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GEp-2γ experiment
and the new JLab Hall-C Focal Plane
Polarimeter
Mehdi MezianeThe College of William & Mary
- APS Meeting April 14, 2008 -
On behalf of the JLab-Gep Collaboration
OUTLINE
• INTRODUCTION
• TWO PHOTONS EXCHANGE (TPEX)
• THE NEW FOCAL PLANE POLARIMETER IN HALL-C
• EXPECTED UNCERTAINTIES
• SUMMARY
Two methods, two different results
ROSENBLUTH AND POLARIZATION TRANSFER MEASURMENTS OF OF THE PROTON
• Recoil polarization and Rosenbluth
ratios are clearly
different in the Born approximation.
• Difference increase systematically with Q2.
pp ME GG /
Something beyond the Born approximation
Two photons exchange
• Either cross section or recoil polarization measurements to extract the proton form factor
THE GEP-2γ (04-019) EXPERIMENT AT JLAB HALL-C
• 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
• We look for a kinematical dependence of to detect a
pepe possible two photons exchange effect in the scatteringpp ME GG /
T matrix: )'()()'(2
2
pukukuQe
T p
MP
FGM
2~~
23~
MKP
F pu
MG~
2~F 3
~Fwith complexand,
In the Born approximation:
are form factors (real) ,~,,~ 2
22 QFQGM 22
2 , QFQGM and 0~3 F
The virtual photon polarization is:
TWO PHOTONS EXCHANGE (TPEX)
2~M
red
G
d
2~~
112 Y
GG
M
E
2
~~
1M
E
GG
red
t dP
2
122Y
MG~
MG~
EG~
MG~
1112
red
l dP
21
2Y
Polarization components:
The reduced cross section is:
MG
FQY ~
~
111
, 322
with
2Y 2YIn redd lP tPand the TPEX contribution enters as while in only as
For a non-zero contribution to the cross section at small the absolute
value of the TPEX contribution will change by a factor 1/ for tP
TWO PHOTONS EXCHANGE (TPEX)
BASIC PRINCIPLES OF THE EXPERIMENT
Measure the polarization of the recoil proton in the scatteringpepe
• In the Born approximation:
Two non-zero polarization components:2
Ml GP MEt GGP and
• Measurement of the ratio
By measuring the asymmetry distribution f after (re)scattering of the recoil proton in the analyser:
fpplyPhAf sin cossin fpp
tyPhA
ly
ty
PhA
PhA
M
E
GG
Analyser 1
Analyser 2
HM
S D
C 1
HM
S D
C 2
FPP
DC
1,2
FPP
DC
3,4
Focal Plane Polarimeter (FPP) in Hall-C
• 2 Chambers with 3 planes eachper polarimeter
u x vConsists in 2 polarimeters in series to increase the efficiency
12 planes of detectionActive area 134*166 (cm2)
Measure coordinates u, x, v
1164 instrumented wires
Need 3 tracking procedures (2 polarimeters and HMS)
• Gas mixture Argon50%/Ethane50%
• Operating HV 2350V (field and cathode wires)
Focal Plane Polarimeter (FPP) in Hall-C
CH2 Analysers
• 145cm *111cm *60cm
• Layered, split (L/R), 1 ton each
Open like a sliding door for straight through tracking
• Independently supported on interleaved frame
Focal Plane Polarimeter (FPP) in Hall-C
EXPECTED UNCERTAINTIES
Fixed Q2, fixed proton momentum
Same analysing power and HMS settings (spin transport) for the 3 kinematics
We measured the transferredpolarization component ratio Pt/Pl and relative Pl separately in ep elastic for a fixed Q2 of 2.5GeV2 at three different kinematics (epsilons) by changingthe beam energy.
Statistical errors onlyStatistics < 0.01 for a ratio of 0.7
LONGITUDINAL POLARIZATION TRANSFERT
The ε dependence of the form factor ratio mainlycomes from Pt
As Q2 is constant
Relative polarization have small systematicuncertainties ≈ 1% (Möller measurements)
Statistical (expected) and systematic errors
A. Afanasev et al.P.G. Blundel et al.
SUMMARY
• Absolute value of TPEX contribution changes by a factor 1/ for Pt
• A new detector: the HALL C focal plan polarimeter
• The form factor ratio ε dependence comes from Pt
• Statistics < 0.01 for a ratio of 0.7