Probing the Properties of the Quark-Gluon Plasma and Studying Strong-Field QCD in Heavy Ion Collisions @

RHIC and the LHC

Prof. Brian A. Cole.Columbia University

PHENIX and ATLAS

The “Big Picture”

Heavy Ion (Au+Au) collision as seen by the STAR time-projection chamber.

Why???

Fundamental InteractionsMatter usually studied in the lab has properties determined by EM interactions.

How would non-Abelian matter be different?

The Big Picture•We have a fundamental theory of strong interactions

•exhibits asymptotic freedom at large momentum transfer.

•What about other limits of QCD?–High temperature–High field strength

nn

nnQCD mtAigFFL

4

1

QCD Thermodynamics (on Lattice)

•Rapid cross-over from “hadronic matter” to “Quark-Gluon Plasma” at T 170 MeV

Energy density, ~ 1 GeV/fm3.

•Only fundamental “phase transition” that can be studied in the laboratory.

– Is the QGP weakly interacting??

Energy Density / T4 Pressure / T4

Relativistic Heavy Ion Collider

Run 1 (2000): Au-Au SNN = 130 GeVRun 2 (2001): Au-Au, p-p SNN = 200 GeVRun 3 (2003): d-Au, p-p SNN = 200 GeVRun 4 (2004): Au-Au SNN = 200, 64 GeV, p-p SNN = 200 GeVRun 5 (2005): Cu-Cu SNN = 200, 64 GeV, p-p SNN = 200 GeV

STAR

Heavy Ion Collision Time History

t

z

RHIC collision space-time history in “parton cascade” model

Initial particle production from strong fields

Rapid thermalization

“Hydrodynamic” evolution

Hadronization (interesting but I won’t cover)

RHIC Initial Conditions

•Au+Au @ 200 GeV per nucleon, = E/m 100.–Au diameter, d 14 fm, contracted d/ 0.2 fm Crossing time < 0.2 fm/c.

•Add quantum mechanics: E ħc / t –Fluctuations with E > 1 GeV are “on shell”These are primarily gluons (~ 200/collision)RHIC is a gluon collider! (10 GeV/fm3)

“Saturation” @ low x•@ High energy nuclei are highly Lorentz contracted– Except for soft gluons

– Which overlap longitudinally

– And recombine producing broadened kT distribution

Generates a new scale: Qs

Typical kT of gluons

•If Qs >> QCD, perturbative calculations possible.

Large occupation #s for kT<Qs classcial fields

•Saturation is a result of unitarity in QCD

QCD: Evolution•Quarks radiate copiously Evolution of proton (e.g.) parton

distribution functions Growth of gluon distribution @ low x

•@ high gluon density recombination starts to limit growth.

•Qs - resolution scale where recombination starts to dominate

•Limiting density Qs

2/s

– ~ Classical field

Saturation @ HERA?

•Measurements of DIS cross-section vs x, Q2 for x < 0.01.

•Plotted vs

•All x dependence in the saturation scale.

– “Geometric scaling”

•Precursor to saturation in PDF evolution

)(2

2

xQ

Q

s

Golec-Biernat and Wusthoff (GBW) Saturation Model (empirical)

RHIC Particle Multiplicities

•Multiplicity @ RHIC on low end of predicted range

•Slow growth with impact parameter (Npart)

– Inconsistent with factorized mini-jet production

–Best described by saturation model

Multiplicity per colliding nucleon pair

But, Have We Created “Matter” ?

•“Pressure” converts spatial anisotropy to momentum anisotropy.

•Requires early thermalization.•Unique to heavy ion collisions•Answer: yes

dN

/d

x

yz

“Elliptic Flow”

•Parameterize variation by “v2” parameter

–

•Compare to “eccentricity”:

•Data consistent w/ hydrodynamic calculations

)2cos(21 2

vd

dN

22

22

xy

xy

Transverse Plane

(Ideal) Hydrodynamics in one slide

Estimating the (Shear) Viscosity•Relevant parameter for determining collective motion of quark-gluon plasma

– viscosity to entropy ratio: /s.

•Finite viscosity leads to dependence of flow strength on pT.

