Exploring Nature’s First Liquid: How the Quark Gluon Plasma Works

outline l What is quark gluon plasma l Tools to study it l Collective effects l Probes of transport in the plasma high momentum gluons and light quarks strong coupling !?! heavy quark probes l What we know l What we don’t know (yet)

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 + +… At high temperature and density: screening by produced color-charges expect transition to plasma of quarks and gluons T c ~ 170 Mev (10 12 K) asymptotic freedom

properties of matter l thermodynamic (equilibrium) T, P,  EOS (relation between T, P, V, energy density) v sound, static screening length l transport properties (non-equilibrium)* particle number, energy, momentum, charge diffusion sound viscosity conductivity plasma has interactions among charges of multiple particles charge spread, screened in characteristic (Debye) length,  D we deal with strong, not EM force: exchange g instead of  * measuring these is new for nuclear/particle physics!

typical plasma diagnostics X-ray tomography: to get temporal and spatial evolution of plasma mode activity, we use 10 soft X-ray cameras with 20 channels each, at 70 kHz max.

important differences l Medium created by colliding beams doesn’t last long lifetime ~ 10 fm/c ~ 3 x sec (3 x attosecond) l Relevant energy scale given by T ~ 200 MeV wavelength of a photon with energy of 200 MeV: ~ 6 fm = 6 x nm 1/1000 th that of gamma rays! Not going to study this plasma with any laser known to man! To study a plasma of gluons, want a probe sensitive to color not charge

RHIC at Brookhaven to heat matter up Collide Au + Au ions for maximum volume  s = 200 GeV/nucleon pair, p+p and d+A to compare STAR

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

heavy ion collision & diagnostics  K  p  n  d, Hadrons qqq baryons & qq mesons reflect (thermal) properties when inelastic collisions stop not possible to measure as a function of time nature integrates over the entire collision history thermal radiation ( ,  e + e -,  +   color-screened QGP  pressure builds up Hard scattered q,g (short wavelength) probes of plasma formed time

a bit of geometry, terminology l baseline for heavy ions are p+p collisions l peripheral collisions (large impact parameter) are like a handful of p+p collisions l central (small impact parameter) collisions produce largest plasma volume, temperature, lifetime l report centrality as fraction of total A+A cross section »0-5% = very central peripheral: few participant nucleons (small N part ) few NN collisions (N coll ) central: large N part & N coll Ncoll near 1000 in ~ head-on Au+Au

study plasma by radiated & “probe” particles l as a function of transverse momentum p T = p sin  with respect to beam direction) 90° is where the action is (max T,  ) p L midway between the two beams: midrapidity l p T < 1.5 GeV/c “thermal” particles radiated from bulk of the medium “internal” plasma probes

1 st we watch it blow up → collective flow (v 2 ) dN/d  ~ v 2 (p T ) cos (2  ) + … “elliptic flow” Almond shape overlap region in coordinate space x y z momentum space

collective flow! hydrodynamics works!! large pressure buildup anisotropy  happens fast fast equilibration! only works with viscosity/entropy ~ 0 “perfect” liquid (D. Teaney, PRC68, 2003) Kolb, et al Hydrodynamics reproduces elliptic flow of q-q and 3q states Mass dependence requires soft EOS, NOT gas of hadrons

Elliptic flow scales with number of quarks transverse KE implication: valence quarks, not hadrons, are relevant pressure field builds early → dressed quarks born of flowing field (strongly interacting liquids lack well-defined, long-lived quasi-particles)

formation of the particles we observe l system expands, so T falls below T c quarks and gluons form hadrons baryons: qqq and mesons q + anti-q l scaling of the flow tells us how simplest mechanism: coalescence co-moving gluons dress quarks co-moving dressed quarks coalesce into hadrons (guided by hadron wave functions) l Nature is kind of lazy…

Turn now to the transmission measurement l The “x-ray laser” at RHIC autogenerated probe! fast q or g scattered early in the collision large Q 2 separates production & propagation final state: jet of high p T particles l p T > 3 GeV/c large E tot (high p T or M) set scale other than T(plasma) describe by perturbative QCD l control probe: photons no strong interaction also produced in Au+Au

The plasma is opaque p-p PRL 91 (2003) Good agreement with pQCD head-on Au+Au N coll = 975  94 

colored objects lose energy, photons don’t VERY opaque!

