Does RHIC produce a strongly coupled quark gluon plasma?
outline l Why should we expect a strongly coupled quark gluon plasma? l Collective motion, opacity of the plasma What do data and lattice say? Underlying degrees of freedom? l What needs to be done next?
Relativistic heavy ion collisions l Create high(est) energy density matter similar to that existing ~1 sec after the Big Bang can study only in the lab – relics from Big Bang inaccessible T ~ 200-400 MeV (~ 2-4 x 10 12 K) ~ 5-15 GeV/fm 3 (~ 10 30 J/cm 2 ) R ~ 10 fm, t life ~ 10 fm/c (~3 x 10 -23 sec) l The goal: characterize the hot, dense medium expect QCD phase transition to quark gluon plasma 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
Energy Density in the Universe high energy density: > 10 11 J/m 3 P > 1 Mbar I > 3 X 10 15 W/cm 2 Fields > 500 Tesla QGP energy density > 1 GeV/fm 3 i.e. > 10 30 J/cm 3
QCD phase transition Color charge of gluons self interaction; theory is non-abelian curious properties at large distance: confinement of quarks in hadrons + +… At high temperature and density: Debye screening by produced color-charges expect transition to free gas of quarks and gluons
Proper non-perturbative QCD - lattice T/T c Karsch, Laermann, Peikert ‘99 /T 4 T c ~ 170 ± 10 MeV (10 12 °K) ~ 3 GeV/fm 3 required conditions to create quark gluon plasma ~15% from ideal gas of weakly interacting quarks & gluons
Guide to expectations l I’ve already tossed around some numbers Based on QCD coupling value + expected temperature l Look more closely at screening, plasma “coulomb” coupling parameter, energy density
Screening by the QGP Follow usual derivation of Debye screening Now put in QGP scales and assumptions: Hadrons with radii greater than ~ D will be dissolved Origin of J/ suppression QGP signature Strong interaction
Plasma coupling parameter l High quark & gluon density estimate = / using QCD coupling strength =g 2 /d d ~1/(4 1/3 T) ~ 3T g 2 ~ 4-6 (value runs with T) ~ g 2 (4 1/3 T) / 3T so plasma parameter l So the quark gluon plasma is a strongly coupled plasma As in warm, dense plasma at lower (but still high) T But strong interaction rather than electromagnetic in this range implies liquid properties of the plasma
Estimate energy density In quark gluon plasma: Energy density for “g” massless d.o.f 8 gluons, 2 spins; 2 quark flavors, anti-quarks, 2 spins, 3 colors 37 (!) Normal nuclear matter: ~ 0.2 GeV/fm 3 ; ~ 0.17 /fm 3 ~ 12 T 4 ~ 2.4 GeV/fm 3
pressure builds up look for evidence for QGP liquid K p n d, Hadrons reflect (thermal) properties when inelastic collisions stop (chemical freeze-out). , e + e -, + Real and virtual photons emitted as thermal radiation. we focus on mid-rapidity (y=0), as it is the CM of colliding system 90° in the lab at collider Hard scattered or heavy q,g probes of plasma formed
Collective “elliptic flow” Origin: spatial anisotropy of the system when created, followed by multiple scattering of particles in the evolving system spatial anisotropy momentum anisotropy v 2 : 2 nd harmonic Fourier coefficient in azimuthal distribution of particles with respect to the reaction plane Almond shape overlap region in coordinate space
The data show Particle emission really is azimuthally anisotropic Magnitude of the anisotropy grows with beam energy, then flattens c.m. beam energy
Hydro. Calculations Huovinen, P. Kolb, U. Heinz v 2 reproduced by hydrodynamics PRL 86 (2001) 402 see a large pressure buildup anisotropy happens fast while system is deformed success of hydrodynamics early equilibration ! ~ 0.6 fm/c Viscosity very small a perfect liquid! central STAR
Collective motions probes early phase Hydrodynamics reproduce magnitude of elliptic flow for , p. BUT correct mass dependence requires softer than hadronic EOS!! Kolb, et al Azimuthal anisotropy Reflects early pressure when overlap is spatially not isotropic
Elliptic flow scales as number of quarks v 2 scales ~ with # of quarks! evidence that quarks are the particles when the pressure is built up
Need large for fast equilibration & large v 2 Parton cascade using free q,g scattering cross sections underpredicts pressure Behavior NOT perturbative! Lattice QCD shows qq resonant states at T > Tc, also implying high interaction cross sections NOT ideal gas of q,g!
The beginning of sQGP: a New QCD Phase Diagram, in which ``zero binding lines” first appeared (ES+I.Zahed hep-ph/030726, PRC) T The lines marked RHIC and SPS show the adiabatic cooling paths Chemical potential B Edward Shuryak on how this can come about
Why is hydro description so good ? Well, it was shown to work for strongly coupled atoms => near zero binding provides large objects, maybe large cross sections? (ES+Zahed,03, same)
Unexpected help from string theorists, AdS/CFT correspondence The viscosity/entropy => 1/4 when g 2 N c ! 1, (D.Son et al 2003), as small as at RHIC! Multiple Coulomb bound states with l» g 2 N c (ES+Zahed, PRD 04)
Another kind of plasma probe at RHIC l A favorite probe in EM plasma studies Hard x-rays Wavelength short compared to characteristic wavelength of the medium e.g. a warm, dense plasma at ~1 keV l QCD DOES have an analog! Hard scattered q, g from initial N-N collisions 5-10 GeV colored probe with short wavelength l Rate and spectrum calculable with pQCD look for effects of the medium as it transits Start by benchmarking the probe!
