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

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

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

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

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

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

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

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