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
Barbara JacakStony BrookMay 18, 2005
reminder: what’s a plasma?
4th state of matter (after solid, liquid and gas) a plasma is:
ionized gas which is macroscopically neutralexhibits collective effects
interactions among charges of multiple particlesspreads charge out into characteristic (Debye) length, D
multiple particles inside this length
“normal” plasmas are electromagneticquark-gluon plasma interacts via strong interaction
color forces rather than EMexchanged particles: g instead of
Plasma coupling parameter?
For high gluon density achieved at RHIC & LHCestimate = <PE>/<KE>
using QCD coupling strength, g<PE>=g2/d d ~1/(41/3T)
<KE> ~ 3Tg2 ~ 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 plasmaAs in warm, dense plasma at lower (but still high) T
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
Properties of interest: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?
nove
l pla
sma
of
str
ong
inte
ract
ion
take a deep breath…
What did we expect for QGP?
What SHOULD we expect?
weakly interactinggas of quarks & gluons
quarks & gluons retain correlations,medium exhibits liquid properties
NB: the (quasi-)bound states are not your mother’s hadrons!
Plasma Diagnostics
Many interesting systems are short-lived!ns for laser-heated plasmasstudy via time integrated observables
(radiation or probes)plasma folks can also measure time dependencecorrelations 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 infoDiagnostic of collective motionsMultiparticle emission Single particles in multiparticle field, acoustic waves
Method using 3 lasers: 1) create shock, 2) x-rays, and 3) probe sample
Sapphire window
Beryllium foil
Metal pusher
Copper
D2
Shock
Radiograph x-rays
X-ray µscope
and streak
camera
Iron foil
1) Shock generating laser
3) Probe laser2) x-ray generating laser
R. Lee, S. Libby, LLNL; RBRC workshop
Shock and interface trajectories are measured by x-ray radiography
Slope of shock front yields Us
Slope of pusher interface gives Up
.
Al
D2
time (ns)
shock front
Al pusher
dista
nce (µ
m)
0.0 5.01.0 2.0 3.0 4.0 6.0 7.0 8.0
0
100
200
300
x
L
Lx
=o
=
Us
Us-U
p
streak camera record
R. Lee, S. Libby, LLNL
P-P0=0UsUp
Collective motion? Pressure: a barometer called “elliptic flow”
Origin: spatial anisotropy of the system when createdmultiple scattering of particles builds pressure collective expansionspatial anisotropy momentum anisotropy
dN/d ~ 1 + 2 v2(pT) cos (2) + …
Almond shape overlap region in coordinate space
2cos2 vx
y
p
patan
y2 x2 y2 x2
x
yz
The data show
Anisotropy amplitude grows with beam energy, then flattens. For LHC first guess – use same v2 at same pT
c.m. beam energy
Hydro. CalculationsHuovinen, P. Kolb,U. Heinz
v2 reproduced by hydrodynamics
STARPRL 86 (2001) 402
• see large pressure buildup! • anisotropy happens fast • early equilibration
central
Hydrodynamics can reproduce magnitudeof elliptic flow for , p. BUT mass dependence →softer than hadronic EOS!!
Kolb, et al
NB: these calculations have viscosity = 0medium behaves as an ideal liquid
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 vibrationalLi2 state → 0 energy, huge cross section
weakly coupled
strongly coupled
Caveat: use hydrodynamic models carefully
proton pion
Hydro models:Teaney(w/ & w/oRQMD)
Hirano(3d)
Kolb
Huovinen(w/& w/oQGP)
nucl-ex/0410003
WHICH are the flowing degrees of freedom?
v2 scales ~ with # of quarks! evidence that quarks are the particles when the pressure is built uppattern same at LHC??
v2 for particles of different mass
flow and thermalization
Data suggest that partons are what flowsquark scaling of v2
requirement of QGP EOS for hydro to reproduce v2
Look “under the hood” in the hydro calculationv2 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 imaginationdo 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
nucl-ex/0502009
YES
v2 ≠ 0 at 90% C.L.
data consistent with heavy q thermalization
“predicted” by Moore&Teaney
hep-ph/0412346
*run4 analysis now
Greco,Ko,Rapp.
PLB595, 202 (2004)
LHC: CGC initial state, even greater pT reach
“external” probes of the medium
hadrons
q
q
hadronsleadingparticle
leading particle
schematic view of jet productionHard 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
AA
AA
ddpdT
ddpNdpR
TNN
AA
TAA
TAA /
/)(
2
2
nucleon-nucleon cross section<Nbinary>/inel
p+p
1st: benchmark the probes in p+p collisions
calculable with perturbative QCD!
Produced photonsProduced photonsProduced pionsProduced pions
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!
look for the jet on the other sideSTAR PRL 90, 082302 (2003)
Central Au + Au
Peripheral Au + Au
near side
away side
peripheral central
22 2 2( ) ( ) (1 cos(2 ))D Au Au D p p B v
Medium is opaque!
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.
Au + Au Experiment d + Au Control
PHENIX preliminary
Are back-to-back jets there in d+Au?
Pedestal&flow subtracted
Yes!
importance of “p”+A comparisonpush hard for it at LHC!
Induced gluon brehmsstrahlung
Au-Au
d-AudAu
pQCD (Vitev): energy loss number of scatterings
Agreement with data:initial gluon density
dNg/dy ~ 1100
~ 15 GeV/fm3
hydro initial state same
Lowest energy radiation sensitive to infrared cutoff.
