The CEBAF IIELIC Upgrade at JLab

1
The CEBAF II/ELIC Upgrade at JLab

• Highly likely to be Absolutely Central to
  Advancing Nuclear Science
• Scientific/Engineering Challenges to Resolve
• Rolf Ent
• 02/15/2003

2
CEBAF Beyond 12 GeV

• Clear scientific case by 12-GeV JLab Upgrade,
  addressing outstanding issues in Hadron Physics
• Unprecedented measurements to region in x (gt 0.1)
  where basic three-quark structure of nucleons
  dominates.
• Measurements of correlations between quarks,
  mainly through Deep-Virtual Compton Scattering
  (DVCS) and constraints by nucleon form factors,
  in pursuit of the Generalized Parton
  Distributions.
• Finishing the job on the transition from hadronic
  to quark-gluon degrees of freedom.
• Over the next 5-10 years, data from facilities
  worldwide concurrent with vigorous accelerator
  RD and design will clarify the key physics and
  machine issues, revealing the relative advantages
  and technical feasibility of these accelerator
  designs and permitting an informed choice of
  design approaches.
• 25 GeV Fixed-Target Facility?
• Electron-Light Ion Collider, center-of-mass
  energy of 20-65 GeV?

3
CEBAF II/ELIC Upgrade - Science

• Science addressed by this Upgrade
• How do quarks and gluons provide the binding and
  spin of the nucleons?
• How do quarks and gluons evolve into hadrons?
• How does nuclear binding originate from quarks
  and gluons?

(x 0.01)
Glue 100
12 GeV
ELIC
4
CEBAF II/ELIC Upgrade - Science

• At present, uncertain what range of Q2 really
  required to determine complete structure of the
  nucleon. Most likely Q2 10 GeV2?
• Upcoming years wealth of data from RHIC-Spin,
  COMPASS, HERMES, JHF, JLab, etc.
• DVCS (JLab-12!) and single-spin asymmetries
  possible at lower Q2
• Range of Q2 directly linked to required
  luminosity
• What energy and luminosity, fixed-target facility
  or collider
• (or both)?
• JLab Science Policy Advisory Group The goal
  must be to find the best match to the science
  needs on the time scale of the next NSAC long
  range plan.
• Upgrade is highly likely to be Absolutely
  Central to Field

5
CEBAF II/ELIC Upgrade - Readiness

• Electron-Light Ion Collider (ELIC)
• RD needed on
• High Charge per Bunch and High Average Current
  Polarized Electron Source
• High Energy Electron Cooling of Protons/Ions
• High Current and High Energy demonstration of
  Energy Recovery
• Integration of Interaction Region design with
  Detector Geometry
• NSAC Report Strong consensus among nuclear
  scientists to pursue RD over the next three
  years to address a number of design issues
• 25-GeV Fixed-Target Facility
• Use existing CEBAF footprint
• Upgrade Cryo modules to 12-GeV design (7-cell
  design, 18 MV/m)
• Change ARC magnets, Switchyard, Hall Equipment
• Significant Scientific/Engineering Challenges
  to Resolve

6
CEBAF II/ELIC Upgrade - Kinematics
ELIC kinematics at Ecm 45 GeV, and beyond the
resonance region.

• Luminosity of up to 1035 cm-2 sec-1
• One day ? 5,000 events/pb
• Supports precision experiments
• DIS Limit for Q2 gt 1 GeV2 implies (Bjorken) x
  down to 5 times 10-4
• Significant results for 200 events/pb for
  inclusive scattering
• If Q2 gt 10 GeV2 required for Deep Exclusive
  Processes, can reach x down to 5 times 10-3
• Typical cross sections factor 100-1,000 smaller
  than inclusive scattering ? still easily
  accessible

7
The Structure of the Proton F2
F2 Structure Function measured over impressive
range of x and Q2. Q2 evolution also constrains
glue!
Still large uncertainty in glue, especially at Q2
lt 20 GeV2
8
The Structure of the Proton FL
Great Opportunity to (finally) determine FL with
good precision (JLab-6 example here in the
nucleon resonance region only)
Complementarity of 25-GeV fixed-target and ELIC
for this example! ? Glue at low Q2
De gt 0.3
gt
gt
20
known
9
The Spin Structure of the Proton

