The Glue that binds us all

Phases of Matter Town Hall meeting, Jan. 12th, 2007

Probing the nature of gluonic matter with EIC: the world’s first eA collider

Raju Venugopalan

Brookhaven National Laboratory

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Talk Outline:

Outstanding questions in QCD at high energies

Lessons and open questions from HERA and RHIC

How these are addressed by measurements with EIC

The discovery potential of eA at EIC

Summary

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QCD explains ~ 99% of the mass of the visible universe

Quenched QCD (no dynamical quark-antiquark pairs)explains hadron mass spectrum to 10%

hep-lat/0304004

Hadron mass spectrum vs quenched lattice results Quenched QCD full QCD

The dynamics of glue is central to our understanding of the structure of matter

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The DIS Paradigm

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θμμ Measure of

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Measure of inelasticity

Measure of momentum fraction of struck quark

quark+anti-quarkmom. dists.

gluon mom. dists

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Where is the glue ?

The proton is dominated for x < 0.01 by glue- which grows rapidly…

What happens when the density of gluons becomes large ?

# of partonsper unit rapidity

momentum fraction of hadron

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Mechanism of gluon saturation in QCD

p, A

Large x - bremsstrahlunglinear evolution (DGLAP/BFKL)

Small x -gluon recombinationnon-linear evolution(BK/JIMWLK)

Saturation scale QS(x) - dynamical scale below which non-linear QCD dynamics is dominant

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CGC: Classical effective theory of QCD describingdynamics of gluon fields in non-linear regime

o Novel renormalization group equations (JIMWLK/BK) describe how the QCD dynamics changes with energy

o A universal saturation scale QS arises naturally in the theory

The Color Glass Condensate

In the saturation regime: Strongest fields in nature!

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Saturation scale grows with energy

Typical gluon momenta are large

Bulk of high energy cross-sections:a) obey dynamics of novel non-linear QCD regimeb) Can be computed systematically in weak coupling

Typical gluon kT in hadron/nuclear wave function

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Saturation scale grows with A

High energy compact (1/Q < Rp) probes interact coherently across nuclear size 2 RA - experience large field strengths

Enhancement of QS with A => non-linear QCD regime reached at significantly lower energy in A than in proton

Pocket formula:

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New window on universal properties of the matter in nuclear wavefunctions

ACan we quantify the various regimes ?

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Evidence of non-linear saturation regime at HERA ?“Linear” pQCD describes inclusive observables well-however hints of non-linear (“higher twist”) at small x and Q2

# partons per unit rapidity

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Kowalski et al.,hep-ph/0606272

Also see Forshaw et al.hep-ph/0608161

Saturation Models-excellent fits to HERA data

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Typical sat. scale is rather low...QS

2 << 1 GeV2

Caveat: Saturation scale extracted from HERA data inconsistent with model assumptions

Model assumes

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Evidence of non-linear saturation regime at RHIC ?

Global multiplicity observables in AA described in CGC models:

Kharzeev,Levin,NardiKrasnitz, RV

Au-Au mult. at eta=0

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DA:

Kharzeev,Kovchegov,TuchinAlbacete,Armesto,Salgado,Kovner,Wiedemann

D-Au pt spectra compared toCGC predictionHayashigaki, Dumitru, Jalilian-Marian

Talk by M. Leitch

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A

Estimates of the saturation scale from RHIC

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Outstanding questions in high energy QCD(QCD Theory Workshop, DC, Dec. 15th-16th, 2006)

What is the nature of glue at high density ? How do strong fields appear in hadronic or nuclear wavefunctions at high energies ?

How do they respond to external probes or scattering ?

What are the appropriate degrees of freedom ?

Is this response universal ? (ep,pp,eA, pA, AA)

An Electron Ion Collider (EIC) can provide definitive answers to these questions.

