Probing the Quark Gluon Plasma What sort of plasma is a qgp?


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Probing the Quark Gluon Plasma

  • What sort of plasma is a QGP?

  • RHIC and its experiments

  • Collective flow

  • Transmission of color-charged probes

  • Transport properties and hadronization

  • Conclusions


reminder: what’s 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, D
    • multiple particles inside this length
  • “normal” plasmas are electromagnetic

    • quark-gluon plasma interacts via strong interaction
      • color forces rather than EM
      • exchanged particles: g instead of 


Map of high energy densities



Plasma coupling parameter?

  • For high gluon density achieved at RHIC & LHC

    • estimate  =
      /
    • using QCD coupling strength, g

    • =g2/d d ~1/(41/3T)
    • ~ 3T
    • g2 ~ 4-6 (value runs with T)
    •  ~ g2 (41/3T) / 3T  so plasma parameter 
    • NB: such plasmas known to behave as a liquid!
  • Correlated or bound q,g states, but not color neutral

  • So the quark gluon plasma is a strongly coupled plasma

    • As in warm, dense plasma at lower (but still high) T


from S. Ichimaru



Other strongly coupled plasmas

  • Inside white dwarfs, giant planets, and neutron stars

    • (n star core may even contain QGP)
  • In ionized gases subjected to very high pressures, magnetic fields, or particle interactions

  • Dusty plasmas in interplanetary space & planetary rings

  • Solids blasted by a laser



take a deep breath…

  • What did we expect for QGP?

  • What SHOULD we expect?



Plasma Diagnostics

  • Many interesting systems are short-lived!

    • ns for laser-heated plasmas
    • study via time integrated observables
    • (radiation or probes)
    • plasma folks can also measure time dependence
    • correlations of probes and/or medium particles
  • Transmission of external probes

    • hard x-rays, electrons. In our case: jets
  • Final state cluster distributions for early state info

    • Diagnostic of collective motions
    • Multiparticle emission
    • Single particles in multiparticle field, acoustic waves


Method using 3 lasers: 1) create shock, 2) x-rays, and 3) probe sample



Shock and interface trajectories are measured by x-ray radiography



We use RHIC at Brookhaven National Laboratory



4 complementary experiments



Collective motion? Pressure: a barometer called “elliptic flow”



The data show



v2 reproduced by hydrodynamics



gas of strongly interacting Li atoms

    • M. Gehm, S. Granade, S. Hemmer, K, O’Hara, J. Thomas Science 298 2179 (2002)
    • excite Feshbach resonance: 38th vibrational
    • Li2 state → 0 energy, huge cross section


Caveat: use hydrodynamic models carefully



WHICH are the flowing degrees of freedom?



flow and thermalization

  • Data suggest that partons are what flows

    • quark scaling of v2
    • requirement of QGP EOS for hydro to reproduce v2
  • Look “under the hood” in the hydro calculation

    • v2 magnitude → start hydro by t = 0.6 fm/c (U. Heinz)
    • technique exactly the same in plasma physics
  • HOW does the system thermalize so fast?

    • collisions? quasi-bound states increase 
    • plasma instabilities? maybe (Arnold, et al; Rebhan …)
  • help to constrain the imagination

    • do heavy quarks thermalize and flow?
    • use massive quarks to probe diffusion in QGP
    • D ~ coll ; small diffusion → large elliptic flow & Eloss


Heavy quark flow?

  • PHENIX measures v2 of non-photonic e± electron ID in Au+Au via RICH + EMCAL

  • measure and subtract photonic sources using converter



“external” probes of the medium



1st: benchmark the probes in p+p collisions

    • calculable with perturbative QCD!


Direct Photon Spectra in Au+Au

  •  does not interact with the color charges

  • data and theory agree → calibrated probe

  • pQCD works in the complex environment of two Au nuclei colliding

  • 0 large, making g easier to measure!



strongly interacting probe: a different story!



Photons shine, Pions don’t



look for the jet on the other side



Could suppression be an initial state effect?

  • Dramatically different and opposite centrality evolution of AuAu experiment from dAu control.

  • Jet Suppression is clearly a final state effect.



Are back-to-back jets there in d+Au?



Induced gluon brehmsstrahlung



So, what do E loss & collectivity tell us?



