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JLab, October 31, 2008 WACS in 12 GeV era
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GPDsWide-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?
p -> p
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JLab, October 31, 2008 WACS in 12 GeV era
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GPDs
WACS: Introduction
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
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JLab, October 31, 2008 WACS in 12 GeV era
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GPDs
GPDs
DIS Form factors
DVCS WACS
tx
N -> N*DVMP
Unification of nucleon structure within GPDs
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JLab, October 31, 2008 WACS in 12 GeV era
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GPDs
GPDs and form factors of WACS
GA at w
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JLab, October 31, 2008 WACS in 12 GeV era
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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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JLab, October 31, 2008 WACS in 12 GeV era
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GPDs
Polarization observables of WACSin GPDs handbag calculations
LO + RT :
photon helicity and PL of the recoil proton hard
softLO:
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Mixed e/beam –> productivity 1300 higher than “clean”
ep events
RCS events
“pion” events
Two body kinematics
1013 photons/sec
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JLab, October 31, 2008 WACS in 12 GeV era
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GPDs
Results of 6 GeV experiment
PRL 94, 242001 (2005)
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JLab, October 31, 2008 WACS in 12 GeV era
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GPDs
Form factors of RCS and partonic structure of the nucleon
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JLab, October 31, 2008 WACS in 12 GeV era
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GPDs
Polarization transfer in Wide Angle Compton Scattering
ep
Very clean data
Calibration to elastic e-p polarization data taken parallel to WACS
Expected small systematic uncertainty
Result is favor to CQM
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JLab, October 31, 2008 WACS in 12 GeV era
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GPDs
ALL- initial state polarization correlation parameter
ALL- KLL diff. as a measure of quark spin flip ~ mq/E
ALL=KLL in QCD
D.Day, BW
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JLab, October 31, 2008 WACS in 12 GeV era
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GPDs
RCS perspective with 12 GeV Jlab beam
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JLab, October 31, 2008 WACS in 12 GeV era
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GPDs
Neutron Compton Scattering
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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JLab, October 31, 2008 WACS in 12 GeV era
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GPDs
Joint Hall A + C projectconsists of 1744 blocksfront face - 1.2 m x 2.2 mresolution ∆E ~ 6%/√Eσposition ~ 1 cm
Ready for experiments
BIGCAL Calorimeter
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JLab, October 31, 2008 WACS in 12 GeV era
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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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JLab, October 31, 2008 WACS in 12 GeV era
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GPDsCalorimeter parameters
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