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

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

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

JLab, October 31, 2008 WACS in 12 GeV era

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GPDs

GPDs and form factors of WACS

GA at w

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

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:

Mixed e/beam –> productivity 1300 higher than “clean”

ep events

RCS events

“pion” events

Two body kinematics

1013 photons/sec

JLab, October 31, 2008 WACS in 12 GeV era

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GPDs

Results of 6 GeV experiment

PRL 94, 242001 (2005)

JLab, October 31, 2008 WACS in 12 GeV era

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GPDs

Form factors of RCS and partonic structure of the nucleon

JLab, October 31, 2008 WACS in 12 GeV era

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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

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

JLab, October 31, 2008 WACS in 12 GeV era

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GPDs

RCS perspective with 12 GeV Jlab beam

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

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

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)

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