Universal features of QCD dynamics in hadrons & nuclei at high energies

Raju Venugopalan

DNP (APS/JPS) meeting, Hawaii, October 13, 2009

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QCD is the “nearly perfect” fundamental theory of the strong interactions (F.Wilczek, hep-ph/9907340)

We are only beginning to explore the high energy, many body dynamics of this theory

What are the right effective degrees of freedom at high energies?

-- gluons & sea quarks, dipoles, pomerons, strong fields?

How do these degrees of freedom interact with each other and with hard probes? -- Multi-Pomeron interactions, Higher Twist effects, Saturation/CGC -- Rapidity Gaps, Energy loss, Multiple Scattering, Color Transparency -- Glasma, Quark Gluon Plasma

What can this teach us about confinement, universal features of the theory (infrared fixed point?) -- hard vs soft Pomerons, Strong Fields, in-medium hadronization

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High Energy QCD

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High Energy QCD-the role of Glue

Self-interacting carriers of the strong force

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are needed to see this picture. Localized energy fluctuation of Gluon Field(D. Leinweber)

Dominate structure of QCD vacuum

MILC Coll.:hep-lat/0304004

Hadron mass spectrum vs quenched lattice results

Quenched QCD full QCD

Nearly all visible matter in the universe is made of Glue

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Measuring Glue: what are our options?

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Gluon jet event in e+e- collisions at LEP

p+A and e+A provide complementary information on role of glue

Need both to test what’s universal and what’s not in QCD processes

Some final states differ dramatically (diffractive/exclusive states)

Nothing matches DIS for precision-vast majority of world data for pdfs (especially for glue) from DIS

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Inclusive DIS

sy

Q

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pk

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⎟⎠⎞

⎜⎝⎛ ′′

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θ

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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HERA data on inclusive DIS

x= fraction of momentum of hadron carried by parton

PartonDensity

Gluon distribution from scaling violations of F2

Proton is almost entirely glue by x=0.01 for Q2 = 10 GeV2

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HERA data on inclusive DIS

# partons per unit rapidity

For Q2 ≤ 5 GeV2, leading twist (“parton gas”) description problematic. Sign of higher twist (multi-parton correlation) effects?

Recent HERA data on FL (H1: Q2 = 12-90 GeV2 ; ZEUS Q2 = 24-110 GeV2)

EIC can add significantly to world FL data set -- even for protons. Important test of QCD evolution

Golec-Biernat, Stasto, arXiv: 0905.1321

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increasing Q2

But… the phase space density decreases-the proton becomes more dilute

Resolving the hadron in the Bjorken limit of QCD -DGLAP evolution

Important for precision/beyond standard model physics

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- Large x

- Small x

IMF picture:Gluon phase space grows- saturates at occupation # f =

Resolving the hadron in the Regge-Gribov limit of QCD-BFKL evolution

Rest frame picture:Scattering amplitude is unity…

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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 (“higher twist”) QCD dynamics is dominant

Gribov,Levin,RyskinMueller,Qiu

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CGC: Classical effective theory of QCD describingdynamical gluon fields + static color sources 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!

McLerran, RVJalilian-Marian,Kovner,WeigertIancu, Leonidov,McLerran

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Saturation scale grows with energy

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

unintegratedgluon dist.from NLL RG evolution

QS(x)

Y=ln(x0/x)0,1,3,9

Exact analogy to physics of “pulled” travelling wave fronts in stat. mech.

Munier,Peschanski

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Evidence from HERA for geometrical scalingGolec-Biernat, Stasto,Kwiecinski

F2 F2D VM, DVCS = Q2 / QS

2 D V

Marquet, Schoeffel hep-ph/0606079

Scaling confirmed by “Quality factor” analysisGelis et al., hep-ph/0610435

Scaling seen for F2D and VM,DVCS for same QS as F2

Recent NLO BK analysis: Albacete, Kovchegov, hep-ph-0704.0612

Recent caveats: Avsar, Gustafson, hep-ph/0702087

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Inclusive DIS in saturation models

q

q P

* z

1-zr

= 0.3; x0 = 3* 10-4

Kowalski, Teaney Machado, hep-ph/0512264

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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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Inclusive diffraction

