Quarks and Gluons in the Nuclear Medium –
Opportunities at JLab@12 GeV and an EIC
Rolf 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”
The QCD
vacuum
Long-distance gluonic fluctuations
Lattice calculation
demonstrates reduction of
chiral condensate q q of
QCD vacuum in presence of
hadronic matter
Leinweber, Signal et al.
Does the quark structure
of a nucleon get modified
by the suppressed QCD
vacuum fluctuations in a
nucleus?
Quarks in a Nucleus
Observation that structure functions are altered in nuclei
stunned much of the HEP community ~25 years ago
A=3 EMC Effect at 12 GeV
Effect well measured,over large
range of x and A, but remains
poorly understood
1) ln(A) or r dependent?
2) valence quark effect only?
Anti-Quarks in a Nucleus
Is the EMC effect a valence
Tremendous opportunity for
quark phenomenon or are sea
experimental improvements!
quarks involved?
RCa
E772
1.0
Deep inelastic electron scattering
gluons
valence
probes only the sum of quarks
and antiquarks  requires
assumptions on the
sea
role of sea quarks
S. Kumano, “Nuclear Modification of Structure
Functions in Lepton Scattering,” hep-ph/0307105
0.5Solution: Detect a final state hadron
x
0.1
in addition
to scattered electron 1.0
 Can ‘tag’ the flavor of the
struck quark by measuring the
hadrons produced: ‘flavor tagging’
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
F2 A
F2 D
g1 A
g1 p
Chiral Quark-Soliton model
(quarks in nucleons (soliton) exchange infinite pairs
of pions, vector mesons with nuclear medium)
Valence
only calculations
consistent with Cloet, Bentz,
Thomas calculations
Miller, Smith
Large enhancement for
x>0.3 due to sea quarks
Valence + Sea
Same
model shows small
effects due to sea quarks
for the unpolarized case
(consistent with data)
Valence only
Sea is not much modified
•
•
•
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
 
g1 A 7 Li
(polarized EMC effect)
g1 p
Curve follows
calculation by
W. Bentz,
I. Cloet,
A. W. Thomas.
Extend measurements on nuclei
to x > 1: Superfast quarks
Fe(e,e’)
5 PAC days
Mean field
Correlated nucleon pair
Six-quark bag
(4.5% of wave function)
Does the quark structure
of a nucleon get modified
by the suppressed QCD
vacuum fluctuations in a
nucleus?
Reminder: EMC effect is effect that quark momenta in nuclei are altered
1)
2)
3)
4)
Measure the EMC effect on the mirror nuclei 3H and 3He
Is the EMC effect a valence quark only effect?
Is the spin-dependent EMC effect larger?
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?
Now: use the nuclear arena to look for QCD
Use the Nuclear Arena
to Study QCD
Total Hadron-Nucleus Cross Sections
K
a
p
p
_
p
Hadron– Nucleus
total cross section
Fit to
Hadron momentum
60, 200, 250 GeV/c
a = 0.72 – 0.78, for p, p, k
a < 1 interpreted as due to the
strongly interacting nature of the
probe
A. S. Carroll et al. Phys. Lett 80B 319 (1979)
Physics of Nuclei: Color Transparency
Traditional nuclear physics expectation:
transparency nearly energy independent.
Quantum ChromoDynamics:
A(e,e’h), h = hadron
1.0
T
Energy (GeV)
Ingredients
• s
hN
h-N cross-section
• Glauber multiple
scattering approximation
(or better transport calculation!)
• Correlations & Final-State
Interaction effects
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
Search for Color Transparency in
Quasi-free A(e,e’p) Scattering
Fit to s = soAa
a
Constant value line fits give good description:
c2/df = 1
Conventional Nuclear Physics Calculation by
Pandharipande et al. (dashed) also gives good
description
 No sign of CT yet
a = constant = 0.75
Close to proton-nucleus
total cross section data
Physics of Nuclei: Color Transparency
Results inconsistent with CT only.
But can be explained by including
additional mechanisms such as
nuclear filtering or charm
resonance states.
AGS
A(p,2p)
Glauber
calculation
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.
2.9
5.1
7.3
9.6
Pp (GeV/c)
Physics of Nuclei: Color Transparency
A(e,e’p+)
6
7
8
9
10
Total pion-nucleus cross section slowly disappears, or …
pion escape probability increases  Color Transparency?
Transparency
 Unique possibility to map out at 12 GeV (up to Q2 = 10)
Physics of Nuclei: Color Transparency
A(e,e’r+) at 12 GeV
(at fixed coherence length)
12 GeV
Using the nuclear arena
How long can an energetic quark remain deconfined?
How long does it take a confined quark to form a hadron?
Formation time tfh
Hadron is formed
Production time tp
Quark is deconfined
Hadron attenuation
CLAS
Time required to produce colorless “prehadron”, signaled by medium-stimulated
energy loss via gluon emission
Time required to produce fullydeveloped hadron, signaled by CT
and/or usual hadronic interactions
Using the nuclear arena
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?
p+
e’
pT
g*
DpT2 = pT2(A) – pT2(2H)
e
L
“pT Broadening”
dE/dx ~ <pT2>L
DE ~ L (QED)
~ L2 (QCD)?
Using the nuclear arena
How long can an energetic quark remain deconfined?
How long does it take a confined quark to form a hadron?
Relevance to RHIC and LHC
Or How do energetic quarks transform into hadrons?
Deep
Scattering
Relativistic
HowHeavy-Ion
quickly does
Collisions
it happen? What
areInelastic
the mechanisms?
p
p
e
e’
Initial quark energy is known
Properties of medium are known
DpT2 (GeV2) ~dE/dx
DpT2 vs. n for Carbon, Iron, and Lead
Hall B Preliminary CLAS
Pb
~ 100 MeV/fm (perturbative formula)
Fe
C
n (GeV)
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 DE ~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 arena
DpT2 reaches a “plateau” for sufficiently large
quark energy, for each nucleus (L is fixed).
DpT2
Projected Data
n
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
Now split in two to ease transition into operations phase
Note → 6 to 18 months
schedule float included
(A) = Actual Approval Date
12 GeV Upgrade: Phases and Schedule
(based on funding guidance provided by DOE-NP in June-2007)
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
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!
Exposing the high-energy (dark) side
of the nuclei
The Low Energy View of Nuclear Matter The High Energy View of Nuclear Matter
• nucleus = protons + neutrons
The visible Universe is generated by
quarks, but dominated by the dark glue!
• nucleon  quark model
• quark model  QCD
Remove
factor 20
29
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.”
How do we understand the visible matter in our universe in terms of the
fundamental quarks and gluons of QCD?
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?
Explore the structure of the nucleon
• Parton distribution
functions
• Longitudinal and transverse
spin distribution functions
• Generalized parton
distributions
•Transverse momentum
distributions
Precisely image the sea quarks
Spin-Flavor Decomposition of the Light Quark Sea
| p
>
u
=
u
d
u
+
u
d
RHIC-Spin region
u
u
u
+
u
d
d
d
+ … Many models
predict
Du > 0, Dd < 0
GPDs and Transverse Gluon Imaging
Deep exclusive measurements in ep/eA with an EIC:
diffractive:
transverse gluon imaging
non-diffractive:
quark spin/flavor structure
J/y, ro, g (DVCS)
p, K, r+, …
[ or J/y, f, r0
p, K, r+, … ]
Are gluons uniformly
distributed in nuclear
matter or are there
small clumps of glue?
Describe correlation of longitudinal momentum
and transverse position of quarks/gluons 
Transverse quark/gluon imaging of nucleon
(“tomography”)
GPDs and Transverse Gluon Imaging
Fourier transform in momentum transfer
x ~ 0.001
x < 0.1
x ~ 0.3
x ~ 0.8
gives transverse size of quark (parton) with longitud. momentum fraction x
EIC:
1) x < 0.1: gluons!
2) x ~ 0  the
“take out” and
“put back” gluons
act coherently.
,g
d
x-x
x+x
GPDs and Transverse Gluon Imaging
Goal: Transverse gluon imaging of nucleon over wide range of x: 0.001 < x < 0.1
Requires: - 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
EIC enables gluon imaging!
Scaled from
2 to 16 wks.
EIC
(16 weeks)
Simultaneous data
at other Q2-values
eA Landscape and a New Electron Ion Collider
Well mapped in e+p
Not so for ℓ+A (nA)
Electron Ion Collider (EIC):
L(EIC) > 100  L(HERA)
eRHIC (e+Au):
Ee = 10 (20) GeV
EA = 100 GeV
seN = 63 (90) GeV
LeAu (peak)/n ~ 2.9·1033 cm-2 s-1
Terra incognita: small-x, Q  Qs
38
high-x, large Q2
ELIC (e+Au):
Ee = 9 GeV
EA = 90 GeV
seN = 57 GeV
LeAu (peak)/n ~ 1.6·1035 cm-2 s-1
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

