1. COMPTON POLARIMETRY Collected data Cavity power Status on counting methods Systematic errors and hardware issues

2. Collected Data Good Runs E beam (GeV) k’ Max (MeV) int (%) A Max (%) 4 He~4502.761292.34.8 LH2~4503.181712.75.5

3. Cavity Power Laser power supply changed in June, tested OK: 1200W Down to 300W few days before the experiment Incident laser power still nominal Laser beam waist measured in agreement with simulation Alignment optimized Back to 500W after optics cleaning Slow exponential decay of cavity power seen during part of the run Cavity mirrors? Two finesse measurements so far, differ by a factor 2  to be done again…

4. Counting Analysis 650  m strips PbW04 4.5 mm gap Differential Semi-integrated

5. Calibration e - Detector E e-  Position of the Compton edge calibrates the detector and determine the vertical gap Y det. Can use the rate or the asym spectra. Discrepancy taken as syst error.  P e /P e ~ 2  Y/Y  Y=200  m   P e /P e = 3.5% @ 3 GeV E e-  Bdl, lever arm after D3 (~4.1m)

6. Calibration e - Detector Compton edge position sensitive to beam motion (coil pulsing)  Extracted Y det is always underestimated Comtpon+Background Background S/B

7. Calibration e - Detector Fit of the asymmetry spectra of the 4 planes is more robust because it uses information from all strips. Systematic error could be pinned down by simulation Stringent cross-check available on line…

8. Calibration cross-check The response function of the  detector is determined using the e - det as an energy tagger  reference run. When fitting the photon spectrum, corrects for gain drift and error in the e- det calibration.  expect =1 for the fit of the reference run itself -1 = 10 -4 !! Had to correct for the fact that  strip planes are 1 cm appart and use survey data for the lever arm after dipole 3 (4.11 m instead of 4.10)

9. Photon Analysis No dependence on the software threshold over a wide range  Systematics of the response function under control.

10. Comparison e - -  03/08/0519/09/05

11. Comparison e - -  01/11/0515/10/05

12. Laser Polarization

14. e - and  data are not compatible  origin found to be electronic pickup: ADC pedestal changes by 4 channels depending on the cavity state! Wasn’t there last year… Could be due to logical signals of cavity ON/OFF, Left/Right present in several electronic crates? Easy to test.

15. Laser Polarization After pedestal correction, all analysis are compatible at 1  level. Compton polarimetry is a tedious way to check pedestal correlations… The correction doesn’t affect the mean value, but reduces the Left Right discrepancy. Left laser polarization is ~0.6% lower than Right polarization?

16. Laser Polarization Laser /4 plates CIP Exit Line Transfer Function Monitoring of the laser polarization downstream the cavity

17. /4 Scans x x y y Wollaston Prism Integrating Sphere S1 Integrating Sphere S2  Incoming Polarization Ellipse TE pol. state TM pol. state Rotatable  /4 plate Slow 9.8 6 8.8 6 Stokes Parameters: P 0,P 1,P 2,P 3

18. Laser Polarization 30 /4 scans performed during helium run. We do find a 0.5% difference in laser polarization states but with opposite sign  Need to check the Left-Right definitions in both analysis… Accumulated cavity power: Left state = Right state at few % level (~500W)

19. Conclusion  Significant improvements in the counting analysis Best agreement between the two methods ever achieved On track for 2% accuracy or better.  Hardware issues: compton is getting old… Should revise all cabling at some point Need cross-talk study before G E n starts Numerous « end of run failed »  remove 600Hz part of the aquisition? Cavity power is low? How long before very low?

20. Systematic Errors SourceLast YearTo DoExpect. PP 0.70%Open cavity?0.7%-1% Resp. Func.1.25%Ana.+Simul.1.1% Dead Time1.00%-~0% Pile up1.00%Simulation0.5% Rad. Corr.0.25%-~0% TOTAL 2.0%1.4%-1.6% Expect to reduce syst. error as well because of better calibration. (3.5% for now) Counting photon Counting electron