Charm via single e± in p+p



p+p single e± as reference for Au+Au → RAA



Is Eloss consistent with that of light quarks?



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

    • The cross section of early q,g collisions must be ~50 times larger than those of free q,g for large v2
  • Quarks and gluons are interacting, but need not be locally (color) neutral like the baryons & mesons. Neutrality scale likely larger, as expected for a plasma.



Study jet fragmentation to probe medium properties



correlation functions of two high pT hadrons



decompose to get jet pair distribution



interpretation? *it’s fun to speculate

  • pQCD energy loss is by gluon radiation

    • mostly collinear with radiating particle
  • various authors now remind us of ionization

    • (Shuryak, Vitev …)
    • more direct interaction of probe parton with medium!
    • drives question “what happens to the lost energy”
  • options:

    • it remains collinear
    • creates a wake in the medium (Fries et al; Shuryak)
    • thermalizes in the medium
  • speed of wake reflects cs in the medium: cosm=cs/c





identify triggers, count partners



How about the screening length?

  • J/

    • Test confinement:
    • do bound c + c survive?
    • or does QGP screening kill them?
    • Suppression was reported in lower
    • energy heavy ion collisions at CERN


data on Au+Au, Cu+Cu being analyzed



so, is there QGP at RHIC?

  • Yes! RHIC creates a strongly coupled, opaque liquid

  • 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:

    •  ~ 15 GeV/fm3
    • dNgluon/dy ~ 1000
    • int large for T < 2-3 Tc
  • Are measuring properties of this new kind of plasma

    • opacity, collision frequency, EOS, screening
    • speed of sound?
    • color and maybe thermal conductivity to be quantified
    • color screening currently being analyzed
  • LHC will make QGP too. (As) strongly coupled?

    • higher , pT reach for hard probes; soft physics at higher T


RBRC workshop on Dec.16, 17 2004

  • Strongly Coupled Plasmas:

  • Electromagnetic, Nuclear and Atomic

  • organizers: B. Jacak, S. Bass, E. Shuryak, T. Hallman, R. Davidson

  • An interdisciplinary “experiment”

  • opportunity to learn from each other

  • form new collaborations/directions

  • http://quark.phy.bnl.gov/~bass/workshop.htm

  • for program, slides



Suppression: an initial state effect?

  • Gluon Saturation

    • (color glass condensate)
  • Wavefunction of low x gluons overlap; the self-coupling gluons fuse, saturating the density of gluons in the initial state. (gets Nch right!)

  • Multiple elastic scatterings (Cronin effect)

    • Wang, Kopeliovich, Levai, Accardi


d+Au central/peripheral



Compare with BRAHMS



Color glass condensate?



But, recombination lurks…

  • shower + medium recombination → reductes soft parton density on deuteron side

  • Can explain fward-bward asymmetry AND RCP (protons) > RCP (mesons) at midrapidity.



From talk of Todd Ditmire (U. Texas)

  • Diagnostic quantity measured

  • Transmission of , hard x-rays density, atomic properties

  • Probe photon interference imaging, expansion velocity

  • Phase shifts of probe photon release velocity of expanding material

  • x-ray reflectivity image shock front

  • spectrum, time structure of hydrodynamic expansion

  • radiated clusters

  • Time-resolved absorption density profile with time

  • Electron radiation plasma oscillations

  • test hydro predictions

  • Anisotropy in radiation test calculations of field gradients



Direct photons in Au+Au

  • pQCD works too (with nuclear Sa/A(xa,r) , TA(r) + observed 0 

  •  can reliably calculate rate & distribution of short wavelength probes of hot, dense partonic matter!



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


Charm production scales with Ncoll*



FONLL Predictions

  • Mateo Cacciari provided a prediction using the Fixed Order Next Leading Logarithm pQCD approach

  • His calculation agrees perfectly with our “poor man’s” HVQLIB+PYTHIA predictions

  • Data exceed the central theory curve by a factor of 2-3

  • Possible explanations:

    • NNLO contribution
    • Fragmentation mechanisms need to be studied in more details


Non-photonic single electron spectra



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

  • Recent paper by Nastase uses mathematics of black holes developed by Hawking, but forces and behavior (and sizes) are quite different



Is enough for fast equilibration & large v2 ?



Is the energy density high enough?



Do see Cronin effect!




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