Start simple p+p collisions p-p PRL 91 (2003) 241803 Good agreement with NLO pQCD QCD works for high p transfer processes! Have a handle on initial NN interactions by scattering of q, g inside N We also need: Parton distribution functions Fragmentation functions 00 These are measured, so known
pQCD in Au+Au? direct photons [ w/ the real suppression] ( pQCD x N coll ) / background Vogelsang/CTEQ6 [if there were no suppression] ( pQCD x N coll ) / ( background x N coll ) Au+Au 200 GeV/A: 10% most central collisions [ ] measured / [ ] background = measured / background Preliminary At high p T, it also works! TOT p T (GeV/c)
“external” probes of the medium hadrons q q leading particle leading particle schematic view of jet production Hard scattering of q,g early. Observe fast leading particles, back-back correlations Before creating hadron jets, scattered quarks induced to radiate energy (~ GeV/fm) by the colored medium -> jet quenching AA nucleon-nucleon cross section / inel p+p
Au-Au s = 200 GeV: high p T suppressed! PRL91, 072301(2003) Proportional to the density of scattering centers (gluons) dN g /dy ~1000
look for the jet on the other side STAR PRL 90, 082302 (2003) Central Au + Au Peripheral Au + Au near side away side peripheral central Medium is opaque!
Suppression: a final state effect? Such a medium is absent in d+Au collisions! But Au provides all initial state effects of q, g wavefunction inside a Au nucleus d+Au is the “control” experiment Hot, dense medium causes
Centrality Dependence l Dramatically different and opposite centrality evolution of AuAu experiment from dAu control. l Jet Suppression is clearly a final state effect. Au + Au Experimentd + Au Control PHENIX preliminary
Are back-to-back jets there in d+Au? Pedestal&flow subtracted Yes! hadrons leading particle suppressed q q ? So this is the right picture for Au+Au
Property probed: density Au-Au d-Au dAu Induced gluon brehmsstrahlung pQCD calculation depends on number of scatterings Agreement with data: initial gluon density dN g /dy ~ 1100 ~ 15 GeV/fm 3 hydro initial state same Lowest energy radiation sensitive to infrared cutoff.
Experimental look at energy density 5.5 GeV/fm 3 (200 GeV Au+Au) well above predicted transition! R2R2 2c Colliding system expands: Energy to beam direction per unit velocity || to beam value is lower limit: longitudinal expansion rate, formation time overestimated E
Study jet fragmentation to probe medium properties Radiated gluons are collinear (inside jet cone) Can also expect a jet “wake” effect, medium particles “kicked” alongside the jet by energy they absorb And expect hard-soft recombination C.M. Ko et al, Hwa & Yang PRC68, 034904, 2003 PRC67, 034902, 2003 nucl-th/0401001 & 0403072 Fries, Bass & Mueller nucl-th/0407102
or EVEN MORE complicated couplings Edward’s conic flow: a pressure wave or “super wake” i.e. medium response to the energy deposited by jets Both consistent with features in data with modest jet fragment energy Correlations of jet fragments with flowing medium Armesto, Salgado & Wiedemann, hep-ph/0405301 Does jet fragmentation have a meaning in presence of medium? Mechanisms mix up medium & radiated partons New tool to see conductivity & correlations in medium at ~1 fm/c??
More partners on same side Near-Side ratio p+p 40-80% Au+Au 0-5% Au+Au STAR Preliminary AuAu/pp nucl-ex/0408007 In 55° and small In large
Current knowledge on properties l Extract from models, constrain by data Energy loss (GeV/fm)7-100.5 in cold matter Energy density (GeV/fm 3 )14-20>5.5 from E T data dN(gluon)/dy~1000From energy loss + hydro T (MeV)380- 400 Experimentally unknown as yet Equilibration time 0 (fm/c) 0.6From hydro initial condition; cascade agrees Opacity (L/mean free path)3.5Based on energy loss theory Equation of state not hadronic Early degrees of freedom apparently partonic Cross section of resonances at high T? Conductivity?
Other strongly coupled plasmas l Inside white dwarf and neutron stars (n star core may even contain QGP) l In ionized gases subjected to very high pressures or magnetic fields l Dusty plasmas in interplanetary space & planetary rings l Solids blasted by a laser l We would like to know: How do these plasmas transport energy? How quickly can they equilibrate? What is their viscosity? >10 can even be crystalline! How much are the charges screened? Is there evidence of plasma instabilities at RHIC? Can we detect waves in this new kind of plasma? novel plasma of strong interaction
Conclusions l Answer: yes! l The adventure will be to figure out its properties Use medium coupling to jet fragmentation to study energy transport (conductivity) Heavy quark bound states to determine screening length Lepton pairs and soft photons from thermal radiation to probe temperature Does RHIC produce a strongly coupled quark gluon plasma?