– Correction (/s) pT2

•From data shown to right, obtain estimate:

/s ~ 0.1

–Very small!

s is “sound attenuation length” mean free path

sTs

3

4

Comparison to “Typical” Vicosities

/s ~ 0.1

•Thus the statement: –QGP is most perfect fluid every created

Lattice QCD Estimate of /s

Nakamura & Sakai, hep-lat/0510100

T/Tc

/s pQCD

AdS/CFT

Shear viscosity in quenched QCD

“Casual” Viscous Hydrodynamics

•Causal fix introduces a new scale, , relaxation time. Uncertainty in weakens /s constraint– Concludes /s < 0.5, but elliptic flow?

P. Romatschke nucl-th/0701032

Comparison of viscous hydro results to meson spectra from PHENIX

Thermalization via Plasma Instabilities?

•pT vs pz anisotropy

– Generates strong local chromo-magnetic fields

– Lorentz forces produce rapid isotropization.

•Pressure from macro-scopic color fields?!

Ene

rgy

dens

ity

•Use self-generated quarks/gluons/photons as probes of the medium (classic physics technique!)

Penetrating Probes of Created Matter

z

t

Collisions between partons

How to directly probe medium ?•Use quarks & gluons from high-Q2

scattering– “Created” at very early times (~ 0.1 fm).– Propagate through earliest, highest matter.

•(QCD) Energy loss of (color) charged particle– ~ Entirely due to radiation– Virtual gluon(s) of quark multiply scatter.

•e.g. GLV (Gyulassy, Levai, Vitev) formalism

Experimentally measure using:(Leading) high-p hadronsDi-jet correlations

Perturbative quantum chromo-dynamics

•Factorization: separation of into– Short-distance physics: – calculable using perturbation theory**

– Long-distance physics: ’s – universal, measured separately.

•Valid @ large momentum transfer – high pT particles

p-p di-jet Event

STARSTAR

a/A

b/B

A

B

ab

From Collins, Soper, Sterman Phys. Lett. B438:184-192, 1998

pQCD – Single Hadron ProductionAdd fragmentation to hadrons

•D(z) – fractional momentum distribution of particles in “jet”

KKP

Kretzer

data vs pQCD

dt

d

z

QzD

QxQxdxdxdp

dE

c

abcaBbaAaba

ˆ),,(

),,(),,(

2

/

2/

2/3

3

0

Phys. Rev. Lett. 91, 241803 (2003)

a/A

b/B

A

B

ab

D(z)

PHENIX Au-Au 0 Spectra

Calculations with no energy loss

Calculations with energy loss

•Observe 20% of expected yield @ high pT

Deduce energy density ~15 Gev/fm3

Compare crit ~ 1 GeV/fm3

100 x normal nucleus energy density!

pT s

pectr

um

Expected

RAA Observed/Expected

Using p-p data as baseline

PHENIX: Au-Au High-pT 0

Suppression

0 Suppression: dE/dx Comparisons

•Quark & gluon dE/dx analysis: Turbide et al (McGill)–Essentially an ab initio calculation–Compared to precision (relatively) data

Prompt Photon Production

•Prompt photons provide an independent control measurement for jet quenching.– Produced in hard scattering processes

–But, no final-state effects (???)

Au-Au Prompt Photon Production

•Photon control measurement shows no quenching

•pQCD calculations OK, quenching a final-state effect

High-pT Single Particle Summary

5 violation of factorization up to 20 GeV/c– In hadron production (jets), but not prompt Hard scatterings occur at expected ratesSuppression from final-state energy loss

To explain data:

Unscreened color charge dn/dy~1000

Energy density ~15 GeV/fm3

> 10 “critical” energy density

Analysis of Single Hadron Data: BDMS-Z-SW

•“Thick medium” energy loss calculation

/fmGeV14~ˆ 2q

Central 200 GeV Au+Au

lengthp

q T2

ˆ

Transport coefficient:

for radiated gluon

•Baier [Nucl. Phys. A715, 209 (2003)]: – C = 2 expected for ideal QGP– 14 GeV/fm2 c = 8-10!! Strong coupling [Eskola et al, Nucl. Phys. A747, 511 (2005)]

4/3ˆ cq

STAR Experiment: “Jet” Observations

Nu

mb

er o

f p

airs

Angle between high energy particles0º 180º

proton-proton jet event

In Au-Au collisions we see only one “jet” at a time !

