the quark gluon plasma at rhic
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The Quark Gluon Plasma at RHIC
outline
What’s a plasma?and why do we expect one from quarks and gluons?
The tools to make and study quark gluon plasma What do we see at RHIC?
collective flowopacity of the matterexcitations of the matter?J/ suppression to search for deconfinement
Conclusionswhat we have foundwhat HAVE we found?
what is 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 lengththey screen each other
plasma size > D
“normal” plasmas are electromagnetic (e + ions)quark-gluon plasma interacts via strong interaction
color forces rather than EMexchanged particles: g instead of
Quarks, gluons, hadrons
6 quarks: 2 light (u,d), 1 sort of light (s)2 heavy (c,b), 1very heavy (t)flavor & color quantum numbers
Quarks are bound into hadronsBaryons (e.g. n, p) have 3Mesons (e.g. ): 2 (q + anti-q)
Colored quarks interact by exchange of gluons Quantum Chromo Dynamics (QCD)
Field theory of the strong interaction parallels Quantum Electrodynamics (QED)
EM interactions: exchanged photons electrically unchargedgluons carry color charge
QCD phase transition
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 gas of free quarks and gluons
non-perturbative QCD – lattice gauge theory
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
Create high(est!) energy density mattersimilar to that existing ~1 sec after the Big Bangcan study only in the lab – relics from Big Bang inaccessible
T ~ 200-400 MeV (~ 2-4 x 1012 K)~ 5-15 GeV/fm3 (~ 1030 J/cm2)R ~ 10 fm, tlife ~ 10 fm/c (~3 x 10-23 sec)
Characterize the hot, dense mediumdoes 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
to get there: collide BIG ions at v ~ c
ideal gas or strongly coupled plasma?
Huge gluon density! estimate = <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
quark gluon plasma should be a strongly coupled plasmaAs in warm, dense plasma at lower (but still high) T
how does it compare to interesting EM plasmas?
> 1: strongly coupled, few particles inside Debye radius
RHIC at Brookhaven National Laboratory
Collide Au + Au ions for maximum volumes = 200 GeV/nucleon pair, p+p and d+A to compare
4 complementary experiments
STAR
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
0
look at radiated & “probe” particles
as a function of transverse momentumpT = p sin
with respect to beam direction90° is where the action is (max T, )
midway between the two beams! pT < 1.5 GeV/c
“thermal” particles from the bulk of the plasma
pT > 3 GeV/c
fast particles – mostly part of jets of hadrons coming from hard scattered q or g
produced very early, probe the plasma
search for collectivity (plasma feature)
dN/d ~ 1 + 2 v2(pT) cos (2) + …
“elliptic flow”
Almond shape overlap region in coordinate space
x
yz
momentum space
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 perfect liquid!
Elliptic flow scales as number of quarks
0 1 2 pT/n (GeV/c)
v2 scales ~ with # of quarks! quarks are the particles when the pressure is built up
even charm quarks flow!
measure D→e± + hadrons
Mcharm = 1.3 GeV
D’s flow to ~2 GeV/c
then expect e from B decays (Mb~4 GeV → shouldn’t flow)
collective flows tell us: RHIC creates matter – not just collection of particles the matter catches even the heavy c quarks!
How to actively probe the deep interior?
p
pT
measure “hard scatterings”of q,g at large pT
“transverse momenta”
Direct Photon Spectra in Au+Au
q + g → q + Should not interact
with the color charges
data and theory agree → calibrated probe
pQCD works in the complex environment of two nuclei (Au+Au ) colliding at high energies
peripheralN
coll = 12.3 4.0
centralN
coll = 975 94
strongly interacting probe: a different story!
suppression persists to 20 GeV/c!
ddpdT
ddpNdpR
TNN
AA
TAA
TAA /
/)(
2
2
nuclear modification factorratio of data on previous slide
so we see: photons shine, pions don’t
Direct photons are not inhibited by hot/dense medium Pions (all hadrons) are inhibited by hot/dense medium
look for the jet on the other sideSTAR PRL 90, 082302 (2003)
Central Au + Au
Peripheral Au + Au
Medium is opaque!
Are back-to-back jets there in d+Au?
Pedestal&flow subtracted
Yes!
QGP properties, so far
Extract from models, constrain by dataEnergy loss <dE/dz> (GeV/fm) 7-10 0.5 in cold matter
Energy density (GeV/fm3) 14-20 >5.5 from ET data
above hadronic e density!
dN(gluon)/dy ~1000 From energy loss, hydro huge!
T (MeV) 380-400
Experimentally unknown as yet
Equilibration time0 (fm/c) 0.6 From hydro initial condition; cascade agrees very fast!
NB: plasma folks have same problem & use same technique
Opacity (L/mean free path) 3.5 Based on energy loss theory
Equation of state? Early degrees of freedom and their ? Deconfinement? Thermalization mechanism? Conductivity?