the opacity sets in between 20 & 62 GeV √s Cu+Cu √s = 200 GeV √s = 62.4 GeV √s = 20 GeV

interaction of radiated gluons with gluons in the plasma greatly enhances the amount of radiation energy loss: induced gluon bremsstrahlung calculate opacity by expanding in the ratio of distance/mean free path answer: L/ ~ 3.5 →  ~ 1400 gluons/dy I. Vitev d+Au Au+Au

weak coupling? l What does 1400 gluons per unit rapidity tell us? rapidity measures velocity along the beam direction y=tanh -1 [p z /E] l It’s a LOT! density implies multiple gluon interactions at once Elliptic flow magnitude not reproducible with only 2- body qq, gg,qg interactions and normal QCD  l Perfect fluid behavior and hydrodynamics success requires FAST thermalization (<1 fm/c) → coupling is strong, not weak perturbation theory probably in trouble

medium response to deposited energy? l study using hadron pairs l high p T trigger to tag hard scattering l second particle to probe the medium Central Au + Au same jet opposing jet collective flow in underlying event

medium responds to the “lost” energy 3<p t,trigger <4 GeV p t,assoc. >2 GeV Au+Au 0-10% preliminary STAR 1 < p T,a < 2.5 < p T,t <4 GeV/c peripheralcentral

calculating transport properties weak coupling limit strong coupling limit perturbation theory not so easy! but… QCD for q,g interaction  strong coupling: AdS/CFT kinetic theory, cascades gravity  supersym 4-d QCD-like theory

in the strong coupling limit Anti de-Sitter/Conformal Field Theory Correspondence N=4 SupersymmetricWeakly coupled type IIB Yang-Mills theory -on AdS 5 xS 5 a field theory similar Dual to gravity near a to QCD black hole Maldacena predict various properties of strongly coupled fluid, also non-equilibrium processes becomes “easy” to calculate evolution of stress-energy tensor Son, Policastro, Starinets

lost energy excites a sound (density) wave? peak observed in data at  +/-1.23=1.91,4.37 → speed of sound cos  m =c s ~ (c s 2 =0.33 in QGP,~0.19 in hadron gas) Gubser,Pufu,Yarom PRL100, ’08 Chesler & Yaffe, (hep-th) mach cone wake

what do heavy quarks do?? l Like putting a rock in a stream and watching if the stream can drag it along… l Measure their correlation with the light hadrons (i.e. v 2 ) l NB: rate of equilibration gives information on the viscosity of the liquid! analogy from J. Nagle

Diffusion of heavy quarks traversing QGP l Prediction: less energy loss than light quarks large quark mass reduces phase space for radiated gluons how many collisions with light quarks??? l Measure via semi-leptonic decays of mesons containing charm or bottom quarks D Au D X e±  K

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

heavy quarks get stopped too! e± from heavy quark decay: ► charm quarks also have huge energy loss ► more than gluon radiation can explain PRELIMINARY Run-4 Run-7 Rapp & van Hees, PRC 71, (2005) minimum-bias ► charm quarks get dragged along by the liquid ► bottom quarks??

an independent measure of viscosity PRL98, (2007) Heavy quark diffusion (Langevin equation) drag force  random force  /unit time  D* ~OK agreement with data slow relaxation ruled out D ~ 4-6/(2  T)   D = / 3  s D =   /S →  /S = (1.3 – 2.0)/ 4 

S. Gavin and M. Abdel-Aziz: PRL 97:162302, 2006 p T fluctuations STAR Comparison with other estimates R. Lacey et al.: PRL 98:092301, 2007 v 2 PHENIX & STAR H.-J. Drescher et al.: arXiv: v 2 PHOBOS conjectured quantum limit estimates of  /s based on flow and fluctuation data indicate small value as well close to conjectured limit l lowest viscosity of anything!

l Collective flow and opacity show that quark gluon plasma is strongly coupled l Energy loss in the plasma is large density is ~1400 gluons/unit rapidity but might not have (quasi)particles at T>T c l Deposited energy shocks the medium. Mach cones? c s ~ (0.35 – 0.4) c (closer to hadron gas than QGP) l Medium is opaque even to very heavy charm quarks Heavy quark diffusion, hadron flow & fluctuations →viscosity  S = (1 – 3)/ 4  near conjectured quantum bound we are STARTING to figure out this plasma

l what are the degrees of freedom above T c ? what is the right way to understand the strongly coupled fluid? can pQCD be fixed up by adding more loops? CFT is NOT the same as QCD! l does the liquid also stop very heavy b quarks short? what does yes/no tell about the coupling? l is the funny jet shape really a sound wave? must verify that we’re seeing shocks in the plasma need to measure the energy transport by knowing the jet’s energy and reconstructing where that goes do “shots” with different T, V and probe l Then we’ll know how the quark gluon plasma works BUT

for further (experimental) progress STAR HFT PHENIX VTX Silicon vertex detectors to separate b and c quarks rapidity NCC MPC VTX & FVTX  HBD EMCAL RHIC II AuAu 20 nb -1  -jet acceptance and ∫ L mechanism of eloss and do it again at higher T at the LHC! need more events on tape

conclusions what I know about AdS/CFT … 1/4  l experiments have started to probe plasma transport see significant momentum transfer to/from medium heavy quarks diffuse, losing energy and gaining flow l sound waves? evidence for wakes??? parameterdata saysAdS/CFT predicts transfer q6-24 GeV 2 /fm4.5 GeV 2 /fm viscosity  /s~0 (via  v2)≥ 1/4  (1.6/4  : lQCD) diffusivity D ~3/(2  T)≥ 1/4  speed of sound c s ~ 0.33 (?)QGP: 1/√0.33* hadrons: ~0.2* ^ ?