d 2s epeX 4pa 2 
y2 
y
2
2
1  y   F2 ( x, Q ) 

FL ( x, Q )
2
4 
2 
2
dxdQ
xQ 

FL at EIC: Measuring the Glue Directly
Longitudinal Structure Function FL
• Experimentally can be determined
directly IF VARIABLE ENERGIES!
• Highly sensitive to effects of gluon
2

d 2s epeX 4pa 2 
y2 
y
2
2
1  y   F2 ( x, Q )  FL ( x, Q )

2
4 
2 
2
dxdQ
xQ 

Explore gluon-dominated matter
Longitudinal Structure Function FL
 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”
At high gluon density, gluon
recombination should compete
with gluon splitting  density
saturation.
Color glass condensate
Oomph factor stands up under scrutiny.
Nuclei greatly extend x reach:
xEIC = xHERA/18 for 10+100 GeV, Au
Diffractive Surprises
‘Standard
Diffractive
DIS event
event’
Detector
No activity
activity in
in proton
proton direction
direction
7 TeV equivalent electron bombarding the proton
… but proton remains intact in 15% of cases …


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 frame
EIC: 10 < n < 2000 GeV
(HERMES: 2-25 GeV)
EIC: can measure heavy flavor energy loss
Using the nuclear arena
DpT2 reaches a “plateau” for sufficiently large quark energy,
for each nucleus (L is fixed). In the pQCD region, the effect
is predicted to disappear (arbitrarily put at n =1000)
DpT2
n
Quarks and Gluons in the Nuclear Medium –
Opportunities at JLab@12 GeV and an EIC
Rolf Ent, ECT-Trento, June 06, 2008
Personal View:
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.
FL at EIC: Measuring the Glue Directly
Longitudinal Structure Function FL
• Experimentally can be determined
directly IF VARIABLE ENERGIES!
• Highly sensitive to effects of gluon
+ EIC alone
+ 12-GeV data
2

d 2s epeX 4pa 2 
y2 
y
2
2
1  y   F2 ( x, Q )  FL ( x, Q )

2
4 
2 
2
dxdQ
xQ 

Gluons in the Nucleus
eRHIC
Note: not all models carefully checked against existing data
+ some models include saturation physics
GPDs and Transverse Gluon Imaging
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
e
k
k'
g*
p
q
g
q'
p'
At small x (large W):
s ~ G(x,Q2)2
Simultaneous measurements over large range in x, Q2, t at EIC!