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

Map of high energy densities

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

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

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

We use RHIC at Brookhaven National Laboratory

s = 200 GeV/A Au+Au, p+p and d+A to compare

4 complementary experiments

STAR

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!

peripheralN

coll = 12.3 4.0

centralN

coll = 975 94

strongly interacting probe: a different story!

Photons shine, Pions don’t

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??

Recall the annoying baryon puzzle…

h/0 ratio shows baryons enhanced for pT < 5 GeV/c

PRELIMINARY

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

Compare with BRAHMS

Overall consistent.

nucl-ex/0411054

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

Charm production scales with Ncoll*

* there is a long-standing problem with c

nucl-ex/0409028

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 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

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