GPDs
p -> p
Wide-Angle Compton Scattering
pi-0 photo-production in 12 GeV era
B. Wojtsekhowski
Outline
•
WACS and other Hard Exclusive Reactions.
•
WACS physics and GPDs. Theoretical framework.
•
Experimental technique. How will we do these experiments?
JLab, October 31, 2008
WACS in 12 GeV era
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WACS: Introduction
GPDs
Part of JLab program of Hard Exclusive Reactions
• Elastic Form Factors: GMP,GEP,GMN,GEN,FF
• WACS: high t in two-photon reaction
• Deeply Virtual Compton Scattering (DVCS)
• Deeply Virtual Meson Production
Common issues:
• Handbag diagram
• Interplay between hard and soft processes
• Threshold for onset of asymptotic regime
• Role of hadron helicity flip
JLab, October 31, 2008
WACS in 12 GeV era
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Unification of nucleon structure
within GPDs
GPDs
DIS
Form factors
DVMP
N -> N*
GPDs
WACS
DVCS
x
JLab, October 31, 2008

WACS in 12 GeV era
t
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GPDs and form factors of WACS
GPDs
GA at
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WACS in 12 GeV era
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w
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GPDs
Cross section of Wide-Angle
Compton Scattering
• Three-quark mechanism
dominates at “asymptopia”
2 hard gluon exchanges.
Constituent counting rules:
d/dt = f(CM)/s6
“complicated” polarization
observables
• Single-quark mechanism
“handbag” diagram dominates.
Form factors:
“simple” polarization observables
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WACS in 12 GeV era
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GPDs
Polarization observables of WACS
in GPDs handbag calculations
photon helicity and PL of the recoil proton
hard
soft
LO:
LO + RT :
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WACS in 12 GeV era
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Mixed e/ beam –> productivity 1300 higher
than “clean” 
1013 photons/sec
Two body kinematics
ep events
RCS event
“pion” events
Results of 6 GeV experiment
GPDs
PRL 94, 242001 (2005)
JLab, October 31, 2008
WACS in 12 GeV era
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Form factors of RCS and partonic structure
of the nucleon
GPDs
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WACS in 12 GeV era
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Polarization transfer in Wide Angle Compton Scattering
GPDs
Very clean data
Calibration to elastic ep polarization data
taken parallel to WACS
Expected small
systematic uncertainty
Result is favor to CQM
ep
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ALL- initial state polarization correlation parameter
D.Day, BW
GPDs
ALL=KLL in QCD
ALL- KLL diff. as a measure of quark spin flip
~ mq/E
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GPDs
JLab, October 31, 2008
RCS perspective with 12 GeV Jlab beam
WACS in 12 GeV era
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Neutron Compton Scattering
GPDs

Need to know
incident photon
energy ->

Limited luminosity

Post-target tagging!

For s = 7, t = 4
GeV2
120 hours beam
time per data point
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WACS in 12 GeV era
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BIGCAL Calorimeter
GPDs
Joint Hall A + C project
consists of 1744 blocks
front face - 1.2 m x 2.2 m
resolution ∆E ~ 6%/√E
σposition ~ 1 cm
Ready for experiments
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GPDs
Summary of the physics plan

Proton WACS – cross section with 11 GeV beam, ALL&KLL

Neutron WACS – cross section

Pi-0 from the proton – cross section, KLL, ALL

Pi-0 from the neutron – cross section

Strange form-factor at large Q2 ( ep->e’p, both in calorimeters)
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Calorimeter parameters
GPDs


Calorimeter size – match of Super Bigbite Spectrometer – 20 cm long
target, 70 msr ( horizontal +/- 5 degree, vertical +/- 12 degree),
energy resolution <10%, angular/position resolution of 1-2 mrad, two
cluster resolution < 10 mrad, need at least 2-3 m between magnet
and the calorimeter. Radiation hardness!!
For example: at distance 5-10 meters from the target:
BigCal 1.2x 2.2 m, 6% energy resolution, 1 cm coordinate resolution,
RMol ~ 3.7 cm, limit for radiation dose of 1 krad.
What need to be done:
a) Study of radiation hardness and resolution at high luminosity
b) Budget and used equipment: PMT, DAQ, HV, crystals
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WACS in 12 GeV era
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