Penetrating Probes of the Quark-Gluon Plasma in

Nuclear Collisions at RHIC and the LHC

Prof. Brian A. Cole.Columbia University

PHENIX and ATLAS

Fundamental InteractionsConsider:

Only “matter” that we can study in the lab has properties determined almost entirely by EM “force”.

Caveat:

nuclei – but the force is not truly fundamental (e.g. no single gauge boson)

RHIC Physics - Background

Size scales•Atom – 10-10 m•Nucleus – 10-14 m•Nucleon – 10-15 m (1 fm)•Quarks, electrons

– Have no “size” per se

Mass Scales (MeV/c2)•Electron – 0.511•Quark (up) ~ 5•Nucleon – 939•Au Nucleus – 1.8 x 105

Physical Constants•ħc 0.2 GeV fm

p Cu Au

Strong Interactions at a GlanceHadrons are composed of quarks

• Quarks carry “color” charge.– Gluons mediate interaction.

• gluons also carry “color”. Gluons couple to gluons

• @ large r gluon field between quarks collapses into “flux tube”“Confining” potential

• What about other combinations? – e.g. penta-quark

– Or, how about chilioi-quark ….qqqqq

QCD Thermodynamics (on Lattice)

•Rapid cross-over from “hadronic matter” to “Quark-Gluon Plasma” at T 170 MeV ( ~ 1 GeV/fm3).

•Energy density / T4 saturates rapidly – Except in real-world case of 2+1 flavor ??

•Pressure / T4 saturates less rapidly.Strongly coupled medium @ RHIC ?

Energy Density / T4 Pressure / T4

“Primordial” Quark Gluon Plasma

The Early Universe, Kolb and Turner

Th

erm

odyn

amic

deg

ener

acy

fact

or

QCD Transitio

n

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

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 QM: E ħc / t –Fluctuations with E > 1 GeV are “on shell”

These are primarily gluons (~ 200)RHIC is a gluon collider! (10 GeV/fm3)

RHIC: Au+Au Collision Simulation

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

Penetrating Probes of Created Matter

z

t

Collisions between partons

Perturbative quantum chromo-dynamics

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

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

•Valid @ large momentum transfer – high pT outgoing particles

p-p di-jet Event

STARSTAR

a/A

b/B

A

B

ab̂

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

pQCD – Single Hadron ProductionAdd fragmentation to hadrons

•D(z) – fractional momentum dist. of particles created by outgoing quark or gluon (i.e. in a jet)

KKP

Kretzer

data vs pQCD

dt

d

z

QzD

QxQxdxdxdp

dE

c

abcaBbaAaba

ˆ),,(

),,(),,(

2

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

a/A

b/B

A

B

ab̂

D(z)

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

Central Arms

PHENIX Experiment @ RHIC

•Two spectrometers measuring at 90°•Two (forward/backward muon) spectrometers•Optimized for rare/penetrating probes.

Drift Chamber

Pad Chambers

RICH

EM Calorimeter

Time of flight

Central Arms In Action

0 The pion is the lightest and most copiously produced meson.

0: dduu

Au-Au 0 Spectra From PHENIX

Calculations with no energy loss

Calculations with energy loss

•Observe only 20% of expected yield @ high pT

Energy density ~15 Gev/fm3

100 x normal nuclear energy density!!

Reminder: critical energy density ~ 1 GeV/fm3

Tra

nsvers

e M

om

en

tum

sp

ectr

um

Expected

RAA Observed/Expected

Using p-p data as baseline

PHENIX: Au-Au High-pT 0

Suppression

constancy for pT > 4 GeV/c for all centralities?

We are now measuring out to truly high pT

0 Suppression: dE/dx Comparisons

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

Crucial Control Measurement: Deuteron-Au

Prompt Photon Production

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

–But, no final-state effects (???)

PHENIX p-p Prompt Production

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

Points: PHENIXCurve: PQCD

Au-Au Prompt Photon Production•Large background to prompt measurement

– Primarily hadron decays (e.g. )

tot

al/

back

grou

nd

Pions are quenched

Photons aren’t

Calculations of hard scattering rates in A+A collisions OK.

High-pT hadron suppression must be due to jet quenching.

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

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 azimuthal anisotropy by “v2” parameter

–

•Compare to “eccentricity”:

•Data consistent w/ hydrodynamic calculations

)2cos(21 2

vd

dN

22

22

xy

xy

Transverse Plane

PHENIX QGP “White Paper”

Already a year old …

“Perfect Fluid?”

My view: Perfect fluid is reasonable interpretation of available data but there is room for skepticism.

Perfect Fluid? (2)What we do know•The matter created in Au+Au collisions must thermalize and generate pressure in time < 1 fm/c.

•“perturbative” quark/gluon scattering cannot achieve such early thermalization.

•Thus, “collective” effects are needed. Possibilities:

– Residual long-range interactions

– Colored bound states (Shuryak)

– “Color instabilities” – e.g. runaway growth of local chromo-magnetic fields

– Thermalization ab initio ? (Hawking-like radiation from deceleration of initial gluons/fields)

(di)Jet Angular Correlations (PHENIX)

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

Di-jet Distortion vs “Impact Parameter”

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”

Inte

nsi

ty

Mach Cone (2)Talk by J. Ruppert at Quark Matter 2005.

Suggestive, but there are other possibilities (Cherenkov, coherent gluon radiation, di-jet quenching, …

Photon Production off the Plasma (?)

qg q

The plasma mediates a jet-photon conversion

Jet-Conversion Photons: Data

•There may already be room for jet-conversion contribution in the PHENIX prompt photon data.

•More detailed studies underway – stay tuned.

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. End of the beginning @ RHIC

RHIC: Penetrating Probes•We are directly observing results of colored probes propagating in colored matter.– Calibrated by photon measurements, d-Au.

– ~ 100x energy density of normal nuclei

– >> energy density for “phase transition”

•Matter exhibits strong collective motion– Due to chromo-magnetic instabilities??

– Due to colored bound states??

•We are studying first “fundamental” matter that interacts non electromagnetically!

•We may be seeing collective response of medium to energetic particles (Mach cone?)

New Opportunity: Pb+Pb @ LHC w/ ATLAS

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”!

Why ATLAS? Calorimetery!

Simulated Pb+Pb Event in ATLAS (No Jet)

Pb+Pb Jet Simulations in ATLAS

Heavy Ion Initial Conditions: Modern

•At very high energies, strong gluon production from gluon fields with large occupation #’s (~ classical)

R. Venugopalan, LHC Heavy Ion Workshop @ PANIC

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 physics– Particle physics – Plasma physics– Fluid/hydro dynamics– Thermal field theory, lattice & non-lattice– String theory (!?) – AdS-CFT correspondence– General relativity ???