A new kind of matter at the Relativistic Heavy Ion Collider?
The Physics of Heavy Ion Collisions
Create very high temperature and density mattersimilar to that existing ~1 sec after the Big Bangdistance between hadrons comparable to that in neutron starsstudy only in the lab – relics from Big Bang inaccessible
Goal is to characterize the hot, dense mediumexpect QCD phase transition to quark gluon plasmadoes medium behave as a plasma?find density, temperature, radiation rate, collision frequency,
conductivity, opacity, Debye screening length?probes: passive (radiation) and those created in the collision
Collide Au + Au ions for maximum volumes = 200 GeV/nucleon pair, p+p and d+A to compare
Quantum Chromo Dynamic phase transition
Colored quarks interact by exchange of gluons
Color charge of gluons gluons interact among themselves theory is non-abelian curious properties at large distance: confinement of quarks in hadrons
+ +…
At high temperature and density: force is screened by produced color-chargesexpect transition to free gas of quarks and gluons
non-perturbative QCD - calculate on lattice
T/Tc
Karsch, Laermann, Peikert ‘99
/T4
Tc ~ 170 ± 10 MeV (1012 °K)
~ 3 GeV/fm3
required conditions to study quark gluon plasma
~15% from ideal gas of weakly interacting quarks & gluons
42
30Tg
Where are we?
1039 1045
1010
1012
QGP at RHIC?
1051 1057
345
332
33
104~)(
200~,37
)2
(~2.1
)3()2(
4)(
mTn
MeVTg
TgTg
Tg
Tn
RHIC at Brookhaven National Laboratory
RHIC collides p, Au ions: 200 GeV/nucleon in c.m.10 times the energy previously available!
4 complementary experiments
STAR
pressure builds up
probing early stage of heavy ion collision
PCM & clust. hadronization
NFD
NFD & hadronic TM
PCM & hadronic TM
CYM & LGT
string & hadronic TM
Kpnd,
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 heavyq,g probes of plasma formed
Study simple complex systems: p+p, “p”+A, A+A collisions
p-p PRL 91 (2003) 241803
Good agreementwith NLO pQCD
is QCD the right theory at RHIC? Perturbative for high p transfer processes? pp collisions: it works!
Have a handle on initial NN interactions by scattering of q, g inside N
We also need:2
/( , )
a Nf x Q
2
/( , )ch a
D z Q
Parton distribution functions
Fragmentation functions
0
pQCD in Au+Au? direct photons
[w/ the real suppression]
( pQCD x Ncoll) / background Vogelsang/CTEQ6
[if there were no suppression]
( pQCD x Ncoll) / ( background x Ncoll)
Au+Au 200 GeV/A: 10% most central collisions
[]measured / []background = measured/background
Preliminary
At high pT, it also works!
TOT
pT (GeV/c)
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
Collective effects? Pressure: a barometer called “elliptic flow”
Origin: spatial anisotropy of the system when created, followed by multiple scattering of particles in the evolving system spatial anisotropy momentum anisotropy
v2: 2nd harmonic Fourier coefficient in azimuthal distribution of particles with respect to the reaction plane
Almond shape overlap region in coordinate space 2cos2 v
x
y
p
patan
y2 x2 y2 x2
v2 measured by the experiments
STAR
v2=0.05
130 GeV: 0.075< pt < 2.0 200 GeV: 0.150< pt < 2.04-part cumulants
200 GeV: 0.2< pt < 2.0
Preliminary
200 GeV: Preliminary
- Consistent results- At 200 GeV better pronounced decrease of v2 for the most peripheral collisions.
STARPreliminary
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 assumes early equilibrationInitial energy density is inputEquation of state from lattice QCDSolve equations of motion
Collective effect probes early phase
Hydrodynamics can reproduce magnitudeof elliptic flow for , p. BUT correct mass dependence requiressofter than hadronic EOS!!
Kolb, et al
NB: these calculations have viscosity = 0medium behaves as an ideal liquid
Pressure felt by all hadrons
v2 scales ~ with # of quarks! evidence for quarks as relevant d.o.f. when pressure built up
Need large for fast equilibration & large v2
Parton cascade using free q,g scattering cross sections underpredicts pressure
Lattice QCD shows qqresonant states at T > Tc, also implying high interaction cross sections
Properties of this stuff?
Pressure built up during collision -> observed collectivityviscosity small, interaction large
Quasiparticles may experience binding, butare not color neutral as at T<Tc.
Do we expect screening?quark & gluon density is high…estimate = <PE>/<KE><PE>=g2/d = g2(~4-6) /(41/3T) <KE> ~ 3T
so plasma parameter
RHIC is in regime of strongly coupled plasmaAs in warm, dense plasma at lower (but still high) TBut strong interaction rather than electromagnetic
“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
pp
AuAubinaryAuAuAA Yield
NYieldR
/
Au-Au s = 200 GeV: high pT suppressed!
PRL91, 072301(2003)
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!
lost energy is thermalized
Away side jet is significantly broadened
preliminary
•recover the energy in hadrons ~ 500 MeV.