cosmological implications of strong coupling? l strong coupling → correlations among quarks near/at phase transition forming hadrons l AFTER inflation! l correlations: ~1fm at T~200 MeV participate in Hubble expansion in universe now → structure at scale of 1 parsec l 1 parsec: relevant scale for structure in dark matter distribution l structures at this scale potentially observable by gravity effects?

l backup slides

jets shock the medium p+p central Au+Au

QGP viscosity is lower than that of liquid He!  /s ~4 times smaller than of helium (but at T ~ K) close to the conjectured limit  /s = ћ/4  P. Kovtun, D.T. Son, A.O. Starinets, Phys.Rev.Lett.94:111601, 2005 motivated by AdS/CFT (Anti-deSitter/Conformal Field Theory) correspondence J. Maldacena: Adv. Theor. Math. Phys. 2, 231, 1998 “near perfect” fluid at RHIC R Lacey, et al Phys.Rev.Lett. 98 (2007) ћ/4  9 ћ/4 

get quantitative I. Vitev, Phys. Lett. B639, 38 (2006) radiative energy loss, weak coupling data & fit: arXiv: gluon density = per unit rapidity long. velocity y=tanh -1 [p z /E]

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

long standing baryon puzzle… baryons enhanced for 1.5 < p T < 5 GeV/c coalescence of thermal quarks from an expanding quark gluon plasma. STAR

baryons come from jets – enhanced by wake? ● the “extra” baryons have jet-like partners ● only in central collisions do we see a dilution from thermal baryons meson-meson baryon- meson Near side ●collectively flowing medium boosts quarks to higher p T ● wake of fast quark builds particle density to coalesce ● baryon enhancement reflects dynamics…?! speculate: Gubser, Pufu, Yarom (hep-th)

heavy quark transport: diffusion & viscosity l diffusion = brownian motion of particles definition: flux density of particles J = -D grad n l integrating over forward hemisphere: D = diffusivity = 1/3 so D = / 3n  D  collision time, determines relaxation time Langevin: equation of motion for diffusion thru a medium drag force  random force  /unit time  D* particle concentration = mean free path note: viscosity is ability to transport momentum  = 1/3  so D =  S → measure D get  ! * G. Moore and D. Teaney, hep-ph/

What about b quarks? PRL 98, (2007) e ± from heavy flavor b quark contribution to single electrons becomes significant. do they also lose energy?

LPM effect up to O (g s ) + (3+1)d hydro + collisions Qin, Ruppert, Gale, Jeon, Moore and Mustafa, Fix initial state by constraining hydro with particle spectra Reproduce observed energy loss vs. centrality using  s = 0.27

more complex jet fragment measurements l 3 – particle correlations consistent with Mach-cone shoulder l sum of jet fragment momentum ▪ increases together on trigger and away sides ▪ momentum loss in punch-thru jet balanced by momentum in the shoulder peak ▪ evidence for wakes? d+Au 0-12% Au+Au Δ2Δ2 Δ1Δ1

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

what does non-perturbative QCD say? Lattice QCD shows heavy qq correlations at T > T c, 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/  may emerge intact Hatsuda, et al. J/  is a mystery at the moment! Others may form in final state if c and cbar find each other

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

plasma basics – Debye screening l distance over which the influence of an individual charged particle is felt by the other particles in the plasma charged particles arrange themselves so as to effectively shield any electrostatic fields within a distance of order D D =  0 kT n e e 2 Debye sphere = sphere with radius D l number electrons inside Debye sphere is large N D = N/V D =  V D V D = 4/3  D 3 1/2 n e = number density e = charge

For all distributions described by temperature T and (baryon) chemical potential   : dn ~ e -(E-  )/T d 3 p Does experiment indicate thermalization? T f ~ 175 MeV

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

The acid test for quark scaling  meson (bound state of s+sbar) mass ~ m proton but flows like the mesons PHENIX

dielectron spectrum vs. hadronic cocktail

R.Rapp, Phys.Lett. B 473 (2000) R.Rapp, Phys.Rev.C 63 (2001) R.Rapp, nucl/th/ low mass enhancement at 150 < m ee < 750 MeV 3.4±0.2(stat.) ±1.3(syst.)±0.7(model) Comparison with conventional theory calculations: min bias Au+Au they include: QGP thermal radiation chiral symmetry restoration

Direct photon v 2

momentum exchange: complicated process The medium is dense and opaque! Quantifying energy deposit..ongoing. Different models include radiation and/or collisions some/no transfer medium→probe varying level of detail of collision geometry different handling of fluctuations different level of incorporating expansion dynamics

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