Quarks and Gluons in the Nuclear Medium – Opportunities at JLab@12

GeV and an EICRolf Ent, ECT-Trento, June 06, 2008

Nuclear Medium Effects on the Quark and Gluon Structure of Hadrons

Main Workshop Topics

Nuclear effects in polarized and unpolarized deep inelastic scattering Nuclear generalized parton distributions

Hard exclusive and semi-inclusive processes

Nuclear hadronization

Color transparency

Future facilities and experiments

The Quark Structure of Nuclei

The QCD Lagrangian and

Nuclear “Medium

Modifications”

Leinweber, Signal et al.

The QCD vacuum

Long-distance gluonic fluctuations

Lattice calculation demonstrates reduction of chiral condensate of QCD vacuum in presence of hadronic matter

qq

Does the quark structure of a nucleon get modified by the suppressed QCD vacuum fluctuations in a nucleus?

Quarks in a Nucleus

Effect well measured,over large range of x and A, but remains poorly understood

1) ln(A) or dependent?

Observation that structure functions are altered in nuclei stunned much of the HEP community ~25 years

ago

2) valence quark effect only?

A=3 EMC Effect at 12 GeV

E772

Is the EMC effect a valence quark phenomenon or are sea quarks involved?

Anti-Quarks in a Nucleus

Solution: Detect a final state hadron in addition to scattered electron

Deep inelastic electron scattering probes only the sum of quarks and anti-quarks requires assumptions on the role of sea quarks

0.5

1.0

gluonssea

valence

0.1 1.0

S. Kumano, “Nuclear Modification of Structure Functions in Lepton Scattering,” hep-ph/0307105

x

RCa

Can ‘tag’ the flavor of the struck quark by measuring the hadrons produced: ‘flavor tagging’

Tremendous opportunity for experimental improvements!

p

A

gg

1

1

2

2

A

D

F

F

g1(A) – “Polarized EMC Effect”• New calculations indicate larger effect for polarized structure function

than for unpolarized: scalar field modifies lower components of Dirac wave function

• Spin-dependent parton distribution functions for nuclei nearly unknown

• Can take advantage of modern technology for polarized solid targets to perform systematic studies – Dynamic Nuclear Polarization

Valence only

Valence + Sea

Miller, SmithValence only calculations consistent with Cloet, Bentz, Thomas calculations

Same model shows small effects due to sea quarks for the unpolarized case (consistent with data)

Large enhancement for x>0.3 due to sea quarks

Sea is not much modified

Chiral Quark-Soliton model(quarks in nucleons (soliton) exchange infinite

pairs of pions, vector mesons with nuclear medium)

• New calculations indicate larger effect for polarized structure function than for unpolarized: scalar field modifies lower components of Dirac wave function

• Spin-dependent parton distribution functions for nuclei nearly unknown

• Can take advantage of modern technology for polarized solid targets to perform systematic studies – Dynamic Nuclear Polarization

p

A

gLig

1

71 (polarized EMC effect)

Curve follows calculation by W. Bentz, I. Cloet, A. W. Thomas.

g1(A) – “Polarized EMC Effect”

Extend measurements on nucleito x > 1: Superfast quarks

Correlated nucleon pair

Six-quark bag (4.5% of wave function)

Fe(e,e’)5 PAC days

Mean field

Does the quark structure of a nucleon get modified by the suppressed QCD vacuum fluctuations in a nucleus?

1) Measure the EMC effect on the mirror nuclei 3H and 3He

2) Is the EMC effect a valence quark only effect?3) Is the spin-dependent EMC effect larger?4) Can we reconstruct the EMC effect on 3He and 4He

from all measured reaction channels?5) Is there any signature for 6-quark clusters?6) Can we map the effect vs. transverse

momentum/size?

Reminder: EMC effect is effect that quark momenta in nuclei are altered

Now: use the nuclear arena to look for QCD

Use the Nuclear Arena to Study QCD

Total Hadron-Nucleus Cross Sections

Hadron– Nucleustotal cross section

Fit to

K

pp_

Hadron momentum60, 200, 250 GeV/c

< 1 interpreted as due to the strongly interacting nature of the probe A. S. Carroll et al. Phys. Lett 80B 319

(1979)

= 0.72 – 0.78, for p, , k

Traditional nuclear physics expectation: transparency nearly energy independent.

T

1.0

Energy (GeV)

Ingredients

• h-N cross-section

• Glauber multiple scattering approximation(or better transport calculation!)

