A study of systematic uncertainties of Compton e-detector at JLab, Hall C and its cross calibration against Moller polarimeter APS April Meeting 2014 Amrendra Narayan Department of Physics & Astronomy Mississippi State University, MS (for Hall C Compton Team)

Compton polarimeter overview Exp. ParameterValue Beam Current 180  A Beam Energy1.16 GeV Laser Wavelength532 nm Cavity Power~ 1.7 kW Chicane bend angle10.1 deg Max. e-Displacement17 mm Compton edge energy46 MeV Laser Table Diagram: Donald Jones (UVA) 2 laser power laser cycle

Compton Asymmetry 3 theoretical asymmetry  (=E  /E  max ) measured asymmetrydetector strip number

Compton: Statistical Precision 4 polarization (%) Entries 179 Mean 0.60 RMS 0.17 Polarization from over 200 hour of electron detector data plotted against Compton run # (The figure shows only statistical error) Histogram of the statistical error in above runs

Simulating Compton scattering  The Compton, background and noise events are simulated using a GEANT3 based model of the experimental conditions  The simulation output is analyzed in the same way as the experimental output  The data analysis tools when applied to the output of the Compton simulation, reproduces the input electron beam polarization to within 0.3 %  Using the FPGA modelling toolkit - MODELSIM, we have simulated the e-detector DAQ and are able to reproduce the measured data collection rates 5

Compton: Systematic Uncertainty Major categories: 1.Detector 2.Data Acquisition 3.Laser and electron beam 4.Analysis 5.Others 6 detector strip efficiency ~ 70% secondary electrons position and orientation  P/P(%) ~ 0.12 % trigger noise deadtime  P/P(%) ~ 0.21 % beam energy laser polarization overlap spot size beam charge asymmetry spin precession through chicane dipole fringe field  P/P(%) ~ 0.13 % * preliminary values * * *

Compton: Systematic Uncertainty Major categories: 1.Detector 2.Data Acquisition 3.Laser and electron beam 4.Analysis 5.Others 7 detector strip efficiency ~ 70% secondary electrons position and orientation  P/P(%) ~ 0.12 % trigger noise deadtime  P/P(%) ~ 0.21 % background subtraction radiative correction helicity correlated energy difference helicity correlated position difference helicity correlated angle difference  P/P(%) = 0.13 %

8 Ideal case : 100% efficiencyRandom efficiency between 0 – 100% The change in polarization due to inefficiency is within statistical uncertainty Detector inefficiency

Simulating DAQ 9 Reproducing the experimental DAQDeadtime: effect of signal rate sim / input rate Except for a few outliers, we could reproduce the experimental response in all strips The signal rate is multiplied by a factor of ‘rate multiplier’. We find an increase in the input rate increases the ratio of lost signals

Systematic uncertainty 10 Systematic UncertaintyUncertainty  P/P (%) Plane-1 Laser polarization0.1%0.1 Plane-to-planeSecondary electrons0.0 Dipole field strength( T)0.01 Beam energy1 MeV0.08 Detector longitudinal position1 mm0.03 Detector rotation (pitch)1 degree0.03 Detector rotation (roll)1 degree0.02 Detector rotation (yaw)1 degree0.04 Detector trigger1/3 – 3/3< 0.19 Detector efficiency0-100%< 0.10 DAQ dead time Detector noiseUp to 0.2% events< 0.10 Fringe field(100%)0.05 Radiative correction20%0.05 Beam position & angle at IP Background subtraction HC position & angle differences HC energy differences Charge asymmetry Spin precession through chicane Total 0.29

Cross Calibration: Challenges MollerCompton Low current High current Invasive measurement Non-invasive measurement Needs beam to be reestablished Measurement is continuous 11 High current Moller measurement causes target heating (depolarization) and increased random coincidences Low current Compton measurement will have very low statistics and will be very sensitive charge normalization and background subtraction => 4.5 uA was chosen as the optimal for the Moller-Compton-Moller cross calibration runs

Moller Polarimeter 12 Detect the scattered & recoil electrons Flip beam spin to measure asymmetry: A meas. ~ P Beam x P Target A M ø ller *image courtesy: Josh Magee pure QED process, well understood

Moller – Compton -Moller 13 Polarization recorded in the two Polarimeters in chronological order. *statistical + fixed (0.6%) systematic uncertainty Low current cross calibration runs shown with adjacent regular high current runs *statistical + fixed (0.6%) systematic uncertainty Compton run number Polarization (%)

14 Summary Conclusion: We found that the Moller and Compton polarimeters were consistent with each other at 4.5 uA We are well within reach of getting systematic uncertainty contained to < 1 % for the independent polarization measurement by the Compton electron detector This work was supported by U.S. DOE, Grant Number: DE-FG02-07ER41528 I sincerely thank Josh Magee (College of William and Mary) for his help with information regarding Moller polarimeter Acknowledgement

Moller Systematics 15

Extra slides 16 A (electronic) noise event can: dilutes the asymmetry can cause loss of a true e-event increase the deadtime The detector inefficiency : varies randomly across all strips results in loss of signal can potentially bias the trigger The Monte Carlo simulation with only Compton electrons hitting 100% efficient strips corresponds to the ideal case and yields us the adjacent ideal asymmetry fit with Pol% = 84.7 ± 0.2