1. THE NEW FOCAL PLANE POLARIMETER
IN HALL C
HALL C USERS MEETING, January 18, 2008
Mehdi MEZIANE, the College of William and Mary

2. OUTLINE INTRODUCTION ROSENBLUTH SEPARATION
POLARIZATION TRANSFER METHOD
HMS TRIGGER
FOCAL PLANE POLARIMETER (FPP)
FPP DRIFT MAPS
FPP EFFICIENCY
CONCLUSION

3. Two methods to obtain the form factors of the proton:
INTRODUCTION
Two methods to obtain the form factors of the proton:
Rosenbluth separation of cross section data which gives
and
Measurement of the
ratios using the recoil polarization
technique
The results of these two techniques are incompatible above a of

4. ROTHENBLUTH / TRANSFERT
POLARIZATION DATA
Recoil polarization and Rosenbluth
ratios are clearly
different.
Difference increase systematically
with Q2.

5. ROSENBLUTH SEPARATION
Cross section:
with
Jlab experiment :
I.A. Quattan et al., Phys. Rev. 94,
(2005).

6. POLARIZATION TRANSFER METHOD
The transferred polarization is:
with
Independent of : the analysing power
the beam polarization

7. ASYMMETRIES
Measure asymmetry distribution after rescattering in the analyser
For 2 helicities of beam (±1) with polarization Pe , relative asymmetry in
the polarimeter is:
With
and
the physical polarizations at the FPP
, the analysing power
, the azymuthal scattering
anlge

8. ASYMMETRIES
The physical asymmetries are calculated from the difference distributions:
φ (°) Di

9. HMS TRIGGER NEW HMS trigger HMS S2 detector is physically REMOVED
Use of HMS S1 detector and a new S0 detector
HMS S2 detector is physically REMOVED
S0 is divided in two parts: S0X1 and S0X2 S1
S0X1
S0X2
S1&S0X1
HMS hut trigger
S1&S0X2
Trigger
Supervisor
&

10. S0 DETECTOR 4 PMTs 2 scintillators 2 wave shifters
Central trajectory
Ebeam = GeV
Ebeam = GeV
Ebeam = GeV
Upper part: S0X1 (part of
inelastic)
Lower part: S0X2 (elastic)

11. HMS TRIGGER NEW HMS trigger HMS S2 detector is physically REMOVED
Use of HMS S1 detector and a new S0 detector
HMS S2 detector is physically REMOVED
S0 is divided in two parts: S0X1 and S0X2 S1
S0X1
S0X2
S1&S0X1
HMS hut trigger
S1&S0X2
Trigger
Supervisor
&

12. Focal Plane Polarimeter (FPP) in Hall Cu x v
4 Chambers with 3 planes each
12 planes of detection
Active area 134*166 (cm2)
2 split CH2 analysers
145 *111* 60 (cm3)
Gas mixture Argon50%/Ethane50%
Operating HV 2350V (field and
cathode wires)

13. Focal Plane Polarimeter (FPP) in Hall C
HMS Drift Chambers:
S0 and S1
CH2 analysers
FPP Drift Chambers

14. Focal Plane Polarimeter (FPP) in Hall C

15. FPP Read Out System twisted pair ribbon cables (1 for 2 cards)
Amplifier/read out cards
1 card for 8 sense wires
ECL logic signals
37 cards per chamber TOTAL of 148 cards
VME based F1-TDC

16. FPP Drift Maps Mapping this distribution Time/distance
Background window
Mapping this distribution
into a flat distance distribution
Time/distance
correlation
Drift Time
+ Subtracting background

17. FPP efficiency: Straight Through
At the beginning of each kinematic 1 hour of straight through data
(WITHOUT analyser) are taken.
The efficiency in FPP1,2 is:
FPP 1, 2 refers to chamber pair A&B and C&D respectively
ε = 86% for FPP1
ε = 85% for FPP2

18. HMS/FPP 2D correlation cm cm Chamber A cm Chamber C cm
Predicted drift distance based on HMS and FPP wire hit information VS
Calculated FPP drift distance

19. FPP efficiency: N5hits / N6hits
The efficiency ε per plane is given by:
> 98.6%
with the ratio:
(6 planes of
detection)
The efficiency per pair of chambers is then :
~ 92%
Then the efficiency for 5 OR 6 planes hit is :
> 99.7%

20. FPP 6 planes efficiency evolution
1) Back to the nominal
HMS threshold
6.8V
5.5V
2) HMS/FPP gas
problem fixed

21. FPP1,2 ASYMMETRIES, Q2 = 2.5 GeV2 Di Q2 = 2.5 GeV2 Ebeam = 3.539 GeV
Di vs φ
φ (°)
Q2 = 2.5 GeV2
Ebeam = GeV
θe = 32.9°
24 M triggers

22. GEP 2γ ERRORS

23. CONCLUSION New HMS trigger with a new S0 detector
Good drift maps computation
FPP Straight through efficiency of 85%
5 OR 6 plane FPP efficiency above 99.7%