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Quark Gluon Plasma: The hottest plasma in the world

Quark Gluon Plasma: The hottest plasma in the world. Barbara Jacak Stony Brook Center of Excellence Symposium Kyoto University October 1, 2007. outline. Heat nuclei: expect a plasma of quarks and gluons Quantum Chromo Dynamics (QCD) predictions Create and study it in the laboratory

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Quark Gluon Plasma: The hottest plasma in the world

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  1. Quark Gluon Plasma: The hottest plasma in the world Barbara Jacak Stony Brook Center of Excellence Symposium Kyoto University October 1, 2007

  2. outline • Heat nuclei: expect a plasma of quarks and gluons • Quantum Chromo Dynamics (QCD) predictions • Create and study it in the laboratory • expected QGP signatures • useful probes of plasma properties • p+p is a crucial benchmark – defines the expected • Surprise! evidence for strong coupling in QGP • it is very opaque (“jet quenching”) • it is a (~perfect) thermalized fluid • it catches even the heavy quarks • collective response to deposited energy • “shines” brightly

  3. asymptotic freedom At high temperature and density: screening by produced color-charges expect transition to gas of free quarks and gluons + +… QCD predicts a phase transition gluons carry color charge  gluons interact among themselves • theory is non-abelian curious properties at large distance: confinement of quarks in hadrons

  4. Phase diagram of Nuclear Matter As with other material, nuclear matter has many phases. Strong interaction → strong coupling unless very dilute or hot

  5. 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

  6. plasma • 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 • weakly or strongly coupled • depends on density (number of screeners) • can behave like a liquid or crystal if strongly coupled • “normal” plasmas are electromagnetic (e + ions) • quark-gluon plasma interacts via strong interaction • color forces rather than EM, exchange g not g • non-abelian plasma

  7. e, pressure builds up Hard scattered q,g (short wavelength) probes of plasma formed p, K, p, n, f, L, D, X, W, d, Hadrons reflect (thermal) properties when inelastic collisions stop (chemical freeze-out). color-screened QGP thermal radiation (g, g* e+e-, m+m-) probing a heavy ion collision we focus on mid-rapidity (y=0) y=1/2 ln[(E+pL)/(E-pL)] CM of colliding system 90° in the lab at collider

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

  9. The Tools STAR specialty: large acceptance measurement of hadrons PHENIX specialty: rare probes, leptons, and photons

  10. STAR events viewed by the 4 experiments

  11. Expected signatures of quark gluon plasma • enhanced strange quark production • if T ≥ ms, g + g → s + s • J/y suppression • color screening breaks up cc pairs • phase transition signatures • if first order: long particle emission time • if second order: critical fluctuations in particles • copious thermal radiation • chiral symmetry restoration • jet quenching NB: These were yes/no questions, subject to complicated S/B. But surprises lurk in the properties of the plasma!

  12. a better way:plasma study by radiated & “probe” particles • as a function of transverse momentum • pT = p sin q (with respect to beam direction) • 90° is where the action is (max T, r) • pL midway between the two beams! • pT < 1.5 GeV/c • “thermal” particles • radiated from bulk of the medium • “internal” plasma probes • pT > 3 GeV/c • jets (hard scattered q or g) • heavy quarks, direct photons • describe by perturbative QCD • produced early→“external” probe

  13. p-p PRL 91 (2003) 241803 Good agreement with pQCD 1st: benchmark probes in p+p collisions p mesons are qq bound states produced in the collisions p0 QCD works at RHIC! can use perturbation theory to calculate high p transfer probe production p+p gives baseline prediction for Au+Au

  14. peripheral Ncoll = 12.3  4.0 central Ncoll = 975  94 jet fragments in Au+Au vs. p+p in central collisions jets are quenched by the plasma

  15. (colored) q & g lose energy, photons don’t

  16. interaction of radiated gluons with gluons in the plasma greatly enhances the amount of radiation magnitude, pT dependence of observed jet quenching tells us: dNg/dy ~ 1100 or 6 <q < 24 GeV2/fm q → <pT2> transfer from medium to hard gluon per unit path length ^ these are huge! ^ jet quenching was expected

  17. e± X D Au Au D p K Experimenters must ask questions! • What happens to heavy quarks traversing QGP? • Prediction: much less energy loss • large quark mass reduces phase space for radiated gluons • Measure via semi-leptonic decays • of mesons containing • charm or bottom quarks

  18. c,b decays via single electron spectrum compare data to “cocktail” of hadronic decays

  19. pQCD: Wicks, Horowitz, Djordjevic, Gyulassy, NuclPhysA784, 426 (2007) Plasmas have collisions among constituents! including it helps larger than expected scattering s → stronger coupling surprise #2: heavy quarks DO lose energy! e± from heavy quark decay BUT – what about e± from B meson decays??!! collisions also increase gluon & u,d quark Eloss