• Correlations & Final-State Interaction effects

hN

Physics of Nuclei: Color Transparency

From fundamental considerations (quantum mechanics, relativity, nature of the strong interaction) it is predicted (Brodsky, Mueller) that fast protons scattered from the nucleus will have decreased final state interactions

Quantum ChromoDynamics:

A(e,e’h), h = hadron

Search for Color Transparency in Quasi-free

A(e,e’p) Scattering

Constant value line fits give good description:2/df = 1

Conventional Nuclear Physics Calculation by Pandharipande et al. (dashed) also gives good description

Fit to = Aa

= constant = 0.75

Close to proton-nucleus total cross section data No sign of CT yet

Physics of Nuclei: Color Transparency

AGSA(p,2p)

Glauber calculation

Pp (GeV/c)5.1 7.3 9.62.9

Results inconsistent with CT only. But can be explained by including additional mechanisms such as nuclear filtering or charm resonance states.

The A(e,e’p) measurements will

extend up to ~10 GeV/c proton momentum,

beyond the peak of the rise in transparency found

in the BNL A(p,2p) experiments.

6 7 8 9 10

Physics of Nuclei: Color Transparency

Total pion-nucleus cross section slowly disappears, or … pion escape probability increases Color Transparency Unique possibility to map out at 12 GeV (up to Q2 = 10)

Total pion-nucleus cross section slowly disappears, or … pion escape probability increases Color Transparency?

A(e,e’+)

Physics of Nuclei: Color Transparency

A(e,e’+) at 12 GeV(at fixed coherence length)

12 GeV

Using the nuclear arenaHow long can an energetic quark remain

deconfined?How long does it take a confined quark to form a hadron?

Formation time tfh

Production time tp

Quark is deconfined

Hadron is formed

Hadron attenuation

CLAS

Time required to produce colorless “pre-hadron”, signaled by medium-stimulated energy loss via gluon emission

Time required to produce fully-developed hadron, signaled by CT and/or usual hadronic interactions

Using the nuclear arena

Le

e’

*

+

pT

pT2 = pT

2(A) – pT2(2H)

“pT Broadening”

dE/dx ~ <pT2>L

E ~ L (QED) ~ L2 (QCD)?

How long can an energetic quark remain deconfined?How long does it take a confined quark to form a hadron?

Or How do energetic quarks transform into hadrons? How quickly does it happen? What are the mechanisms?

How long can an energetic quark remain deconfined?How long does it take a confined quark to form a hadron?

Or How do energetic quarks transform into hadrons? How quickly does it happen? What are the mechanisms?

Deep Inelastic ScatteringRelativistic Heavy-Ion Collisions

Initial quark energy is knownProperties of medium are known

e e’

Using the nuclear arena

Relevance to RHIC and LHC

ppTT22 vs. vs. for Carbon, Iron, and Lead for Carbon, Iron, and Lead

C

Pb

Fe

pp

TT22 (

GeV

(G

eV

22))

(GeV)(GeV)

~ 100 MeV/fm (perturbative formula)

~d

E/d

x

Preliminary CLASHall B

Production length from JLab/CLAS 5 GeV data (Kopeliovich, Nemchik, Schmidt, hep-ph/0608044)

What we have learned• Quark energy loss can be estimated

• Data appear to support the novel E ~L2 ‘LPM’ behavior• ~100 MeV/fm for Pb at few GeV, perturbative formula

• Deconfined quark lifetime can be estimated, ~ 5 fm @ few GeV

Outstanding questions• Higher energy data to confirm “plateau” for heavy (large-A) nuclei • Much more theoretical work needed to provide a quantitative basis for jet quenching at RHIC/LHC?

Using the nuclear arenapT

2 reaches a “plateau” for sufficiently large quark energy, for each nucleus (L is fixed). pT

2

Projected Data

DOE Project Critical Decisions

• CD-0 Approve Mission Need

• CD-1 Approve Alternative Selection and Cost Range• Permission to develop a Conceptual Design Report• Defines a range of cost, scope, and schedule options

• CD-2 Approve Performance Baseline• Fixes “baseline” for scope, cost, and schedule• Now develop design to 100%• Begin monthly Earned Value progress reporting to DOE• Permission for DOE-NP to request construction funds

• CD-3 Approve Start of Construction• DOE CD3 (IPR/Lehman) review scheduled for July 22-24

• DOE Office of Science CD-3 Approval meeting in late Sept 2008

• CD-4 Approve Start of Operations or Project Close-out

DOE CRITICAL DECISION SCHEDULE

CD-0 Mission Need MAR-2004 (A)

CD-1 Preliminary Baseline Range FEB-2006 (A)

CD-2 Performance Baseline NOV-2007 (A)

CD-3 Start of Construction SEP-2008

CD-4A Accelerator Project Completion and Start of Operations

DEC-2014

CD-4B Experimental Equipment Project Completion and Start of Operations

JUN-2015

(A) = Actual Approval Date

Note → 6 to 18 months schedule float included

Note → 6 to 18 months schedule float included

Now split in two to ease transition into operations phase

2004-2005 Conceptual Design (CDR) - finished

2004-2008 Research and Development (R&D) - ongoing

2006 Advanced Conceptual Design (ACD) - finished

2006-2009 Project Engineering & Design (PED) - ongoing

2009-2014 Construction – starts in ~1/2 year!Parasitic machine shutdown May 2011 through Oct.

