1. The Physics Program of Jefferson Lab at 12 GeV
Elton S. Smith
Jefferson Lab
Orsay, 2007
Gluonic Excitations
3-dim view of the Nucleon
Quarks in nuclear matter
Project Status

2. Nuclear Physics with Energy Upgraded JLab
Understand how hadrons are constructed from the quarks and gluons of QCD
Understand the QCD basis for the nucleon-nucleon force
Explore the limits of our understanding of nuclear structure
The transition from the nucleon-meson to the quark-gluon description
JLab’s Scientific Mission
We know that QCD works, but we still need to understand how.
Must address critical issues in “strong” QCD
Do quarks and gluons play any direct role in Nuclear Matter?

3. JLab accelerator CEBAF
Continuous Electron Beam
Energy 0.8 ─ 5.7 GeV
200 mA, polarization 75%
1499 MHz operation
Simultaneous delivery 3 halls A B C

4. Upgrade magnets and power supplies
CHL-2
Upgrade magnets and power supplies 12
6 GeV CEBAF
add Hall D
(and beam line)
Enhance equipment in existing halls

5. Overview of 12 GeV Physics Program
Hall D – exploring origin of confinement by studying exotic mesons
Hall B – understanding nucleon structure via generalized parton distributions
Note BSM not included in project
Hall C – precision determination of valence quark properties in nucleons and nuclei
Hall A – short range correlations, form factors, hyper-nuclear physics, future new experiments 5

6. Gluonic Excitations
Dynamical role of Glue
Confinement

7. Quarks are confined inside colorless hadrons
Quarks combine to “neutralize” color force q q q q q
mesons
baryons
glueball meson
hybrid meson
Configurations outside the standard quark model q
pentaquark
molecules

8. Bonding of hydrogen molecule
Electronic energy levels in molecules are created by stable configurations of electrons between the atoms

9. Molecular binding and configuration of electrons
ground state
excited state
“Cloud” of electrons creates potential which bonds
H2 atoms into diatomic molecule. Excited electronic
states correspond to excitations of the electron cloud.

10. Normal Mesons – qq color singlet bound states
Spin/angular momentum configurations & radial excitations generate the known spectrum of light quark mesons.
Starting with u - d - s we expect to find mesons grouped in nonets - each
characterized by a given J, P and C.
Spin 0
Spin 1 K0 K+ K* r w f p0 h’ p− p+ h K− K0
JPC = 0– – – 1+ – 2++ …
Allowed combinations
JPC = 0– – 0+ – 1– – …
Not-allowed: exotic

11. Quark binding and configuration of gluons
JKM, Nucl. Phys. B (Proc. Suppl.) 63A-C (1998) 326
Confinement arises from flux tubes and their excitation leads to a new
spectrum of mesons
From G. Bali

12. Hybrid Mesons
Hybrid mesons
1 GeV mass difference
Normal mesons

13. Quantum Numbers of Hybrid Mesons
Excited Flux Tube
Quarks
Hybrid Meson
like
Exotic
Flux tube excitation (and parallel quark spins) lead to exotic JPC
like

14. Mass Predictions Lowest mass expected to be p1(1−+) at 1.9±0.2 GeV
Lattice
GeV
GeV
GeV

15. How do exotics decay? Possible daughters: L=1: a,b,h,f,…
simple decay modes such as ,, … are suppressed.
The angular momentum in the flux tube stays in one of
the daughter mesons (L=1) and (L=0) meson, e.g:
Example: p1→b1p
flux tube L=1
quark L=1
wp → 4p

16. Hybrid Decays
Sensitivity to a variety of decay modes removes dependence on model predictions.
For example, for hybrids:
favored
Measure many decay modes!
not-favored
To certify results, checks will be made among different final states for the same decay mode, for example:
Should give
same results

17. Strategy for Exotic Meson Search
Use photons to produce meson final states
tagged photon beam with 8 – 9 GeV
linear polarization to constrain production mechanism
Use large acceptance detector
hermetic coverage for charged and neutral particles
typical hadronic final states: f1h KKh KKppp b1p wp pppp rp ppp
high data acquisition rate
Perform partial-wave analysis
identify quantum numbers as a function of mass
check consistency of results in different decay modes

18. Coherent Bremsstrahlung
12 GeV electrons
Incoherent &
coherent spectrum
flux
This technique provides requisite energy, flux and polarization
tagged
with 0.1% resolution
40%
polarization
in peak
photons out
collimated
electrons in
spectrometer
diamond
crystal
Eg (GeV)

19. Coherent Bremsstrahlung
GlueX / Hall D Detector
Lead Glass
Detector
Solenoid
Coherent Bremsstrahlung
Photon Beam
Tracking
Target
Cerenkov
Counter
Time of
Flight
Barrel
Calorimeter
Note that tagger is
80 m upstream of
detector
Electron Beam from CEBAF

20. Finding an Exotic Wave
An exotic wave (JPC = 1-+) was generated at level of 2.5 % with 7 other waves. Events were smeared, accepted, passed to PWA fitter.
Mass
Input: MeV
Width
Input: 170 MeV
Output: /- 3 MeV
Output: /- 11 MeV
Double-blind M. C. exercise
Statistics shown here correspond to a few days of running.