How can this happen ? Jet quenching!

q

q

Analyze by measuring (azimuthal) angle between pairs of particles

Di-jet Distortion vs “Impact Parameter”

(di)Jet Angular Correlations (PHENIX)

•PHENIX (nucl-ex/0507004): moderate pT

Origin of di-jet Distortion?Mach cone?•Jets may travel faster than the speed of sound in the medium.

•While depositing energy via gluon radiation.

•QCD “sonic boom”

Mach Cone (2)

•Ideal QGP, cs

2 = 1/3

•Cos M = cs

M = 55º

•Detailed calculation taking into account evolving speed of (gluon) sound from hydrodynamics.

–

•But, other possible mechanisms proposed. /2 pcs

What I Didn’t Show You•Charm quarks also are quenched

–And show rapid thermalization!

•Large charm quark elliptic flow signal–Can only be established at the quark level.

•Large baryon excess for 2 < pT < 5 GeV/c– Hadron formation by quark recombination

•We see final state particle flavor distributions consistent with “freeze-out” from chemically equilibrated system.–We are rapidly approaching stage where QGP is ONLY viable interpretation of data

RHIC Physics•Since start of RHIC, substantial progress on development of a rigorous foundation for understanding stages of a heavy ion collision:–Particle production from strong gluon fields–Thermalization (ideas but not yet understood)–Hydrodynamic evolution–Hadronization

•With jets as calibrated probe–But jet quenching is still not completely understood

•We are probing the properties of the QGP – with surprising results–Strong coupling – why?–Speed of sound?

Comparison to “Typical” Vicosities

•But what is this “viscosity bound”?– Calculation of viscosity using string theory– Huh????

/s ~ 0.1

AdS/CFT Correspondence

Main idea•Duality between string theory in anti-DeSitter space and conformal field theories.– Weakly coupled string theory strongly coupled CFT

•Example conformal theory: – N=4 supersymmetric Yang-Mills

– Which is not QCD (e.g. no running of s)

– But similar enough??

•AdS/CFT now being applied to many apsects of RHIC physics

– Viscosity, /s.– Jet quenching

– “Sound” waves

Why Heavy Ions @ LHC?

•Low x – Gluon production from saturated initial state

•Energy density – ~ 50 GeV/fm3 (?)•Rate – “copious” jet production above 100 GeV•Jets – Full jet reconstruction •Detector – necessary detector “for free”!

Heavy Ion Initial Conditions: Modern

•At LHC we (think we) will be able to study “classical” gluon fields in nuclei–And their quantum evolution

A+A Multiplicity vs Energy

•LHC measurements will provide an essential test of whether we understand the mechanism responsible for bulk particle production.–e.g. does saturation correctly extrapolate?

RHIC200 GeV

Saturation?

Something else?

Saturation: Geometric Scaling to A-A?

•Extension of GBW analysis to NMC nuclear targets

•Using kT factorization calculate mult. (parton-hadron duality)

•Compare to PHOBOS data

Armesto, Salgado, Wiedemann Phys. Rev. Lett. 94 :022002,2005

Why should it work here?

Elliptic Flow @ LHC

• LHC data will provide an essential test of our understanding of elliptic flow data @ RHIC–And test whether QGP is still strongly coupled–Extremely high priority given the possible relevance of AdS/CFT.

• Large ATLAS acceptance a big advantage

??

Can change horizontal scale by x2 @ LHC

Why Jets @ LHC? Rate @ High pT

•Can access jet energies in excess of 100 GeV•Complete jet measurements greater precision in use of jet tomography as a probe

80 GeV Jet in Pb+Pb

Jets as Color Antennas

•Studies of modified jets in heavy ion collisions may shed light on a “fundamental” problem in (particle) physcs

•A high-energy quark/gluon acts like a “color antenna”

•In vacuum, radiation strongly affected by quantum interference.