• comparable to <pT> of the bulk medium
Au+Au 0-5%
pT > 200 MeV
STAR Preliminary
Property probed: density
Au-Au
d-AudAu
Induced gluon brehmsstrahlung pQCD calculationdepends on 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.
*deuteron-gold control experiment with no suppression
A puzzle
p/ ~1 at high pT
in central collisions
> p+p, d+Au> peripheral Au+Au
>jets in e+e-collisions
PRL 91 (2003) 172301
Hydro. expansion at low pT + jet quenching at high pT.
coalescence of boosted quarks into hadrons
enhances mid pT hadrons baryons especially
pQCD spectrum shifted by 2.2 GeV
Teff = 350 MeV
R. Fries, et al
Are extras from the thermal medium?
But jet-like properties too…
Baryons scale
Ncoll !
Mesons, including are suppressed Effect governed by quark content
~RA
A
pions
protons
are baryons from jets or not?
• measure probability of jet-like partner for baryons & mesons in pT range of excess
•decrease for baryons in most central Au+Au
• expected from recombination of purely thermal quarks
Allow fast quark to grab partner from medium
Many baryons ARE from jets, but medium modifies those jets
Summary of medium properties
Extract from models, constrain by jet suppression & v2
Values from I. Vitev; others consistent
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
Is there quark gluon plasma at RHIC? qualified “yes”Intense debate in community about standard of proof
Evidence is mounting that RHIC creates a strongly coupled, opaque plasma
With aid of hydrodynamic, l-QCD and p-QCD models: ~ 15 GeV/fm3
dNgluon/dy ~ 1000
int large for T < 2-3 Tc Medium modifies hadronization of moderate energy jets Will learn screening properties via cc bound states Are poised to characterize this new kind of plasma
radiation rate, conductivity, collision frequency Color Glass Condensate? Maybe at x ~ 10-3 (y>2)
conclusions
Saturation of gluons in initial state(colored glass condensate)
Wavefunction of low x (very soft) gluons overlap and the self-coupling gluons fuse.
Saturation at higher x at RHIC vs. HERA due to nuclear size
suppressed jet cross section; no back-back pairsr/ggg
Mueller, McLerran, Kharzeev, …
d + Au collisionscent/periph. (~RAA)
Color screening properties?
Lattice predictions for heavy quarks
40-90%most central Ncoll=45
0-20%most central Ncoll=779
20-40%most central Ncoll=296
Data inconclusive, but x50 underway
NB: need temperature too!
PHENIX PRELIMINARY
Open charm: baseline is p+p collisions
fit p+p data to get the baseline for d+Au and Au+Au.
Measure charm via semi-leptonic decay to e+ & e-
, photon conversions are measured and subtracted
Curves are the p+p fit, scaled by the number of binary collisions
No large suppression as for light quarks!
PHENIX PRELIMINARY
Implications of the results for QGP
Ample evidence for equilibration initial dN(gluon)/dy ~ 1000, energy density ~ 15
GeV/fm3, energy loss ~ 7-10 GeV/fm
Very rapid, large pressure build up requiresparton interaction cross sections 50x perturbative
How to get 50 times pQCD ?
• Lattice indicates that hadrons don’t all melt at Tc!c bound at 1.5 Tc Asakawa &
Hatsuda, PRL92, 012001 (2004)
charmonium bound states up to ~ 1.7 Tc Karsch; Asakawa&Hatsuda
, survive as resonances Schaefer & Shuryak, PLB 356 , 147(1995)
q,g have thermal masses at high T. s runs up at T>Tc? (Shuryak and Zahed)would cause strong rescattering
qq meson
spectral function
E. Shuryak
Implications of the results for QGP
Ample evidence for equilibration v2 & jet quenching measurements constrain initial gluon
density, energy density, and energy loss parton interaction cross sections 50x perturbative
parton correlations at T>Tccomplicates cc bound states as deconfinement probes!
Hadronization by coalescence of thermal,flowing quarksv2 & baryon abundances point to quark recombination
as hadronization mechanismJet data imply must also include recombination between
quarks fromjets and the thermalized medium medium modifies jet fragmentation!
Also see evidence for equilibrated final state
Calculate hadron yields in Grand Canonical ensembleObserved hadron ratios in agreement with thermal ratios!T(chemical freeze-out) ~ 175 MeV
Locate RHIC on phase diagram
Baryonic Potential B [MeV]
0
200
250
150
100
50
0 200 400 600 800 1000 1200
AGS
SIS
SPS
RHIC
quark-gluon plasma
hadron gas
neutron stars
early universe
thermal freeze-out
deconfinementchiral restauration
Lattice QCD
atomic nuclei
From fit of yields vs. mass (grand canonical ensemble):
Tch = 176 MeV B = 41 MeV
These are the conditions when hadrons stop interacting
T
Observed particles “freeze out” at/near the deconfinement boundary!
v2 recombination of flowing quarksnucl-ex/0305013
above p forpT < 2 GeV/c, in agreement with hydrodynamicsThen crosses over.Values ~ saturateat high pT
geometry?
v2/quark ~ constant create hadronsby coalescence of quarks from boosted distribution
The yellow band represents the set of alpha values consistent with the data at the 90% Confidence Level.