  20. z y x Almond shape overlap region in coordinate space Plasmas have collective excitations. Do we? momentum space dN/df ~ 1 + 2 v2(pT) cos (2f) + … “elliptic flow”

  21. Kolb, et al Hydrodynamics reproduces elliptic flow of q-q and 3q states Mass dependence requires softer than hadronic EOS!! v2 is large & reproduced by hydrodynamics • large pressure buildup • anisotropy  happens fast • fast equilibration! Surprise #3: must use viscosity ~ 0 “perfect” liquid (D. Teaney, PRC68, 2003)

  22. Elliptic flow scales with number of quarks transverse KE implication: quarks, not hadrons, are the relevant degrees of freedom when the pressure is built up

  23. initial collision of saturated gluon phase? plasma instability (a growing mode transfers energy from particles into wave field)? physics of extremely strong coupling? hydrodynamic flow of quarks! • long expected, works at RHIC as long as: • viscosity per particle (h/S) vanishingly small • initial condition: thermalized system in < 0.6 fm/c • use QGP equation of state for first few fm/c • NB: learning properties by constraining hydrodynamical calculation + data constraint is standard in plasma physics! Surprise 4: HOW can system thermalize in <0.6 fm/c??? parton scattering insufficient (with pQCD cross section)

  24. Strong coupling suggests applying methods of string theory to QGP slide: R. Granier de Cassagnac Maldacena Policastro,Son, Starinets

  25. AdS/CFT also used to calculate energy loss • Gubser and collaborators • Liu, Rajagopal and Wiedemann • Teaney and Casalderrey-Solana • L. Yaffe and collaborators • Surprise #5: the matter is so strongly coupled that string theorists can have fun with it →large drag force from plasma on heavy quark! NB: this plasma is dominantly made of gluons

  26. Heavy quarks flow!! What do they tell about transport properties of QGP? do the heavy quarks equilibrate?? • This is like putting a rock in a stream • and watching if the stream • can drag it along… • Rate of equilibration gives information on the viscosity of the liquid! analogy from J. Nagle

  27. mechanism of energy loss? nucl-ex/0611018 accepted in PRL Radiative energy loss alone: fails to reproduce v2HF. Heavy quark transport model has better agreement with both RAA and v2HF. Small relaxation time t or diffusion coefficient DHQinferred for charm. But, agreement with data is not perfect…

  28. transport: diffusion is related to viscosity • diffusion = brownian motion of particles definition: flux density of particles J = -D grad n • integrating over forward hemisphere: D = diffusivity = 1/3 <v> l so D = <v>/ 3ns D  collision time, determines relaxation time data say: it is small! particle concentration l = mean free path note: viscosity is ability to transport momentum h = 1/3 r <v> l so D = h/r ~ h/S → measure D get h! heavy quark + Langevin model → D ~ few times 1/(4p) NB: h/S≥ 1/(4p) from string ↔“QCD” correspondence

  29. strongly coupled dusty plasma B. Liu and J. Goree, cond-mat/0502009 minimum observed in other strongly coupled systems – kinetic part of h decreases with G while potential part increases minimum h at phase boundary? quark gluon plasma Csernai, Kapusta & McLerran PRL97, 152303 (2006)

  30. Color screening? the famous J/y suppression J/Y (bound state of c and cbar quarks) Tests color screening length (lD) : • do bound c + c survive the medium? • or does QGP screening kill them? note: it’s not so clear what to expect other observations: strong coupling, collective motion, many collisions…

  31. J/y Karsch, Kharzeev, Satz, hep-ph/0512239 y=0 PHENIX PRL98 (2007) 232301 y~1.7 40% of J/y from c and y’ decays they are screened but direct J/y not? screening length: onium spectroscopy suppression at RHIC very similar to that at SPS! why?? more suppressed at y 0

  32. what does non-perturbative QCD say? Hatsuda, et al. Lattice QCD shows heavy qq correlations at T > Tc, also implying that interactions are not zero Big debate ongoing whether these are resonant states, or “merely” some interactions Color screening – yes! but not fully… Some J/y may emerge intact J/y is a mystery at the moment! Others may form in final state if c and cbar find each other

  33. so far we have seen • medium is very opaque to light quarks and gluons • to heavy quarks too • system (dominated by gluons) thermalizes very quickly • collective hydrodynamic behavior • vanishingly small viscosity • i.e. doesn’t support shear stress • large cross sections & strong coupling • forget about perturbation theory after 0.6 fm/c(!) • J/y suppression does not follow the energy density what happens to the energy deposited in the plasma?