RapidityGap

Big surprise at HERA:~ 15% of all events are hard diffractive (MX > 3 GeV) events

In rest frame: 50 TeV electron hits proton - in 1/7 events proton remains intact

“Pomeron”

MX

Diffractive structure functions in DIS measure quark and gluon content of Pomeron

Collins

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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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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

Kowalski, Lappi, RV

c ~ 1 in saturation modelfit to HERA extrapolated tonuclei

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Evidence of geometrical scaling in nuclear DISFreund et al., hep-ph/0210139

Nuclear shadowing: Geometrical scaling

Data scale as a function of = Q2 / QS2

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Evidence of non-linear saturation regime @ RHIC ?

Global multiplicity observables in AA described in CGC models:Input is QS(x,A)

Kharzeev,Levin,Nardi600 1200

Krasnitz, RV

PHOBOS central Au+Au mult. vs models

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DA:

Kharzeev,Kovchegov,TuchinAlbacete,Armesto,Salgado,Kovner,WiedemannBlaizot, Gelis, RV

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

Review: Jalilian-Marian, Kovchegov, hep-ph/0505052

Forward pp @ RHIC as wellBoer, Dumitru, PRD 74, 074018 (2006)

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Extrapolation of BK-fit to RHIC LF data to LHCdn/dy|_{y=0} = 1500-2250 in A+A at LHC

Gelis,Stasto, RV, hep-ph/0605087

Natural explanation for limiting fragmentation + deviations in CGC

Jalilian-Marian

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Why Physics at an e+A collider is interesting

Not your grand aunt’s e+A: world’s first such collider, first measurementsof a range of final states… eg. rapidity gaps, jets, impactparameter dependent distributions

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Terra Incognita

Large x and Q2 : Precision study of propagation of colored probes in extended QCD medium. QCD showering and fragmentation in nuclei

Small x: Explore physics of strong, non-linear color fields

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What are the measurements?

Precision inclusive measurements of structure functions

:

: (Inclusive diffraction)

Semi-inclusive measurements of final state distributions

Exclusive final states

Multiple handles: x, Q2, t, MX2 for light and heavy nuclei

Would like answers to: What is the momentum distribution of gluons in matter What is the space-time distribution of gluons in matter How do fast probes interact with the gluonic medium? What is the nature of color neutral exchanges (Pomerons)

Tools/Measurements:

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EIC: 10 GeV + 100 GeV/n - estimate for 10 fb-1

Gluon distribution from FL @ eRHICEskola,Paukkunen,Salgado

Quark and Glue contribution toHadron-hadron inclusiveCross-section

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Inclusive diffraction In pQCD, expect exponential suppression of large gaps -- parametrize data with diffractive structure functions

which obey QCD evolution…not universal

In CGC, diffractive structure functions depend on universal dipole cross-section squared. Diffractive cross-section ~ 25% of total cross-section!

Kowalski,Lappi,Marquet,RV

Interesting pattern of enhancement and suppression-can be tested

Large = small MX

Small = large MX

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Exclusive final states in DIS

Brodsky et al. Frankfurt,Koepf,Strikman

In the dipole model:Kowalski,Motyka,Watt

Extract b dist. of glue In nuclei?

Claim: can extract t dependence in photo-production of J/ down to t = 10-4 GeV2 Caldwell-Kowalski

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Universal gluodynamics & energy dependence of QS

Small x QCD RG eqns. predict (fixed b) QS

approaches universal behavior with increasing energy (Y) for all hadrons and nuclei-can the approach to this behavior be tested ?

A.H. Mueller, hep-ph/0301109

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In eA DIS, 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 productionMcDermott,Guzey,Frankfurt,Strikman; Review: Frankfurt, Strikman, Weiss

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Semi-inclusive DIS

Virtual photon with short coherencelength scatters off quark or gluon (photon-gluon fusion) in medium- jet propagates through medium

Vastly extended reach at EIC relative to HERMES, Jlab, EMC

Precision studies of heavy quark energy loss

Accardi,Dupre,Hafidi,

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What does a heavy ion collision look like ?