No large suppression as for light quarks!
Do see Cronin effect!
“Cronin” enhancement more pronounced in the charged hadron measurement
Possibly larger effect in protons at mid pT
Implication of RdAu? RHIC at too high x for gluon saturation…
(h++h-)/2
0
How about Color Glass Condensate?
Pt (GeV/c) Pt (GeV/c)
Rda
Rda
Peripheral d+Au (like p+p)
Central: Enhancednot suppressed PHENIX preliminary
y=0
Xc(A)
pQCD
BFKL, DGLAP
G-sat.
>2
RHIC
Log Q2
No CGC signalat mid-rapiditySo, perhaps
But at forward rapidity reach smaller x
y = 3.2 in deuteron direction x 10-3 in Au nucleus
Strong shadowing, maybe even saturation?
d Au
Phenix Preliminary
Why no big energy loss for heavy quarks?
no x4 suppressionfrom peripheral to central,as predicted fordE/dx=-0.5GeV/fm!
But (we squirm) - Is 40-70% peripheral enough? error bars still big!
Why no energy loss for charm quarks?
“dead cone” predicted by Kharzeev and Dokshitzer, Phys. Lett. B519, 199 (1991)
Gluon bremsstrahlung:kT
2 = 2 tform/transverse momentum of radiated gluon
pT in single scatt. mean free path
~ kT / gluon energy But radiation is suppressed below angles 0= Mq/Eq
soft gluon distribution is
dP = sCF/ d/ kT2 dkT
2/(kT2+ 2 0
2) 2not small forheavy quarks!causes a dead cone
Look at “transverse mass” mT2 = pT
2 + m02
— 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 beamResult of pressure built up
Simple quark counting:K-/K+
= exp(2s/T)exp(-2q/T)
= exp(2s/T)(pbar/p)1/3
= (pbar/p)1/3
local strangeness conservation K-/K+=(pbar/p)
= 0.24±0.02 BRAHMS = 0.20±0.01 for SPS
Good agreement with statistical-thermal model of Beccatini et al. (PRC64 2001) w/T=170 MeV
At y=0
From y=0 to 3
PRL 90 102301 Mar. 2003
Evidence for equilibrated final hadronic stateBRAHMS
Au+Au at sNN=200GeVv2 of mesons & baryons
v2
1) High quality M.B. data!!!
2) Consistent between PHENIX and STAR
pT < 2 GeV/c v2(light) > v2(heavy)
pT > 2.5 GeV/c v2(light) < v2(heavy)
Model: P.Huovinen, et al., Phys. Lett. B503, 58 (2001)
QCD Phase Transition
Basic (i.e. hard) questionshow does process of quark confinement work?how nature breaks symmetries massive particles from ~
massless quarks transition affects evolution of early universe
latent heat & surface tension matter inhomogeneity in evolving universeequation of state compression in stellar explosions
Study simple complex systems: p+p, “p”+A, A+A collisions
did something new happen at RHIC?
Study collision dynamics (via final state)
Probe the early (hot) phase
Equilibrium?hadron spectra, yields
Collective behavior?i.e. pressure and expansionelliptic, radial flow
vacuum
QGP
Must create probes in the collision itself: predictable quantity, interact differently in QGP vs. hadron matter
fast quarks/gluons, J/fast quarks/gluons, J/, D mesons, D mesonsthermal radiation
we look for physics beyond simple superposition of NNat low momentum/large distance scales:
See jet suppression! a final state effect?
Hadronic absorption of fragments: Gallmeister, et al. PRC67,044905(2003)Fragments formed inside hadronic medium
Energy loss of partons in dense matterGyulassy, Wang, Vitev, Baier, Wiedemann…
PCM & clust. hadronization
NFD
NFD & hadronic TM
PCM & hadronic TM
CYM & LGT
string & hadronic TM
Hadron gas
1AuAuR Absent in d+Au collisions!d+Au is the “control” experiment
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
• Nuclear shadowing
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
decreases dAuR
Broaden pT :
What about heavy quarks?
J/Test confinement: do bound c + c survive? or does QGP screening kill them?
Open CharmExtra heavy quarks from hot, dense gluon gas?Do the c quarks lose energy like the light quarks?
Need (a lot) more statistics (currently being collected)
But can take a first look…
J/ suppression was observed at CERN at s=18 GeV/A
Fewer J/ in Pb+Pb than expected!Interpret as color screening of c-cbar
by the mediumInitial state processes affect J/ tooso interpretation heavily debated...
collaboration
J/yield
Energy loss is initial or final state?
Dramatically different and opposite centrality evolution of AuAu experiment from dAu control.
Jet Suppression is final state effect of dense medium
Au + Au Experiment d + Au Control
PHENIX preliminary