 GeV/c AuAu ee 200 GeV/c CuCu ee 200 GeV/c CuCu mm 62 GeV/c

  36. At RHIC: J/ymm muon arm 1.2 < |y| < 2.2 J/yee Central arm -0.35 < y < 0.35 ! Factor ~3 suppression in central events Data show the same trend within errors for all beams and even at √s=62 GeV

  37. RAA vs Npart: PHENIX and NA50 • NA50 data normalized at NA50 p+p point. • Suppression similar in the two experiments, although the collision energy is 10 times higher (200GeV in PHENIX & 17GeV in NA50)

  38. What suppression should we expect? Models that were successful in describing SPS data fail to describe data at RHIC - too much suppression -

  39. can get better agreement with data if add formation of “extra” J/y by coalescence of c and anti-c from the plasma (not necessarily unique or correct explanation!)

  40. quarks & gluons retain correlations, medium exhibits liquid properties J/y may survive better NB: the (quasi-)bound states are not your mother’s hadrons! take a deep breath… • What did we expect for QGP? • What SHOULD we expect? weakly interacting gas of quarks & gluons

  41. conclusions Evidence that RHIC creates a strongly coupled, opaque plasma energy density & equation of state not hadronic! must search for plasma phenomena, not asymptotic freedom • With aid of hydrodynamics, l-QCD and p-QCD models: • e ~ 15 GeV/fm3 • dNgluon/dy ~ 1000 • sint large for T < 2-3 Tc • can get at properties of this new kind of plasma • opacity, collision frequency, EOS, screening, speed of sound, conductivity • Open questions • Do heavy quarks really thermalize in the QGP? • Initial temperature (direct g radiation from plasma?) • Discovery announcement soon?

  42. Energy  to beam direction per unit velocity || to beam pR2 2ct0 Is the energy density high enough? PRL87, 052301 (2001) Colliding system expands: • e 5.5 GeV/fm3 (200 GeV Au+Au) well above predicted transition! value is lower limit: longitudinal expansion rate, formation time overestimated

  43. Does final state reflect a thermal distribution? Assume all distributions described by a temperature Tand a (baryon) chemical potential m: dn ~ e -(E-m)/T d3p One ratio (e.g., p / p ) determines m / T : pbar/p = e -(E+m)/T /e -(E-m)/T = e -2m/T Second ratio (e.g., K / p ) provides T  m;predict others Tf ~ 175 MeV

  44. black holes at RHIC? • Not the usual ones that come to mind! • energy and particles get out (we see them) • rate of particle production scales from non-QGP producing collisions – so no evidence of eating ANY external mass/energy • This experiment has been done MANY times by nature • high energy cosmic rays impinging on atmosphere • Recent paper by Nastase uses mathematics of black holes developed by Hawking, but forces and behavior (and sizes) are quite different

  45. Possibility of plasma instability → anisotropy • small deBroglie wavelength q,g point sources for g fields • gluon fields obey Maxwell’s equations • add initial anisotropy and you’d expect Weibel instability • moving charged particles induce B fields • B field traps soft particles moving in A direction • trapped particle’s current reinforces trapping B field • can get exponential growth • (e.g. causes filamentation of beams) • could also happen to gluon fields early in Au+Au collision • timescale short compared to QGP lifetime • but gluon-gluon interactions may cause instability to saturate → drives system to isotropy & thermalization

  46. do heavy quarks thermalize? elliptic flow? energy loss!

  47. r/ ggg d + Au collisions cent/periph. (~RAA) Saturation of gluons in initial state(colored glass condensate) Mueller, McLerran, Kharzeev, … Wavefunction of low x (very soft) gluons overlap and the self-coupling gluons fuse. Saturation at higher x at RHIC vs. HERA due to nuclear size  suppressed jet cross section; no back-back pairs

  48. Open charm: baseline is p+p collisions PHENIX PRELIMINARY Measure charm s via semi-leptonic decay to e+ & e- p0, h, photon conversions are measured and subtracted fit p+p data to get the baseline for d+Au and Au+Au.

  49. Implications of the results for QGP • Ample evidence for equilibration • initial dN(gluon)/dy ~ 1000, energy density ~ 15 GeV/fm3, energy loss ~ 7-10 GeV/fm • Very rapid, large pressure build up requires • parton interaction cross sections 50x perturbative s

  50. How to get 50 times pQCD s? spectral function • Lattice indicates that hadrons don’t all melt at Tc! • hc bound at 1.5 Tc Asakawa & Hatsuda, PRL92, 012001 (2004) • charmonium bound states up to ~ 1.7 Tc Karsch; Asakawa&Hatsuda • p, s survive as resonances Schaefer & Shuryak, PLB 356 , 147(1995) • q,g have thermal masses at high T. as runs up at T>Tc? (Shuryak and Zahed) • would cause strong rescattering qq  meson

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