2011

Accelerator shutdown start mid-May 2012

Accelerator commissioning start mid-May 2013

2013-2015 Pre-Ops (beam commissioning)

Hall A commissioning start October 2013

Hall D commissioning start April 2014

Halls B and C commissioning start October 2014

12 GeV Upgrade: Phases and Schedule

(based on funding guidance provided by DOE-NP in June-2007)

The Gluon Structure of Nuclei

Gluons dominate QCD• QCD is the fundamental theory that describes structure and

interactions in nuclear matter.• Without gluons there are no protons, no neutrons, and no

atomic nuclei• Facts:

– The essential features of QCD (e.g. asymptotic freedom, chiral symmetry breaking, and color confinement) are all driven by the gluons!

– Unique aspect of QCD is the self interaction of the gluons– 98% of mass of the visible universe arises from glue– Half of the nucleon momentum is carried by gluons

• However, gluons are dark: they do not interact directly with light

high-energy collider!

29

The Low Energy View of Nuclear Matter• nucleus = protons + neutrons• nucleon quark model • quark model QCD

The High Energy View of Nuclear MatterThe visible Universe is generated by quarks, but dominated by the dark glue!

Removefactor 20

Exposing the high-energy (dark) side of the nuclei

EIC science has evolved from new insights and technical

accomplishments over the last decade

• ~1996 development of GPDs• ~1999 high-power energy recovery linac technology • ~2000 universal properties of strongly interacting

glue • ~2000 emergence of transverse-spin phenomenon• ~2001 world’s first high energy polarized proton

collider• ~2003 RHIC sees tantalizing hints of saturation• ~2006 electron cooling for high-energy beams

NSAC 2007 Long Range Plan “An Electron-Ion Collider (EIC)

with polarized beams has been embraced by the U.S. nuclear science community as embodying the vision for reaching the next QCD frontier. EIC would provide unique capabilities for the study of QCD well beyond those available at existing facilities worldwide and complementary to those planned for the next generation of accelerators in Europe and Asia. In support of this new direction:

We recommend the allocation of resources to develop accelerator and detector technology necessary to lay the foundation for a polarized Electron Ion Collider. The EIC would explore the new QCD frontier of strong color fields in nuclei and precisely image the gluons in the proton.”

Explore the new QCD frontier:strong color fields in

nuclei  

- How do the gluons contribute to the structure of the nucleus?

- What are the properties of high density gluon matter?

- How do fast quarks or gluons interact as they traverse nuclear matter? Precisely image the sea-quarks

and gluons in the nucleon

- How do the gluons and sea-quarks contribute to the spin structure of the nucleon?

- What is the spatial distribution of the gluons and sea quarks in the nucleon?

- How do hadronic final-states form in QCD?

How do we understand the visible matter in our universe in terms of the fundamental quarks and gluons of QCD?

Explore the structure of the nucleon • Parton distribution

functions• Longitudinal and transverse spin distribution functions• Generalized parton distributions•Transverse momentum distributions

RHIC-Spin region

Precisely image the sea quarksSpin-Flavor Decomposition of the Light Quark Sea

| p = + + + …>u

u

d

u

u

u

u

d

u

u

dd

dMany

models predict

u > 0, d < 0No competition foreseen!

GPDs and Transverse Gluon ImagingDeep exclusive measurements in ep/eA with an EIC:

diffractive: transverse gluon imaging J/, o, (DVCS) non-diffractive: quark spin/flavor structure , K, +, …

[ or J/, , 0

, K, +, … ]

Describe correlation of longitudinal momentum and transverse position of

quarks/gluons

Transverse quark/gluon imaging of nucleon

(“tomography”)

Are gluons uniformly distributed in nuclear matter or are there small clumps of glue?

GPDs and Transverse Gluon Imaging

gives transverse size of quark (parton) with longitud. momentum fraction x

EIC:1) x < 0.1: gluons!

x < 0.1 x ~ 0.3 x ~ 0.8

Fourier transform in momentum transfer

x ~ 0.001

2) ~ 0 the “take out” and “put back” gluons act coherently.