21. Summary of Exotic Hybrid Mesons
The physics goal of GlueX is to map the spectrum of hybrid mesons starting with those with the unique signature of exotic JPC quantum numbers.
Study hybrid mass with masses up to 2.5 GeV
Expected sensitivity to exotic signals is at the level of a few percent of the production of normal hadrons

22. 3-dimensional view
of the Nucleon
Deep Exclusive Scattering

23. 1-dim view from Deep Inclusive Scattering
What have we learned?
nucleon has a substructure made of quarks
quarks are spin ½ objects
Longitudinal momentum distribution of quarks q(x), Dq(x)
50% of the nucleon momentum is carried by quarks, the remainder by gluons
less than 25% of the nucleon spin is carried by quark helicity

24. Generalized Parton Distributions
GPD’s provide access to fundamental quantities such as the quark orbital angular momentum that have not been accessible

25. Interpretation of the GPD’s
Analogy with form factors
Charge ↔ Form Factor å ò = × - M r m R to
relative
measured e d q F i cm ) ( 3
Parton Distribution ↔ GPD’s
Ref. Burkardt CM i b iq R
from
distance a at x
fraction
momentum
with
quarks of
density
parton is f
where r to
relative
measured e d q H ^ × - = å ò ) , ( @ 2

26. 3-dim picture Elastic Scattering Deep Inelastic Deep Exclusive
from Belitsky and Mueller
Elastic Scattering
Deep Inelastic
Deep Exclusive

27. Measurements of the GPDs are now feasible
Measuring the GPD’s
Key experimental capabilities include:
CW (100% duty factor) electron beams
permits fully exclusive reactions
Modern detectors
permit exclusive reactions at high luminosity
Adequate energy
10 GeV to access the valence quark regime
Measurements of the GPDs are now feasible

28. Kinematics for deeply exclusive experiments
no overlap with other
existing experiments
compete with other
experiments

29. Deep Exclusive Scattering
Inclusive Scattering Compton Scattering
Deeply Virtual Compton Scattering (DVCS)
Probes the nucleon quark structure and correlations
at the amplitude level

30. ) ( DVCS Bethe-Heitler + 2 Beam Spin Asymmetry e e’ p g dydtd dx d = T
GPD’s B
dydtd dx d = ( ) BH
DVCS T * 2 + Q e y x 3 1 8 p a f s
Beam Spin Asymmetry ~

31. CLAS12 - DVCS/BH- Beam Asymmetry
Ee = 11 GeV
Q2=5.5GeV2
xB = 0.35
-t = 0.25 GeV2
Luminosity = 720fb-1

32. CLAS12 - DVCS/BH Beam Asymmetry
e p epg
E = 11 GeV
DsLU~sinfIm{F1H+..}df
Selected
Kinematics
L = 1x1035
T = 2000 hrs
DQ2 = 1 GeV2
Dx = 0.05

33. Meson Production as a Filter
Use quantum numbers of meson to select appropriate combinations of parton distributions in nucleon.
Pseudoscalers (polarized)
p0: Duv - ½ Ddv
h : Duv - ½ Ddv + 2Dsv
Vector Mesons (unpolarized)
rL0: u + u + ½ (d + d); g
wL0: u + u - ½ (d + d); g
fL0: s + s; g g*
Y(z)GjY(y)
Y(z)GiY(y) p p

34. Summary of Nucleon Structure
These measurements of exclusive reactions will be possible due to the increased luminosity available at Jefferson Lab.
The Q2 range will double in the region of valence quarks over that accessible today.
Insight into the structure of the nucleon will be obtained by studies of the Generalized Parton Distribtuions which provide information about the transverse position distribution of quarks within the nucleon.

35. Quarks in Nuclear Matter
EMC Effect

36. Nucleons and Pions or Quarks and Gluons?
From a field theoretic perspective, nuclei are a separate solution of QCD Lagrangian
Not a simple convolution of free nucleon structure with Fermi motion
‘Point nucleons moving non-relativistically in a mean field’ describes lowest energy states of light nuclei very well
But description must fail at small distances
In nuclear deep-inelastic scattering, we look directly at the quark structure of nuclei
This is new science, and largely unexplored territory
New experimental capabilities to attack long-standing physics issues

37. The QCD Lagrangian and Nuclear “Medium Modifications”
vacuum
Long-distance gluonic fluctuations
Lattice calculation demonstrates reduction of chiral condensate of QCD vacuum in presence of hadronic matter
Leinweber, Signal et al.