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The Electron Ion Collider

Quantitative QCD studies in largely “terra incognita” small x-large Q2 regime

Variable ep c.m energy up to 100 GeV and high luminosity (~100 times HERA) unpolarized e-p scattering

pol. e-pol. p - highest energies and collider mode for the first time (parallel Town Hall discussion & tomorrow)

First eA collider with wide range of nuclear beams and c.m. energy up to 63 (90) GeV/ nucleon Precision studies of QCD in nuclear media & very high parton densities

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What are the measurements with EIC ?See Thomas Ullrich’s talk

Momentum distributions of gluons and quarks in nuclei

Space-time distributions of quarks and gluons in nuclei Extract space-time dist. of nuclear glue from exclusive final states

Interaction of fast probes with nuclear media

First semi-inclusive measurements: charm and bottom dists. & energy loss in nuclei

Role of color neutral (Pomeron) excitations in scattering off nuclei Semi-hard (M ~ QS

A ) diffractive final states predicted to be > 30 % of cross-section

Gluon dists. measured for x < 0.01 in nuclei for first time

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Strong color fields are vastly more accessible in eA at EIC relative to ep at HERA

Nuclear profile more uniform-study centrality dependence

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The nuclear oomph factor…

Saturation scale significantly enhanced in nuclei

~ 6 enhancement in central Aurelative to min-bias proton

Matchespocket formulato 10%

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EIC can cleanly access cross-over region from weak field to novel strong field QCD dynamics

Weak fieldregime

Q2 >> QS2

Strong fieldregime

Q2 << QS2

Qualitative change in final states: eg.,1/Q6 1/Q2 change in elastic vector meson production!McDermott,Guzey,Frankfurt,Strikman

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p/D-A and AA are complementary probes to eA

Universality:

Soft color exchange between protonand nucleus breaks factorization atorder 1 / Q4

RHIC DA and LHC AA/pA -significant discovery potential Universality => genuine discovery will require complementary probes

Qiu,Sterman

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Summary

EIC with variable energies, nuclear beams and high luminosity is a powerful tool to access and study universal properties of QCD at high parton densities

These studies have profound ramifications for our understanding of QCD dynamics at the LHC-especially in heavy ion collisions

The ability of EIC to distinguish between model predictions for measurements is discussed in thefollowing talk by Thomas Ullrich.

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EXTRA SLIDES

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Inclusive measurements

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θμμ Measure of

resolution power

Measure of inelasticity

Measure of momentum fraction of struck quark

quark+anti-quarkmom. dists.

gluon mom. dists

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Diffractive measurements

Color singlet multi-glue(Pomeron ) exchange

Very sensitive to glue mom. dists.

Extract spatial (impact parameter) dists. of gluon fieldsDeg. of freedom: classsical fields, Pomeron interactions?

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DIS highlights

Bjorken scaling: the parton model.

Scaling violations: QCD- asymptotic freedom, renormalization group; precision tests of pQCD.

Rapid growth of gluon density at small x, significant hard diffraction.

Measurement of polarized structure functions: the “spin crisis”.

QCD in nuclei: EMC effect, shadowing, color transparency,…

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II: Extracting gluon distributions in pA relative to eA

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Direct photons

Open charm

Drell-YanAs many channels…but more convolutions, kinematic constraints-limit precision and range.

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Dramatic breakdown of factorization between ep and pp for diffractive final states

At the Tevatron:

Predictions obtained with HERA diffractive pdfs overestimate CDF

data by a factor of about 10

Alvero,Collins,Terron,Whitmore

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A dependence of saturation scale - estimates from fits to HERA and NMC data

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A dependence

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In pQCD, survival probability ~ 1

Dipole Survival Probability

Data from

Space-time dist. of strong Color Fields!

A 0.3 fm qq dipole survives only 20% of the time scattering off center of the proton!

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Dominant impact parameters in DIS scattering off a proton

b (GeV-1)

Strong color fields are localized here

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Hubble

Hubble is taking beautiful pictures of dark matter binding Galaxies…

Can EIC obtain similar pictures of glue bindingvisible matter ?