GEp-2? experiment (E04-019)

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GEp-2? experiment (E04-019) UPDATE
- HALL C USER Meeting January 30, 2009 -
On behalf of the JLab-Gep Collaboration
Mehdi Meziane The College of William Mary
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OUTLINE

• INTRODUCTION
• THE GEp-2? (04-019) EXPERIMENT AT JLAB HALL-C
• TWO-PHOTON EXCHANGE (TPEX)
• THEORETICAL PREDICTIONS
• ELASTIC EVENTS SELECTION
• CONCLUSION

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ROSENBLUTH AND POLARIZATION TRANSFER MEASURMENTS
OF OF THE PROTON

• Either cross section or recoil polarization
• measurements to extract the proton
• form factor

• Recoil polarization and Rosenbluth

ratios are clearly
different in the Born approximation.

• Difference increase systematically
• with Q2.

Two methods, two different results
Something beyond the Born approximation

Two-photon exchange
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THE GEP-2? (04-019) EXPERIMENT AT JLAB HALL-C

• We look for a kinematical e dependence of
  to detect a

possible two-photon exchange effect in the
scattering

• In the Born approximation we dont have such
  dependence

• We carried out the experiment last year

• Q2 2.5 GeV2 for 3 values of e 0.15, 0.63 and
  0.78

• Requires lt0.01 statistics for a ratio of 0.7

• Systematics cancelled out because Q2 and pp fixed

• Measure separatly the two observables Pt/Pl and
  relative Pl

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TWO-PHOTON EXCHANGE (TPEX)
T matrix
In the Born approximation
form factors (real) respectively
are
and
The virtual photon polarization is
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TWO-PHOTON EXCHANGE (TPEX)
Transverse polarization component
Longitudinal polarization component
Reduced cross section
Born Approx.
Beyond Born Approx.
with
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THEORETICAL PREDICTIONS HADRONIC CALCULATIONS
Only nucleon intermediate states are taken into
account
P.Blunden et al., Phys.Rev.Lett.91 142304 (2003)
The inclusion of ? reduces the TPEX correction
(Figure)
P.Blunden et al., Phys.Rev.C72 034612 (2005)
Higher nucleon resonances has smaller effect
S.Kondratyuk et al., Phys.Rev. C75038201(2007)
Structure function method 2? effects small,
higher orders change Rosenbluth slope (Figure)
Yu. Bystricky, E.A.Kuraev, E. Tomasi-Gustafsson
Phys. Rev. C75, 015207 (2007)
Proton off-shell form factors make almost no
difference
D.Borisuyk, A.Kobushkin arXiv0804.4128
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THEORETICAL PREDICTIONS GPD CALCULATIONS
TPEX contributions are calculated assuming
factorization of the soft nucleon-quark part, and
the hard electron-quark interaction, where the
TPEX takes place via the box diagram.

• Absolute correction to FF ratio Ge/Gm
• slow Q2 variation, strong effects at low e
• valid for high Q2 or high e

A.Afanasev et al., Phys.Rev.D72013008 (2005)
GPD models Gauss on Fig., smaller effect with
Regge, or non-zero quark mass
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THEORETICAL PREDICTIONS
hadronic (elastic) dominated by correction to
GM GPD (includes inelastic) dominated by Y2g and
correction to GE Both theories describe
Rosenbluth data but have opposite predictions for
GE/GM
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GEP-2? at JEFFERSON LAB HALL C

• Key idea
• fixed Q2
• same spin transport
• same analysing power.

precision limited only by statistics ( 1),
unlike Rosenbluth, very small p.t.p
systematics Ay , h cancel out in the Pt/Pl
ratio Q2 fixed, Pp fixed, spin precession fixed
Ee, GeV pp Ee ?p, deg ?e e range ltQ2gt
1.867 2.068 0.527 14.13 106 .130-.160 2.49
2.839 2.068 1.507 30.76 45.3 .611-.647 2.49
3.549 2.068 2.207 35.39 32.9 .765-.786 2.49
80µA beam current 85 pol. 20cm LH target
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DETECTORS
Focal Plane Polarimeter FPP

• 2 analyzers in serie
• 2 sets of drift chamber pairs

Electromagnetic Calorimeter BigCal
1744 channels
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KINEMATIC FORMULA
Proton momentum calculated from the electron
angle
with
Proton momentum calculated from the proton angle
HMS proton momentum
and p0 2.0676 GeV/c
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ELASTIC EVENTS SELECTION
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PRELIMINARY RESULTS
The 2 theoretical curves have been normalized by
a different coefficient to fit the data.
No evidence of TPEX effect at a percent level
PRELIMINARY
Prove the validity of the Born approximation for
data obtained by the polarization technique at a
percent level.
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CONCLUSION

• Fixed Q2 and pp

Measure separatly two observables relative Pl
and the ratio Pt/Pl

• Same analyzing power

• Same spin transport

• Upper estimate of the background of 3.2

• Need a Monte-Carlo simulation to fit the
  background shape

• No evidence of effects beyond the Born
  approximation

• Prove the validity of the polarization method
  for the proton form factor measurements