  34. Central Au + Au same jet opposing jet collective flow in underlying event experimental probe of the medium response • study using hadron pairs • high pT trigger to tag hard scattering • second particle to probe the medium

  35. on away side: same distribution of particles as in p+p but ~5 times fewer! X at high momentum, jets punch through central collisions Phys.Rev.Lett. 97 (2006) 162301 STAR

  36. 3<pt,trigger<4 GeV pt,assoc.>2 GeV Au+Au 0-10% preliminary STAR Surprise #7: particles at lower pT look funny central peripheral 1 < pT,a < 2.5 < pT,t <4 GeV/c

  37. strong coupling: try AdS/CFT answer: yes it does! Gubser, Pufu, Yarom 0706.4307(hep-th) Chesler & Yaffe, 0706.0368(hep-th) lost energy excites a sound (density) wave? FROM DATA PEAK LOCATION =+/-1.23=1.91,4.37 → cs ~ 0.33 (√0.33 in QGP, 0.2 in hadron gas) UNEXPECTED! IS IT RIGHT? relative excitation of sound and diffusion modes in intense study data → sound mode very dominant

  38. Is the medium hot enough to radiate? measure g* → e+e- pairs p+p and Au+Au normalized to p0 region • Au+Auagrees with p+p at resonances (w, f) • Enhancement for • 0.2 < mee < 0.8 GeV • Also excess r: p+p→ r duringhadron gas phase • Agree at 1.2 < m < 3 GeVand J/ by coincidence (J/ scales as p0due to scaling as Ncoll + suppression) arXiv:0706.3034)

  39. submitted to Phys. Rev. Lett arXiv:0706.3034 Low mass dilepton excess at RHIC is large yield excess grows faster than Np large excess below r q+q →g*→ e+e- ? thermal radiation

  40. Low-mass: Comparison with theory A. Toia low mass enhancement at 150 < mee < 750 MeV3.4±0.2(stat.) ±1.3(syst.)±0.7(model) calculations: min bias Au+Au they include: QGP thermal radiation chiral symmetry restoration R.Rapp, Phys.Lett. B 473 (2000) R.Rapp, Phys.Rev.C 63 (2001) R.Rapp, nucl/th/0204003

  41. Plasma at RHIC is dense, hot and weird • the quark gluon plasma is strongly coupled • very opaque to fast quarks & gluons, heavy quarks too • viscosity is vanishingly small • thermalization takes < 0.6 fm/c • can apply methods of string theory for QGP properties • partons seem to leave wakes in the plasma • plasma and/or hadron gas shines brightly • MYSTERIES: • what is the fate of B quarks in the plasma? • how can the system thermalize so fast? • interaction mechanism for heavy quarks? • what’s going on with J/y and color screening? hot gluons show surprising features & their spin is weird too!

  42. QGP energy density • > 1 GeV/fm3 i.e. > 1030 J/cm3 Energy density of matter high energy density: e > 1011 J/m3 P > 1 Mbar I > 3 X 1015W/cm2 Fields > 500 Tesla

  43. backup slides

  44. should narrow rapidity dist. … does it? central peripheral are J/y’s regenerated late in the collision? c + c coalesce at freezeout → J/y R. Rapp et al.PRL 92, 212301 (2004) R. Thews et al, Eur. Phys. J C43, 97 (2005) Yan, Zhuang, Xu, PRL97, 232301 (2006) Bratkovskaya et al., PRC 69, 054903 (2004) A. Andronic et al., NPA789, 334 (2007) J/y is a mystery at the moment!

  45. QCD direct g thermal g? How hot is the medium, anyway? • at RHIC Tinit ~ 1.5 – 2 Tc (~ 300 MeV) • indirect! flow, energy loss constrain initial conditions • will measure T via radiation of g & g* → e+e- real photons inclusive/hadron decays

  46. Calc. by W.Vogelsang and M.Stratmann • “std” scenario, G(Q2=1GeV2)=0.4, is excluded by data on >3 sigma level: 2(std)2min>9 • Uncertainties from functional from ΔG(x) are not included. Reducing these requires measurement at lower x. Spin of the proton: the surprises continue! arXiv 0704.3599 (accepted for publication) 

  47. upgrades extend range! Gehrmann-Stirling models direct photons NCC GSC: G(xgluon= 01) = 1 G(xgluon= 0.020.3) ~ 0 GRSV-0: G(xgluon= 01) = 0 G(xgluon= 0.020.3) ~ 0 GRSV-std: G(xgluon= 01) = 0.4 G(xgluon= 0.020.3) ~ 0.25 GSC: G(xgluon= 01) = 1 GRSV-0: G(xgluon= 01) = 0 GRSV-std: G(xgluon= 01) = 0.4 • charm and bottom identification by displaced vertices • p0, g, Jet identification with larger acceptance must examinelower x

  48. baryon puzzle… baryons enhanced for 1.5 < pT < 5 GeV/c

  49. excess continues to ~ 5 GeV/c STAR

  50. Greco, Ko, Levai: PRC 68 (2003)034904 speculate: Enhancement can be explained by recombination of thermal quarks from an expanding quark gluon plasma. baryons from quark coalescence ►collectively flowing medium focuses them at higher pT ►surplus of energy ahead of fast quark yields extra particle density ►enhances leading baryon in jets formation of baryons – coalescence of quarks

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