How to get fast equilibration & large v2 ?
parton cascade using free q,g scattering cross sections doesn’t work! need x50 in medium
Lattice QCD shows qqresonant states at T > Tc, also implying high interaction cross sections
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
Quarks and gluons are interacting, but are not locally (color) neutral like the baryons & mesons. Neutrality scale likely larger, as expected for a plasma.
strongly coupled gas of atoms
M. Gehm, S. Granade, S. Hemmer, K, O’Hara, J. ThomasScience 298 2179 (2002)
weakly coupled
strongly coupled ↓ ellipticflow!
what about the heavier quarks?
e± in Au+Au vs <Ncoll>*p+p
peripheral collisions
central collisions
c quark suppression is nearly as large as for pions!
where does the lost energy go?
transferred to the plasma? does the medium respond?
look at “away side” jet’s particles near thermal pT
M.Miller, QM04
(1/N
trig)d
N/d
()
STAR Preliminary
cGeVp
cGevpassocT
trigT
/42.0
/64
a sonic boom?
g radiates energykick particles in the plasmaaccelerate them along the jet
+/-1.23=1.91,4.37 → cs ~ 0.33 (√0.33 in QGP, 0.2 in hadron gas)
PHENIX
dN
/d(
)
jury is still out…
How about the screening length?
J/Test confinement:
do bound c + c survive? or does QGP screening kill them?
RHIC
NA50 and NA60 show suppression in Pb+Pb & In+In
suppression follows system size
Normal nuclear absorption from p+A data: = 4.18±0.35 mb
At CERN (√s = 17 GeV):
At RHIC:
CuCu
200 GeV/c
AuAu
200 GeV/c
dAu
200 GeV/c
J/ muon arm
1.2 < |y| < 2.2
mea
sure
d/e
xpec
ted
At RHIC:
CuCu
200 GeV/c
AuAu
200 GeV/c
dAu
200 GeV/c
AuAuee
200 GeV/c
CuCuee
200 GeV/c
J/ muon arm
1.2 < |y| < 2.2
J/ eeCentral arm
-0.35 < y < 0.35
At RHIC:
CuCu
200 GeV/c
AuAu
200 GeV/c
dAu
200 GeV/c
AuAuee
200 GeV/c
CuCu
62 GeV/c
J/ muon arm
1.2 < |y| < 2.2
J/ eeCentral arm
-0.35 < y < 0.35
Factor ~3suppression
in central events
CuCuee
200 GeV/c
At RHIC:
J/ muon arm
1.2 < |y| < 2.2
J/ eeCentral arm
-0.35 < y < 0.35
Factor ~3suppression
in central events
Data show the same trend within errors for all beams and even at √s=62 GeV
RAA
vs Npart
: PHENIX and NA50
NA50 data normalized at NA50 p+p point.
Suppression similar in the two experiments, although the collision energy is 10 times higher (200GeV in PHENIX & 17GeV in NA50)
What suppression should we expect?
Models that were successful in describing SPS datafail to describe data at RHIC
- too much suppression -
can get better agreement with data
if add formation of “extra” J/ by coalescence of c and anti-c from the plasma
(not necessarily unique or correct explanation!)
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 propertiesJ/ may survive better
NB: the (quasi-)bound states are not your mother’s hadrons!
Evidence that RHIC creates a strongly coupled, opaque plasma 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 can get at properties of this new kind of plasma
opacity, collision frequency, EOS, screening, speed of sound, conductivity
Open questionsDo heavy quarks really thermalize in the QGP?Initial temperature (direct radiation from plasma?)Discovery announcement soon?
conclusions
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
Assume all distributions described by a temperature T and a (baryon) chemical potential : dn ~ e -(E-)/T d3p
One ratio (e.g., p / p ) determines / T : pbar/p = e -(E+)/T / e -(E-)/T = e -2/T
Second ratio (e.g., K / ) provides T predict others
Does final state reflect a thermal distribution?
Tf ~ 175 MeV
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
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
do heavy quarks thermalize?
elliptic flow?
energy loss!
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)
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
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
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!
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!
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
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
Predicted signatures of QGP
Debye screening of color charges (Matsui & Satz)Suppresses rate of c-c bound state (J/
Strangeness enhancement (Rafelski & Mueller)Tplasma ~ masss-quark
Enhances strange hadron yields (e.g. K/ increased)
Jet quenching (Gyulassy & Wang)Dense medium induces coherent gluon radiationDecreased yield of jets of hadrons from hard scattered
quarks or gluons
Where does the lost energy go?
Away side jet is significantly broadened
preliminary
• recover the energy in hadrons ~ 500 MeV• comparable to <pT> of the bulk medium• lost energy is thermalized
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.
The physics of the quark gluon plasma
Energy densities characteristic of the early universeWhy expect a quark gluon plasma?
Experimental study at RHICExperiments and probes of the hot, dense matter
What happens?Energy loss of fast quarks, gluonsCollective motion: hydrodynamic flow
First glimpse of the matter’s properties Underlying degrees of freedom & hadronization Conclusions & open questions
“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)
Centrality Dependence
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
Evolution of the Universe
Nucleosynthesis builds nuclei up to HeNuclear Force…Nuclear Physics
Universe too hot for electrons to bindE-M…Atomic (Plasma) Physics 104
K
E/M Plasma
Too hot for quarks to bind!!! Too hot for quarks to bind!!! 1012 KStandard Model (N/P) Physics
Quark-Gluon
Plasma??
Too hot for nuclei to bind 1010 KNuclear/Particle (N/P) Physics Hadron
Gas
SolidLiquidGas
Today’s Cold UniverseGravity…Newtonian/General
Relativity
pressure builds up
probing a 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 q,g(short wavelength) probes of plasmaformed
The Scope of the Tools (!)
STARspecialty: large acceptancemeasurement of hadrons
PHENIXspecialty: rare probes, leptons,
and photons
Energy density of matter
high energy density: > 1011 J/m3
P > 1 MbarI > 3 X 1015W/cm2 Fields > 500 Tesla
QGP energy density > 1 GeV/fm3
i.e. > 1030 J/cm3
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