• From NLO-QCD analysis of DIS measurements (SMC
  analysis)
• DS 0.38 (in AB scheme)
• DG 1.01.9 ,,
• quark polarization Dq(x)
• ?first 5-flavor separation from HERMES (see
  later)
• transversity dq(x)
• ?a new window on quark spin
• ?azimuthal asymmetries from HERMES and
  JLab-6
• gluon polarization DG(x)
• ?RHIC-spin and COMPASS will provide some
  answers!
• orbital angular momentum L
• ?how to determine? ? GPDs

½ ½ DS DG Lq Lg
-0.6
CEBAF II/ELIC Upgrade can solve this puzzle due
to large range in x and Q2 and precision due to
high luminosity
10
Orbital Angular Momentum
Analysis of hard exclusive processes leads to a
new class of generalized parton distributions
Four new distributions
helicity conserving ? H(x,x,t),
E(x,x,t) helicity-flip
? H(x,x,t),
E(x,x,t)

• skewedness parameter x
• mismatch between quark momenta
• ?sensitive to partonic correlations

• New Roads
• Deeply Virtual Meson
• Production _at_ Q2 gt 10 GeV2
• ? disentangles flavor and
• spin!
• r and f Production give access
• to gluon GPDs at small x (lt0.2)

3-dimensional GPDs give spatial distribution of
partons and spin

• Angular Momentum Jq ½ DS Lq !

Can we achieve same level of understanding as
with F2?
11
Hadronization as a Tool
First 5-flavor fit to Dq(x) (Ds(x) Ds(x)
assumed)
_
Quark Polarization from Semi-Inclusive DIS
-
-

• Goal Flavor Separation
• of quark and antiquark helicity distributions
• Technique Flavor Tagging
• The flavor content of the final state
  hadrons is related to the flavor of the struck
  quark through the agency of the fragmentation
  functions

Chiral-Quark Soliton Model ?Light sea quarks
polarized but Data consistent with zero
12
Nuclear Binding

• Natural Energy Scale of QCD O(100 MeV)
• Nuclear Binding Scale O(10 MeV)
• Does it result from a complicated detail of near
  cancellation of strongly attractive and repulsive
  terms in N-N force, or is there another
  explanation?

How can one understand nuclear binding in
terms of quarks and gluons?
Complete spin-flavor structure of modifications
to quarks and gluons in nuclear system may
be best clue.
13
Quarks in a Nucleus
Can pick apart the spin-flavor structure of EMC
effect by technique of flavor tagging, in the
region where effects of the space-time structure
of hadrons do not interfere (large n!)
EMC Effect
F2A/F2D
10-4 10-3 10-2 10-1
1

x
Nuclear attenuation negligible for n gt 50 GeV
?hadrons escape nuclear medium undisturbed
Space-Time Structure of Photon
14
Basis of the ELIC Proposal

• A Linac-Ring Collider Design (with additional
  Circulator Ring)
• Linac
• CEBAF is used for the (one-pass) acceleration of
  electrons
• Energy recovery is used for rf power savings and
  beam dump requirements
• Ring
• Figure-8 storage ring is used for the ions for
  flexible spin manipulations of all light-ion
  species of interest
• Circulator ring for the electrons may be used to
  ease high current polarized photoinjector
  requirements

Luminosity of up to 1035 cm-2 sec-1
15
ELIC Layout
One accelerating one decelerating pass through
CEBAF
16
ELIC Physics Specifications

• Center-of-mass energy between 20 and 65 GeV
• (with energy asymmetry of 10)
• Ee 3 GeV on Ei 30 GeV up to Ee 5 GeV on Ei
  100 GeV worked out in detail (gives Ecm up to
  45 GeV)
• Ee 7 GeV on Ei 150 GeV seems o.k., but
  details to be worked out (extends Ecm to 65 GeV)
• CW Luminosity up to 1035 cm-2 sec-1
• Ion species of interest protons, deuterons, 3He,
  light-medium ions
• Proton and neutron
• Light-medium ions not necessarily polarized
• Longitudinal polarization of both beams in the
  interaction region (Transverse polarization of
  ions Spin-flip of both beams)

17
Luminosity Potential with ELIC
x10,000
18
The same electron accelerator can also provide 25
GeV electrons for fixed target experiments for
physics

