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

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

3. 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

4. THE GEP-2γ (04-019) EXPERIMENT AT JLAB HALL-C
We look for a kinematical ε dependence of to detect a
possible two-photon exchange effect in the scattering
In the Born approximation we don’t have such dependence
We carried out the experiment last year:
Q2 = 2.5 GeV2 for 3 values of ε: 0.15, 0.63 and 0.78
Requires <0.01 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

5. TWO-PHOTON EXCHANGE (TPEX)
T matrix:
with
complex
and , q1 q2 N q1 q2 N
In the Born approximation:
are
form factors (real) respectively
and
The virtual photon polarization is:

6. TWO-PHOTON EXCHANGE (TPEX)
Transverse polarization component
Longitudinal polarization component
Reduced cross section
Born Approx.
Beyond Born Approx.
with

7. THEORETICAL PREDICTIONS: HADRONIC CALCULATIONS
Only nucleon intermediate states are taken
into account
P.Blunden et al., Phys.Rev.Lett.91: (2003)
The inclusion of Δ reduces the TPEX correction (Figure)
P.Blunden et al., Phys.Rev.C72: (2005)
Higher nucleon resonances has smaller effect
S.Kondratyuk et al., Phys.Rev. C75:038201(2007)
Structure function method: 2γ effects small, higher
orders change Rosenbluth slope (Figure)
Yu. Bystricky, E.A.Kuraev, E. Tomasi-Gustafsson
Phys. Rev. C75, (2007)
Proton off-shell form factors make almost
no difference
D.Borisuyk, A.Kobushkin arXiv:

8. 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 ε
valid for high Q2 or high ε
A.Afanasev et al., Phys.Rev.D72: (2005) – GPD models: Gauss on Fig., smaller effect with Regge, or non-zero quark mass

9. 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

10. GEP-2γ at JEFFERSON LAB HALL C
Key idea:
fixed Q2
same spin transport
same analysing power. e e’ p
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’
θp, deg θe
ε range
<Q2>
1.867
2.068
0.527
14.13
106
2.49
2.839
1.507
30.76
45.3
3.549
2.207
35.39
32.9
80μA beam current
85% pol.
20cm LH target

11. DETECTORS Focal Plane Polarimeter FPP Analyser 2 FPP DC 1,2 Analyser 1
HMS DC 1
HMS DC 2
FPP DC 1,2
FPP DC 3,4
Focal Plane Polarimeter FPP
2 analyzers in serie
2 sets of drift chamber pairs
Electromagnetic Calorimeter BigCal
1744 channels

12. KINEMATIC FORMULA Proton momentum calculated from the electron angle:
with
Proton momentum calculated from the proton angle:
HMS proton momentum:
and p0 = GeV/c

13. ELASTIC EVENTS SELECTION
Green: (θe - θp) and (φe- φp) cut
pθp
pθe
(PHMS-pθe)/p0
Black: all events
pθp
pθe
(PHMS-pθe)/p0
Q2 = 2.49 GeV2
ε = (smallest)
Ebeam = GeV
p0 = GeV/c
Blue: normalized background
pθp
pθe
(PHMS-pθe)/p0
Red: background
pθp
pθe
(PHMS-pθe)/p0
σ = 0.28%
σ = 0.22%
σ = 0.18%
Bgd=3.2%
Bgd=3.3%
Different shapes of the elastic peaks
Ratio elastic/inelastic changes with
the kinematic variables
Upper estimate of the background of 3.2% with ±3σ cuts
Need a Monte-Carlo simulation to fit the
background shape

14. 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.

15. 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