How to get there? l Experimental side – upgrade facility (~2009-2015) increase RHIC luminosity by ~40 by electron cooling of heavy ion beams l Capabilities of large detectors (2 steps between now & 2015) technology for rare features in high muliplicity events secondary decay vertices background rejection triggering, readout capabilities data analysis infrastructure (already write 0.5 pB/year) l Theory progress (over next 5-10 years) Large scale computing resources lattice QCD, hydrodynamic & transport simulations Ideas & people to develop new approaches
l Heavy quark probes High mass high E to produce (made at 1 st step in collision) Screening properties via (c-anticharm) bound states Do the heavy quarks thermalize? Lose energy? higher luminosity sufficient statistics of rare, heavy quarks lattice calculation under relevant conditions l Initial quark gluon plasma temperature detector upgrade for background rejection lattice, hydro simulations (with relevant , coupling) l Characterize this new kind of plasma radiation rate, conductivity, collision frequency Need rare probes, including tagged jets detector & accelerator improvements; simulations l Consistent theoretical picture of quark gluon plasma, heavy ion collision to connect with data Need large scale computational resources for numerical simulation Scientific objectives:
Theoretical tools l Large scale computations of fundamental properties Solve QCD on the lattice High T & dynamical quarks: need large computers l Simulations Hydrodynamics (2D mostly, 3D developing) pressure, viscosity, energy gradients, equation of state Cascade, non-equilibrium transport models pre-equilibrium stage, thermalization approach parton cascade for early phase hadron cascade for final state interactions QCD inspired models Hadronization (when T falls below Tc) not understood; collective effects must survive it l Constrain with data to extract plasma properties
What’s currently known about properties? l Pressure built up very rapidly during collision observed collective flow viscosity small (known from hydro. calculation) interaction large l Quasiparticles probably exist above Tc may experience binding, but not as normal hadron not color neutral as at T
"name": "What’s currently known about properties.",
"description": "l Pressure built up very rapidly during collision observed collective flow viscosity small (known from hydro. calculation) interaction large l Quasiparticles probably exist above Tc may experience binding, but not as normal hadron not color neutral as at T
Predicted signatures of QGP l Debye screening of color charges (Matsui & Satz) Suppresses rate of c-c bound state (J/ l Strangeness enhancement (Rafelski & Mueller) T plasma ~ mass s-quark Enhances strange hadron yields (e.g. K/ increased) l Jet quenching (Gyulassy & Wang) Dense medium induces coherent gluon radiation Decreased yield of jets of hadrons from hard scattered quarks or gluons l But real goal is to determine the properties of the plasma
Color screening? Lattice predictions for heavy quarks 40-90% most central N coll =45 0-20% most central N coll =779 20-40% most central N coll =296 NB: need temperature too!
Hydro. Calculations Huovinen, P. Kolb, U. Heinz v 2 reproduced by hydrodynamics STAR PRL 86 (2001) 402 see large pressure buildup anisotropy happens fast early equilibration ! central Hydrodynamics assumes early equilibration Initial energy density is input Equation of state from lattice QCD Solve equations of motion
More energetic baryons in Au+Au p/ ~1 at high p T in central collisions > p+p, d+Au > peripheral Au+Au >jets in e+e- collisions PRL 91 (2003) 172301
Hydrodynamic expansion common velocity boost heavier particles boosted to higher p T enhances mid p T hadrons baryons especially Do the hadrons get boosted? or Coalescence of boosted quarks into hadrons? pQCD spectrum shifted by 2.2 GeV T eff = 350 MeV R. Fries, et al Are extras from the thermal medium?
equilibrated final state, including s quarks Hadron yields in agreement with Grand Canonical ensemble T(chemical freeze-out) ~ 175 MeV (multi)strange hadrons enhanced over p+p, but QGP hypothesis not unique explanation
Locate RHIC on phase diagram Baryonic Potential B [MeV] 0 200 250 150 100 50 020040060080010001200 AGS SIS SPS RHIC quark-gluon plasma hadron gas neutron stars early universe thermal freeze-out deconfinement chiral restauration Lattice QCD atomic nuclei From fit of yields vs. mass (grand canonical ensemble): T ch = 176 MeV B = 41 MeV These are the conditions when hadrons stop interacting T Observed particles “freeze out” at/near the deconfinement boundary!
Look at “transverse mass” m T 2 = p T 2 + m 0 2 — is distribution e -E/T ? i.e. Boltzmann distribution from thermalized gas? , K, p, pbar spectra indicate pressure yes ! Protons are flatter velocity boost to beam Result of pressure built up
RHIC at Brookhaven National Laboratory RHIC collides p, Au ions: 200 GeV/nucleon in c.m. 10 times the energy previously available!