•But, in medium thermal gluons “regulate” radiation.

LHC Physics Opportunity•Create & study quark-gluon plasma at T = 0.8~GeV

•Study particle production from strong gluon fields.

•New program with w/ new discoveries ~ guaranteed– If RHIC is any guide …

•pT reach, rates, detector capabilities at LHC allow for qualitatively different (better!) measurements.

•Overlap w/ many other sub-fields of physicsParticle physics Plasma physicsFluid/hydro dynamicsThermal field theory, lattice & non-latticeString theory (!?) – AdS-CFT correspondenceGeneral relativity (gluon production as Unruh radiation?)

PHENIX: Cu-Cu 0 RAAR

AA

PHENIX p-p Prompt Production

•Absolute comparison, no fudge factors.•pQCD very well reproduces prompt cross-section.

Points: PHENIXCurve: PQCD

“Centrality” Dependence of Suppression

•Measure yield above 4.5 GeV/c.

•Find suppression relative to p-p.

•Plot vs nuclear overlap.– Npart = number of

“participating” nucleons

•Test against “best” energy loss model– GLV = Gyulassy,

Levai, Vitev

•Good agreement

PHENIX: High-pT 0 v2 (Reaction Plane)

•Clear observation of decreasing v2 @ high pT

From parallel session talk by D. Winter

V2(pT): Energy Loss Calculationsv

2PQM: Dainese, Loizides, Paic, Eur. Phys. J. C38:461 2005

PHENIX: Reaction Plane Angle Dependence(2)

•For PHENIX reaction plane resolution & chosen bin sizes, trig bin 4 has smallest flow effects.

•Even without subtracting flow contribution, a dip is seen for central collisions.

Look in bin #4

PHENIX Preliminary

PHENIX: Reaction Plane Angle Dependence

•Study (di)jet correlations vs angle of trigger hadron relative to reaction plane– J. Bielcikova et al,

Phys. Rev. C69:021901, 2004

trig = trig - –6 bins from 0 to /2.

•Flow systematics change completely vs trig

•Can study dependence of distortion on geometry.Shoulder and dip seen in all trig bins.

trig

?

From Poster by J. Jia

Jet Conversion Photons

•There is a new source of “hard” photons in QGP– High pT quarks/gluons convert into photons in

medium

•This extra contribution must be present – @ large enough t, incident jet sees unscreened partons

•What about at low-t ?– In principle, pole in the t channel produces “large”

•But medium screens @ low-t & regulates pole.– Jet-conversion rate sensitive to screening mass.– And potentially also to quark/gluon thermal masses.

Jet Quenching: Photon Bremstrahlung

•For light quarks (and gluons??), in-medium energy loss dominated by radiation.– Interference between vacuum & induced radiation.

– For large parton pT (> ~10 GeV/c) coherence crucial.

•Unfortunately, we can’t measure the gluons.•But we could measure photon bremstrahlung!Direct measurement of medium properties.

Put it all together …

•Extremely rich mixture of physics contributing to the photon spectrum in ~ 4-10 GeV/c range.

•How to unravel all of the different pieces?

Shamelessly stolen from Simon’s talk.

PHENIX

Measuring the Initial ??Bjorken Hydrodynamics:

•For 0 = 0.2 fm (kt ~ 1 GeV)

– Bj = 20 GeV/fm3 !!!

– dNg/dA ~ 6/fm2

•But, estimate too model dependent.

•Need experimental probe of initial state …

~ 150 fm2 Formation time

dy

dE

AT

Bj0

11

dydz 0ycosh

Quark-Gluon ThermodynamicsLattice QCD•Sudden change in # DOF in strongly interacting matter

•Critical temperature (Karsch et al)

– Tc = 150 – 170 MeV

•Energy density: = 0.3 – 1.3 GeV / fm3

– Large uncertainty due to T4 dependence.

–But accessible in heavy ion collisions

Karsch, hep-lat/0106019

T/Tc

/T4

Quark-Gluon Plasma

Hard high-Q2 processes are abundant at collider energy

• Production of hard partons is a standard candle, unaffected by medium

1 Q rmedium,tmedium Q T• Hard partons interact with medium during propagation