1. G0 Beam Polarization T. Horn, D. Gaskell Jefferson Lab
Absolute Polarization at 362 MeV
Mott Analysis and Background Subtraction
G0 collaboration meeting
October 2007

2. Mott/Moller Measurements
Polarization measured by Moller and Mott except at lowest energy
Absolute Polarization at 362 MeV?
Systematic difference between Moller/Mott
Seems to have increased from ~2% to ~4% since 2001
Energy
Time Period
Type
687
Sp 2006
F2006
Sp2007
Mott
Moller
362
Su 2006
Mott only

3. Mott Analysis – Issues Asymmetries determined from ADC spectrum
Mott spectra for detectors used in polarization calculation
Asymmetries determined from ADC spectrum
No gain calibrations
Coarse binning
Resolution of one detector worse by ~50% compared to the rest
Increased sensitivity to potential background contribution
Got worse over time

4. Mott Analysis Method Checks
Event selection depends on the elastic peak location
Mott Analyzer:channel with maximum number of counts
Improved method: determine from a fit to the data for each run
Not a very large effect on the consistency
still coarse binned

5. Mott Analysis Method Checks
Asymmetry typically calculated from an integration from centroid location to infinite sigma
Another test: restrict integration to ±1σ around peak
Here, fit driven by a few outliers

6. Mott Background Contribution
Asymmetry under elastic peak suggests background contribution may be significant
How to quantify?

7. Mott Background Contribution
Introduce a simple, but stable model that under-predicts the background, but is consistent between detectors with good and bad resolution
“No background subtraction” model
Estimate an “upper limit” of what a full background subtraction would give
Straight line background subtraction
One scenario – not final

8. Mott Background Determination
To determine the absolute background subtracted polarization use a GEANT simulation of the Mott
In progress
Other cross check with elastic data from Hall A/LEDEX
Results expected earliest next year