So, what do E loss & collectivity tell us?
Medium is opaque to colored probes Thermalization must be very fast (< 1fm/c) Hydrodynamic, energy loss models constrained with data:
Energy loss <dE/dz> (GeV/fm) 7-10 0.5 in cold matter
Energy density (GeV/fm3) 14-20 >5.5 from ET data
dN(gluon)/dy ~1000 200-300 at SPS
T (MeV) 380-400 Experimentally unknown as yet
Equilibration time0 (fm/c) 0.6 Parton cascade agrees
Opacity (L/mean free path) 3.5
Charm via single e± in p+p
PHENIX preliminary
please measure cc in p+p at LHC too!!
exceeds NLO and phenomenological
predictions
by how much? a bit controversial. I think factor 2-3.
p+p single e± as reference for Au+Au → RAA
energy loss of
charm quarks!
Eloss + flow →
small diffusion coeff
short time btwn charm collisions
(NB likely some
e± from B decays)
RAA
pT (GeV/c)
Is Eloss consistent with that of light quarks?
RAA
pT (GeV/c)
q consistent w/ light quark eloss
non-pert.
effects on
“normal” g
radiation
calculation from:
Dainese, Armesto, Wiedemann
data say:
same transport coefficient,
smaller hadron suppression
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 neighborsThe 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
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 & YangPRC68, 034904, 2003PRC67, 034902, 2003nucl-th/0401001 & 0403072
Fries, Bass & Muellernucl-th/0407102
correlation functions of two high pT hadrons
Elliptic flow component measured vs. BBC reaction plane
decompose to get jet pair distribution
Away-side jets broadened
non-Gaussian!
~2 dip at & peak at 1.25 rad around hard parton thru medium
integrating entire away side recovers jet partners
Casalderry, Shuryak, Teaney say 1.1 rad cone
hep-ph/0411315
interpretation? *it’s fun to speculate
pQCD energy loss is by gluon radiationmostly 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 collinearcreates a wake in the medium (Fries et al; Shuryak)thermalizes in the medium
speed of wake reflects cs in the medium: cosm=cs/c
= 1/√3 in non-interacting QGP, ~ 0.45 in hadron gas = 1/3 a mixture of the two??
Jet partner likely for trigger baryons as well as mesons! Same side: slight decrease with centrality for baryonsDilution from boosted thermal p, pbar?
identify triggers, count partnersnucl-ex/0408007
hadron formation time(lab frame) f ~ Rh (Eh/mh)for 2.5 GeV pT; Rh~1 fmf ~ 9-18 fm/c for pions ~ 2.7 fm/c for baryons Baryons formed inside!
pick up q from wake?
trigger: 2.5-4 GeV/c; partner 1.7-1.5
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
currently being analyzed; first look not conclusive
RHIC
data on Au+Au, Cu+Cu being analyzed
40-90%most central Ncoll=45
0-20%most central Ncoll=779
20-40%most central Ncoll=296 South Muon Arm
6062+/-195 J/
343+/-82’ (6%)
Cu+Cu, 2005 run
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, screeningspeed of sound? color and maybe thermal conductivity to be quantifiedcolor screening currently being analyzed
LHC will make QGP too. (As) strongly coupled?higher , pT reach for hard probes; soft physics at higher T
so, is there QGP at RHIC?
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
Thanks for support from RBRC & NSF!
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
Levin, Ryshkin, Mueller, Qiu, Kharzeev, McLerran, Venugopalan,
Balitsky, Kovchegov, Kovner, Iancu …
probe rest frame
r/ggg
dAu AuAuR R RdAu~ 0.5D.Kharzeev et al., hep-ph/0210033
1dAuR Broaden pT :
d+Au central/peripheral
Aud
Au
1.5 < pT (GeV/c) < 4.0
Suppression at forward η and enhancement in the back η.
PTH = Punch Through HadronsHDM = Hadronic Decay Muon
PHENIX nucl-ex/0411054
x~0.2-0.3
x~0.2x10-3
Color glass condensate?
Hadron Punch Through
Slightly better agreement with BRAHMS data“normal” shadowing cannot explain (R. Vogt hep-ph/0405060)
…could be sign of CGC
Kharzeev, hep-ph/0405045
Centrality, pT
dependence
~ correct
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.
Hwa, Yang and Fries nucl-th/0410111
BRAHMS data
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 fieldsgluon fields obey Maxwell’s equationsadd initial anisotropy and you’d expect Weibel instability
moving charged particles induce B fieldsB field traps soft particles moving in A directiontrapped particle’s current reinforces trapping B fieldcan 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 lifetimebut gluon-gluon interactions may cause instability to
saturate → drives system to isotropy & thermalization
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
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 naturehigh 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
Is enough for fast equilibration & large v2 ?
Parton cascade using free q,g scattering cross sections underpredicts pressure must increase x50
Lattice QCD shows qqresonant states at T > Tc, also implying high interaction cross sections
Is the energy density high enough?
5.5 GeV/fm3 (200 GeV Au+Au) well above predicted transition!
PRL87, 052301 (2001)
R2
2c
Colliding system expands:
dy
dE
cRT
Bj 22
11
02
Energy tobeam direction
per unitvelocity || to beam
value is lower limit: longitudinal expansion rate, formation time overestimated