A study of systematicuncertainties 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. Parameter ValueBeam Current 180 A

Beam Energy 1.16 GeV

Laser Wavelength 532 nm

Cavity Power ~ 1.7 kW

Chicane bend angle 10.1 deg

Max. e-Displacement 17 mm

Compton edge energy 46 MeV

Laser Table Diagram: Donald Jones (UVA)

2

lase

r po

wer

laser cycle

3

Compton Asymmetryth

eore

tica

l asy

mm

etry

(=E/Emax

)

QEDe APPNN

NNA

exp

me

asu

red

as

ymm

etry

detector strip number

4

Compton: Statistical Precisionpo

lariz

atio

n (%

)

Entries 179Mean 0.60RMS 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

6

Compton: Systematic Uncertainty

Major categories:

1. Detector

2. Data Acquisition

3. Laser and electron beam

4. Analysis

5. Others

• 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

*

*

*

7

Compton: Systematic Uncertainty

Major categories:

1. Detector

2. Data Acquisition

3. Laser and electron beam

4. Analysis

5. Others

• 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% efficiency Random efficiency between 0 – 100%

The change in polarization due to inefficiency is within statistical uncertainty

Detector inefficiency

9

Simulating DAQ

Reproducing the experimental DAQ Deadtime: effect of signal rate

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64-30

-20

-10

0

10

20

30

detector strip

𝑒𝑥𝑝𝑡−

𝑠𝑖𝑚

𝑒𝑥𝑝𝑡

(%)

sim

/ in

put r

ate

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 Uncertainty Uncertainty DP/P (%) Plane-1

Laser polarization 0.1% 0.1

Plane-to-plane Secondary electrons 0.0

Dipole field strength (0.0011 T) 0.01

Beam energy 1 MeV 0.08

Detector longitudinal position 1 mm 0.03

Detector rotation (pitch) 1 degree 0.03

Detector rotation (roll) 1 degree 0.02

Detector rotation (yaw) 1 degree 0.04

Detector trigger 1/3 – 3/3 < 0.19

Detector efficiency 0-100% < 0.10

DAQ dead time

Detector noise Up to 0.2% events < 0.10

Fringe field (100%) 0.05

Radiative correction 20% 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

11

Cross Calibration: Challenges

Moller Compton

• Low current • High current

• Invasive measurement • Non-invasive measurement

• Needs beam to be reestablished

• Measurement is continuous

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

12

Moller Polarimeter

eeee

Detect the scattered & recoil electrons

Flip beam spin to measure asymmetry: Ameas. ~ PBeam x PTarget AMøller

*image courtesy: Josh Magee

pure QED process, well understood

13

Moller – Compton -Moller

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

Compton run number

Pola

riza

tion

(%)

Pola

riza

tion

(%)

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

15

Moller Systematics

16

Extra slides

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