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

39. Quark Structure of Nuclei: The EMC Effect
Observation that structure functions are altered in nuclei stunned much of the HEP community 23 years ago.
~1000 papers on the topic; the best models explain the curve by change of nucleon structure, BUT more data are needed to uniquely identify the origin
What is it that alters the quark momentum in the nucleus? x
JLab 12
Explain what the EMC effect is.
Point out the three regions and their conventional explanations.
Cannot be fully explained by conventional nuclear physics
Amazing that we still don’t know the answer after nearly a quarter of a century
Coverage extends at least to x=1.5
J. Ashman et al., Z. Phys. C57, 211 (1993)
J. Gomez et al., Phys. Rev. D49, 4348 (1994)

40. EMC Effect - Theoretical Explanations
Quark picture
Multi-quark cluster models
Nucleus contains multinucleon clusters (e.g., 6-quark bag)
Dynamical rescaling
Confinement radius larger due to proximity to other nucleons
Call it an increased confinement radius, not a bigger nucleon.
Note connection between clusters and SRC
Hadron picture
Nuclear binding
Effects due to Fermi motion and nuclear binding energy, including virtual pion exchange
Short range correlations
High momentum components in nucleon wave function

41. Quarks in a Nucleus
Observation that structure functions are altered in nuclei stunned much of the HEP community ~25 years ago
r scaling
A scaling
A=3 EMC Effect
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?

42. Anti-Quarks in a Nucleus
Is the EMC effect a valence
quark phenomenon or are sea quarks involved?
Tremendous opportunity for experimental improvements!
0.5
1.0
gluons
sea
valence
0.1
S. Kumano, “Nuclear Modification of Structure Functions in Lepton Scattering,” hep-ph/ x
RCa
E772
Deep inelastic electron scattering probes only the sum of quarks and anti-quarks  requires assumptions on the role of sea quarks
Solution: Detect a final state hadron in addition to scattered electron
 Can ‘tag’ the flavor of the struck quark by measuring the hadrons produced: ‘flavor tagging’

43. “Free” Neutron Structure – The BONuS Experiment
Goal: Measure Structure Functions F2 on Nearly Free Neutrons
Technique:
e + D  e’ + pS + X
With backward going pS as low as 70 MeV/c to guarantee:
nearly free neutron struck
no complications due to FSI
or resonance decay
CLAS
RTPC p e-
Also gives information on nuclear complications
= nuclear physics!
Also possible to extend measurements to 3He and 4He to measure all electroproduced particles and reconstruct nuclear EMC effects from pieces!

44. Summary of quarks in nuclei
The properties of nucleons change when they are imbedded inside nuclei
High luminosity beams at Jefferson lab and the kinematic range of the 12 GeV Upgrade will allow measurements of the structure of nucleons in nuclei with unprecedented accuracy.
The program will be used to understand the reasons for changes in the momentum distribution of quarks in nuclei.

45. DOE Generic Project Timeline
We are here
DOE Reviews
Almost half-way between CD1 and CD2

46. 12 GeV Upgrade: Project Reviews
Independent Project Review - June 2006 (DOE)
35% Hall D design review – July 2006
Cryomodule review - September 2006
Superconducting Magnet Review – September 2006
Arc Magnet review - November 2006
60% Hall D design review - January 2007
Project Status Review – January 2007 (DOE):
“The 12 GeV Upgrade Project is on track in their preparations and readiness for CD-2 approval in September 2007.”
June (?): Baseline Readiness (CD-2) Review, stage I
Conducted by Lehman office (DOE - IPR)
August: Baseline Readiness (CD-2) Review, stage II
Conducted by OECM (DOE – EIR)
The project is on track for CD-2 Approval
Next

47. Schedule Overview

48. Highlights of the CEBAF 12 GeV Upgrade Physics Program
Exploration of QCD and confinement: Existence and properties of exotic mesons
Totally New View of Hadron (and Nuclear) Structure: Generalized Parton Distributions
New Paradigm for Nuclear Physics: Quark structure of nuclei
Not covered in this talk:
Revolutionize Our Knowledge of Spin and Flavor Dependence of Valence Parton Distribution Functions
Finalize Our Knowledge of Distribution of Charge and Current in the Nucleon
Precision Tests of the Standard Model
Project is on track with construction starting in less than two years!