# Presentation: 609993

## of the Proton in Exclusive Processes  

- of the Proton in Exclusive Processes * *

- Volker D. Burkert
- Charles Hyde-Wright
- Xiangdong Ji

-  From 1-D “structure” to 3-D “dynamics”
-  GPD-sensitive results in DVCS at < 6 GeV
-  The dedicated 6 GeV program at Jlab
-  Prospects for the 12 GeV CEBAF Upgrade
-  What will we learn about the nucleon @ 12 GeV?
-  Summary

- Special Meeting of PAC23 on Physics at 12 GeV

## Slide 2

## Form factors, parton distributions, GPDs

- A. Belitsky and D. Muller, Nucl.Phys.A711(2002)118

- Deeply Virtual Exclusive Processes

-  =

- xB

- 2 - xB

- (in the Bjorken regime)

- e.g. Deeply Virtual Compton Scattering (DVCS)

- hard processes

- Deeply Virtual Meson Production

## X. Ji, 

- X. Ji,

## Modeling Generalized Parton Distributions

- Modeling Generalized Parton Distributions

- Accessed by beam helicity asymmetry

- Quark distribution q(x)

-  -q(-x)

- Accessed by cross sections

## Scaling cross sections for photon and meson production

-   The scaling cross section for photons dominate at high Q2 over meson production.

- M. Vanderhaeghen, P.A.M. Guichon, M. Guidal, PRD60, (1999), 94017

- 1/Q4

- 1/Q6

- 1/Q6

- 

## Kinematics coverage for deeply exclusive experiments

## DVCS in the Interference with Bethe-Heitler (BH)

- Measures DVCS in the interference with the known  BH process:

************determined by Dirac & Pauli
-  form factors
**T****DVCS**: determined by GPDs

## Measurement of exclusive DVCS with CLAS 

- Beam Spin Asymmetry

-  1999 data, E=4.2GeV

- A. Belitsky et al.

- Phys. Rev. Lett. **87 **(2001)

## uncorrected data

- sinsin2

- <xB>  = 0.25
- <Q2> = 1.6 GeV2
- <-t>  = 0.25 GeV2 

- 

- not for distribution

-  - twist-2
-  - higher twist

-  2000 data, E=5.65GeV, 10% sample

## Measurement of exclusive DVCS with CLAS 

-  2000 data, E=5.65GeV, 10% sample

- A. Belitsky et al., hep-ph/0112108
- xB=0.25, Q2=2GeV2,-t=0.25GeV2

- <xB>  = 0.25
- <Q2>  = 1.6 GeV2
-  <-t>  = 0.25 GeV2 

- uncorrected data
- not for distribution

- 

- 

- 

- ALL

- 

## Exclusive ep       ep production

- Compare with GPD formalism and models

##  The JLab Program @ 6 GeV

-  The JLab Program @ 6 GeV

-      Full reconstruction of final state ep       ep
- 	 ALU,  for several Q2 bins xB ,, t- dependence
- Im(TDVCS)
- Kinematical dependence of DVCS observables and GPDs
- Twist-2 and higher twist contributions, QCD corrections
- Modeling of GPDs

- CLAS and Hall A

-  DVCS

- CLAS

-  DVMP

- o,   production at W > 2 GeV, Q2 = 1.0 - 4.5 GeV2
-    production

## Kinematics coverage for deeply exclusive experiments

*ELFE*

## The JLab GPD Program @ 12 GeV

**DVCS**:
-     Precise Q2 dependence of Im(TDVCS) amplitude
- 	=> twist-2/3 contributions in sin(), sin() of   and ALU
-      and t- dependence of , ALU  => linear combination of GPDs
-     Diff. cross section: , t-dependence Re(TDVCS) =>  moments of GPDs
-     Asymmetries with polarized targets => different sensitivity to GPDs

**DVMP:**
**   **Establish kinematics range where theory is tractable
-     Flavor separation of  GPD-dependent observables ()
-     Separation of spin-dependent and spin-independent GPDs ()   **	**

## DVCS/BH projected for CLAS++  at 11 GeV

- 972 data points
- measured
- simultaneously

## DVCS with CLAS++  at 11 GeV

- 2% of all data points that are measured simultaneously.