2) ~ 0 x - x +

d

GPDs and Transverse Gluon ImagingGoal: Transverse gluon imaging of nucleon over wide range of x: 0.001 < x < 0.1Requires: - Q2 ~ 10-20 GeV2 to facilitate interpretation

- Wide Q2, W2 (x) range - Sufficient luminosity to do differential measurements in Q2, W2, t

Q2 = 10 GeV2 projected data

Simultaneous data at other Q2-values

EIC enables gluon imaging!

Scaled from 2 to 16 wks.

EIC(16 weeks)

38

eA Landscape and a New Electron Ion Collider

Well mapped in e+pNot so for ℓ+A (A)

Electron Ion Collider (EIC):L(EIC) > 100 L(HERA)

eRHIC (e+Au):Ee = 10 (20) GeVEA = 100 GeVseN = 63 (90) GeVLeAu (peak)/n ~ 2.9·1033 cm-2 s-1

ELIC (e+Au):Ee = 9 GeVEA = 90 GeVseN = 57 GeVLeAu (peak)/n ~ 1.6·1035 cm-2 s-1

Terra incognita: small-x, Q Qs

high-x, large Q2

F2 : Sea (Anti)Quarks Generated by Glue at Low x

F2 will be one of the first measurements at EIC

nDS, EKS, FGS:pQCD based models with different amounts of shadowing

Syst. studies of F2(A,x,Q2): G(x,Q2) with precision distinguish between models

),(2

),(2

14 2

22

2

2

4

2

2

2

QxFy

QxFy

yxQdxdQ

dL

eXep

Longitudinal Structure Function FL

• Experimentally can be determined directly IF VARIABLE ENERGIES!• Highly sensitive to effects of gluon

FL at EIC: Measuring the Glue Directly

),(2

),(2

14 2

22

2

2

4

2

2

2

QxFy

QxFy

yxQdxdQ

dL

eXep

Explore gluon-dominated matter

At high gluon density, gluon recombination should compete with gluon splitting density saturation.

What is the role of gluons and gluon self-interactions in nucleons and nuclei? NSAC-2007 Long-Range Plan Report.– The nucleus as a “gluon amplifier”

Color glass condensate

Oomph factor stands up under scrutiny.Nuclei greatly extend x reach:xEIC = xHERA/18 for 10+100 GeV, Au

Longitudinal Structure Function FL

Diffractive Surprises‘Standard DIS event’

Detector activity in proton direction

7 TeV equivalent electron bombarding the proton … but proton remains intact in 15% of cases …

Diffractive event

No activity in proton direction

Predictions for eA for such hard diffractive evens range up to: ~30-40%... given saturation models

Look inside the “Pomeron” Diffractive structure functions Diffractive vector meson production ~ [G(x,Q2)]2

Explore the transition from partons to hadrons

• What governs the transition of quarks and gluons in pions and nucleons? NSAC-2007– Fragmentation and parton energy loss– The nucleus as a “femto-meter stick”

Nuclear SIDIS: Suppression of high-pT hadrons analogous but weaker than at RHIC Clean measurement in ‘cold’ nuclear matter

Energy transfer in lab rest frameEIC: 10 < < 2000 GeV

(HERMES: 2-25 GeV)EIC: can measure heavy flavor energy loss

Using the nuclear arena

pT2 reaches a “plateau” for sufficiently large quark

energy, for each nucleus (L is fixed).

pT2

In the pQCD region, the effect is predicted to disappear (arbitrarily put at =1000)

Quarks and Gluons in the Nuclear Medium – Opportunities at JLab@12

GeV and an EICRolf Ent, ECT-Trento, June 06, 2008

JLab 12 GeV Upgrade: The 12 GeV Upgrade, with its 1038+ luminosity, is expected to allow for a complete spin and flavor dependence of the valence quark region, both in nucleons and in nuclei.

Electron Ion Collider (eRHIC/ELIC)Provide a complete spin and flavor dependence of the nucleon and nuclear sea, study the explicit role that gluons play in the nucleon spin and in nuclei, open the new research territory of “gluon GPDs”, and study the onset of the physics of saturation.

Personal View:

Longitudinal Structure Function FL

• Experimentally can be determined directly IF VARIABLE ENERGIES!• Highly sensitive to effects of gluon

+ 12-GeV data+ EIC alone

FL at EIC: Measuring the Glue Directly

),(2

),(2

14 2

22

2

2

4

2

2

2

QxFy

QxFy

yxQdxdQ

dL

eXep

eRHIC

Gluons in the Nucleus

Note: not all models carefully checked against existing data + some models include saturation physics

GPDs and Transverse Gluon Imaging

k

k'

*q q'

p p'

e

A Major new direction in Nuclear Science aimed at the 3-D mapping of the quark structure of the nucleon.

Simplest process:Deep-Virtual Compton Scattering

Simultaneous measurements over large range in x, Q2, t at EIC!

At small x (large W): ~ G(x,Q2)2