• Implement 5-pass recirculator, at 5 GeV/pass, as
  in present CEBAF

(straightforward upgrade, no accelerator RD
needed)

• Exploring whether collider and fixed target
  modes can run simultaneously (can in alternating
  mode)

19
RD Strategy

• Several important RD topics have been identified
• Our RD strategy is multi-pronged
• Conceptual development
• Circulator Ring concept promises to ease high
  current polarized photoinjector and ERL
  requirements significantly
• Additional concepts for luminosity improvements
  are being explored
• Analysis/Simulations
• Electron cooling and short bunches
• Beam-beam physics
• ERL physics
• Experiments
• CEBAF-ER The Energy Recovery experiment at CEBAF
  to address ERL issues in large scale systems
  (March 2003)
• JLab FEL (10mA), Cornell/JLab ERL Prototype
  (100mA), BNL Cooling Prototype (100mA) to address
  high current ERL issues

20
CEBAF II/ELIC Upgrade Conclusions

• An excellent scientific case is developing for a
  high luminosity, polarized electron-light ion
  collider will address fundamental issues in
  Hadron Physics
• The (spin-flavor) quark-gluon structure of the
  proton and neutron
• How do quarks and gluons form hadrons?
• The quark-gluon origin of nuclear binding
• Highly Likely to be Absolutely Central to
  Field
• JLab design studies have led to an approach that
  promises luminosities as high as 1035 cm-2 sec-1,
  for electron-light ion collisions at a
  center-of-mass energy between 20 and 65 GeV
• This design, using energy recovery on the JLab
  site, can be cost-effectively integrated with a
  25 GeV fixed target program for physics
• ELIC and a fixed-target program have a
  complementary physics reach.
• Planned RD will address open readiness issues
• Significant Scientific/Engineering
  Challenges to Resolve

21
New Spin Structure Function Transversity
(in transverse basis)
-
dq(x)

• Nucleons transverse spin content ? tensor
  charge
• No transversity of Gluons in Nucleon ?
  all-valence object
• Chiral Odd ? only measurable in semi-inclusive
  DIS
• first glimpses exist in data (HERMES, JLab-6)
• COMPASS, RHIC-spin plans
• Future Flavor decomposition?

Estimates for TESLA-N (Ecm 22 GeV)
22
Generalized Parton Distributions

• What could be the role of a 25 GeV Fixed Target
  facility?
• Enhances phase space to Q2gt10 GeV2
• Comfortable to do flavor separation of GPDs
• Also L/T separations to verify how hard process
  really is!

Deep Virtual Meson Production
sL dominant, and Q-6?
23
How Do Quarks and Gluons form Hadrons?
In semi-inclusive DIS a hadron h is detected in
coincidence with the scattered lepton

• Study quark-gluon substructure of
• nucleon target
• ? parton distribution functions q(x,Q2)
• hadron formation (or hadronization)
• ? fragmentation functions D(z,Q2)

Process both of interest in its own right and as
a tool (see also next slides)
24
Hadronization
(Or The Long-Range Dynamics of Confinement)
What do we know?
What dont we (exactly) know?
The Lund String Model Phenomenological
description in terms of color-string breaking and
parton clustering Evolution of the fragmentation
functions

• Spin Transfer
• Is the spin of the struck quark communicated to
  the hadronic final state?
• Single-Spin Asymmetries
• How important is intrinsic transverse momentum?
• Space-Time Structure
• How long does it take to form a hadron?

Present and future studies (HERMES, JLab-6,
JLab-12) use the nuclear radius as a yardstick to
measure the time scale of hadron formation
25
Sea-Quarks and Gluons in a Nucleus
Drell-Yan ? No Nuclear Modifications to
Sea-Quarks found at x 0.1 Where is the
Nuclear Binding?
Constraints on possible nuclear modifications of
glue come from 1) Q2 evolution of nuclear ratio
of F2 in Sn/C (NMC) 2) Direct measurement of
J/Psi production in nuclear targets
F2
G

• Compatible with EMC effect?
• Precise measurements possible of
• Nuclear ratio of Sn/C (24 GeV)
• J/Psi production (ELIC)

Flavor tagging can also (in principle) disentangle
sea-quark contributions