**Q****2****=5.5GeV****2**
** ****x****B**** = 0.35**
** ****-t = 0.25 GeV****2 **

**Q****2****=2.75GeV****2**
** ****x****B**** = 0.35**
** ****-t = 0.25 GeV****2 **

## DVCS/BH twist-2 with CLAS++  at 11 GeV 

- DVCS/BH twist-2 with CLAS++*** ****** ***at 11 GeV

- Q2, xB, t  ranges
- measured
- simultaneously.
- Measure ALU,
-  and DVCS
- simultaneously

- Sensitivity to five GPD models (***ADD references)***

## DVCS/BH  interference projected for Hall A

- DVCS/BH  interference projected for Hall A

- L=1037cm-2s-1
- 400 hours
- MAD spectrometer
- ECAL @ -12.5o,
- 300 cm distance
- SC array @ -7o

- E = 11 GeV
- Q2 = 6 GeV2
- xB = 0.37

## DVCS/BH  interference projected for Hall A

- L=1037cm-2s-1
- 400 hours
- MAD spectrometer
- ECAL @ -12.5o,
- 300 cm distance
- SC array @ -10.5o

- E = 11 GeV
- Q2 = 7 GeV2
- xB = 0.50

- DVCS/BH  interference projected for Hall A

## Beam spin asymmetry projected data for Hall A

- Beam spin asymmetry projected data for Hall A

- AsinBsin
- A: twist-2
- B: twist-3

## Single Target

-  DVCS/BH asymmetries
- Projections for CLAS++

## CLAS++

- Deeply Virtual  Production at 11 GeV

- CLAS++

- L/T Separation via
-   decay
- distribution and SCHC

-  Test the Q2 evolution of   L, T => factorization

-  Rosenbluth separation => test SCHC assumption

## / production with transverse polarized target 

- / production with transverse polarized target

- 2    (Im(AB*))/

- T

- t/4m2) - Re

- UT

-  A ~ (euHu - edHd)

- B ~ (euEu - edEd)

- Asymmetry depends linearly
- on the GPD* **E*, which enters in
- Ji’s sum rule. High xB gives
- important contribution.

- L has similar sensitivity to proton quark spin

## From Observables to GPDs 

- Procedures to extract GPDs from experimental data are currently under intense development.

-    Fit parametrizations of GPDs to large sets of data.
-   Constraint by “forward” parton distribution
-   Polynomiality conditions
-   Elastic form factors
-   Meson distribution amplitudes

-    Approximations for certain kinematics (small , t), allow  extraction of dominant GPDs (e.g. H(,t)) directly

-    Partial wave expansion techniques (Polyakov, A. Shuvaev).
-  generalized quark distributions given by sum over t-channel exchanges
-  make connection to total quark spin by analysing S- and D-wave exchanges in t-channel

- “Tomographic” Images of the Proton’s Quark Content

- (In the Infinite Momentum Frame)

## Summary

-  Knowledge of GPDs will allow to construct “tomographic” respresentations of the nucleon’s quark and spin distributions in transverse space and infinite momentum frame.
-  GPDs can be accessed in deeply virtual exclusive processes at moderately high Q2. Broad theoretical support is available to extract the fundamental physics.
-  DVCS + DVMP () give access to the total quark contributions to the nucleon spin.
-  A broad program for DVCS and DVMP is proposed for Jlab covering a wide range of kinematics in several channels, largely unaccessible at other labs.
-  The upgrade energy of 12 GeV allows to reach high Q2. We can make use of asymmetry and cross section measurements to determine GPDs.
-  Data  from non-optimized experiments at Jlab @ 4-6 GeV and DESY show feasibility of the program.

- The program of Deeply Exclusive Experiments at Jefferson Lab is continuing the breakthrough experiments to study the internal nucleon structure at a new level. It has the potential to revolutionize hadronic physics with electromagnetic probes.