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Color Glass

Condensates

Initial

Singularity

Glasma sQGP - perfect fluid

Hadron Gas

t

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What does a heavy ion collision look like ?

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Color Glass

Condensates

Initial

Singularity

Glasma sQGP - perfect fluid

Hadron Gas

t

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Glasma flux tubes from small x dynamics

After: boost invariant Glasma flux tubes of size 1/QS

Krasnitz,Nara, RV; Lappi

Before: transverse E & B“Weizsacker-Williams fields

Lappi,McLerran,NPA 772 (2006)

parallel color E & B fields

Kharzeev, Krasnitz, RV, Phys. Lett. B545 (2002)

generate Chern-Simons topological charge

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Imagining the Glasma through long range rapidity correlations

Causality dictates:

Au+Au 200 GeV, 0 - 30%PHOBOS preliminary

ηΔφΔ dd

Nd

N

1 ch2

trig

< 1 fm for Δy > 4

At LHC can probe color field dynamics for << 1 fm…Very sensitive to gluon correlationsin wave fn.

Theory @ EIC

High EnergyPhysics

(LHC,LHeCCosmic Rays)

Lattice Gauge Theory

CondensedMatter Physics

(B-E Condensates, Spin Glasses Graphene)

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Inclusive Diffraction-IIImpact parameter dipole (CGC) models give -sq. fits ~ 1 to HERA e+p inclusive and diffractive cross-section: H. Kowalski, C. Marquet, T. Lappi and R. Venugopalan,

Phys. Rev. C 78, 045201 (2008).

= parton mom./Pomeron mom. xP= Pomeron mom./ Hadron mom.

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A

Estimates of the saturation scale from RHIC

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The unstable Glasma • Small rapidity dependent quantum fluctuations of the LO Yang-Mills fields grow rapidly as

• E and B fields as large as EL and BL at time

Romatschke, RV:PRL 96 (2006) 062302

Possible mechanism for rapid isotropization Problem: collisions can’t ‘catch up’

Turbulent “thermalization” may lead to “anomalously” low viscosities

Asakawa, Bass, Muller; Dumitru, Nara, Schenke, Strickland

Significant energy loss in Glasma because of synchroton like radiation?

Shuryak, Zahed; Zakharov; Kharzeev

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P and CP violation: Chiral Magnetic Effect

Kharzeev,McLerran,Warringa

L or B

+ External (QED) magnetic field

Chiral magnetic effect

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= STAR Preliminary

Possible experimental signal of charge separation (Voloshin, Quark Matter 2009)

Topological fluctuations-sphaleron transitions in Glasma

NCS = -2 -1 0

1 2

Effect most significant, for transitions at early times

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Semi-inclusive DIS

At small x:

Hadron distributions and multiplicities sensitive to QS(x,A)

Ratio to x = 10-2 proportional to ratio of QS

2(x,A)

Marquet,Xiao,Yuan

Kang,Qiu

Need more quantitative studiesfor EIC kinematics

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Forming a Glasma in the little Bang Glasma (\Glahs-maa\): Noun: non-equilibrium matter between Color Glass Condensate (CGC)& Quark Gluon Plasma (QGP)

Problem: Compute particle production in QCD with strong time dependent sources

Gelis, Lappi, RV; arXiv : 0804.2630, 0807.1306, 0810.4829

Solution: for early times (t 1/QS) -- n-gluon production computed in A+A to all orders in pert. theory to leading log accuracy

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New window on universal properties of the matter in nuclear wavefunctions

A

Can we quantify the various regimes ?

Iancu, RV, hep-ph/0303204

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Strong color fields may be more accessible in eA collisions relative to ep

Nuclear profile more uniform-can study centrality dependence of distributions

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Outline of talk

Whither the “perfect” theory ? - QCD at high energies

QCD coherence at small x => Universality - Saturation in hadrons & nuclei;

the Color Glass Condensate picture

Exploring the structure of high energy nuclei with EIC - entering terra incognita

From Glue to Glasma & QGP - how multi-parton correlations in nuclei generate extreme states of quark-gluon matter