a study of systematic uncertainties of compton e-detector at jlab, hall c and its cross calibration...
TRANSCRIPT
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 %
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Ideal case : 100% efficiency Random efficiency between 0 – 100%
The change in polarization due to inefficiency is within statistical uncertainty
Detector inefficiency
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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
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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
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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
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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
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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
(%)
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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
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Moller Systematics
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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