prex q2analysis - hallaweb.jlab.org
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PREX Q2Analysis
Kiadtisak Saenboonruang
University of Virginia
Hall A Analysis Workshop
December 8, 2010
PREX
“Pb Radius Experiment”
Experiment Overview• A clean measurement of the neutron skin of 208Pb through
parity violating electron scattering
• Run from March-June 2010
• For PREX(elastic scattering of 208Pb target),
� ��~0.009��
� spectrometer angle �~88 ���
�� Analysis• �� is the square of the four-momentum transferred from
the incident electron to the target proton
• �� � ��� � �������� �
• For elastic scattering, �, ��, � are related
• Four ways of calculating ��(elastic scattering)
1. �� � 2��� 1 � cos �2. �� � 2��"#�1 � cos � �3. �� � 2�′�"′#�1 � cos � �4. �� � 2 �� � ��� where m is target nucleus mass
where "# �'
'(�)
*��'+,-. / �
and "′# �'
'+�)0
*��'+,-. / �
Challenges in �� determination
For Eq 4: �� � 2 �� � ���, • � � ��~201• 2�~2001 and 2�′~2001• 2 � � �� ~2801 → 1400% uncertainty
THIS EQUATION WILL NOT WORK!!!
For Eq 1-3: �� � ������5���5����, ��
• 2�� � �678
692���:�
678
69�2�′��:�
678
6/2���
• 2�and2��contribute~0.002% uncertainty
• Most uncertainty is from �• Spectrometer angle from survey gives δ�~0.8mrad
•F78
78 � 2F/
/→ δ�� � 2%
• We need δ�� G 1% → 2� G 0.5%or0.4 ���
Pointing Measurement• A way to measure spectrometer angle with better accuracy
than survey
• Using the fact that;
� for different scattering angles, scattering energy
is different.
� for different mass, the same scattering angle, scattering
energy is different.
�� �� � �I�1 :
�I2JK
�
1 :2�LMN��
�2�
JK
How do we do pointing?With waterfall target (for several OP settings)
� Select events from central sieve slit’s hole
� Find the separation of 16O and 1H ground state energies
� Find central angle � using the least χ2 method
16O16O
1H
1H
Pointing Results • Angles between target and central sieve slit’s hole
• These angles give scattering energies to be better than 50 keV
• Given that we have 3 settings to use, 2�~351 → δ�~0.35 ���• 2�~0.4% → 2��~0.8% (within required 1% uncertainty)
LHRS 88.4 mrad
RHRS 86.1 mrad
P0 ∆R�STUT� ∆R�VWXYUXYZ� Difference
1.063 GeV 4.217 MeV 4.229 MeV 12 keV
1.058 GeV 4.185 MeV 4.220 MeV 35 keV
1.047 GeV 4.245 MeV 4.199 MeV 46 keV
0.0855
0.0860
0.0865
0.0870
0.0875
0.0880
0.0885
0.0890
Angle (rad)
Spectrometer
Analysis Values
Survey ValuesLHRS RHRS
Central Angle Comparison
Optics Reconstruction• With pointing measurement, we can have accurate �� for central hole
• Need to consider �� for the whole acceptance � Optics Reconstruction
• Optics Reconstruction
� Transform focal plane variables (x,y,θ,φ) to target variable (δ,y, θ,φ)
Note: 2 �[+[\
[\
Angle Reconstruction
LHRS
RHRS
Momentum Reconstruction• Crucial for pointing measurement (need to be better than 40 keV)
• Look at energy difference of 12C ground state and first excited state
Note: Nominal value is 4.44 MeV
Preliminary Q2 Analysis• Cut used
� Single hit
(“L.tr.n==1”)
� Trigger type
(“P.evtype ==1 or P.evtype==5”)
� Angle cut
(“abs(L.tr.tg_th)<0.07 and abs(L.tr.tg_ph)<0.07”)
� Particles hit PREX detector
(“P.loQadcL > pedestal and P.upQadcL > pedestal”)
Q2 Dependence on Trigger Type• 2 triggers used:
1. T1: Trigger above VDC planes
2. T5: S0 Trigger above PREX detectors
• By average, T5 gives Q2 ~0.2% higher than T1
LHRS RHRS
T1
T1
T1
T1
T1
T1
T5
T5
T5
T5
T5
T5
Q2 Dependence on Trigger Rate
LHRS RHRS
500kHz 150kHz
57kHz 9.3kHz
1.6kHz 0.3kHz
500kHz 150kHz
57kHz 9.3kHz
1.6kHz 0.3kHz
• VDC fails to perform for high rates
• In normal operation, VDC can work up to ~101]^/` �
• For PREX, the active area on VDC is 20 cm2
• For high rates, GEM will solve the problem
0.008800
0.009000
0.009200
0.009400
0.009600
0.009800
0.010000
0.010200
1 10 100 1000 10000 100000 1000000
Q2
Rate (Hz)
Q2 vs Rate
500 kHz
150 kHz
57 kHz9.3 kHz300 Hz
Gas Electron Multiplier (GEM)• Gaseous ionization detector
• Perform up to 10 MHz/cm2
• Resolution ~70b
VDC Projection on GEM Correlation
Q2 by Geometric Cut• To ensure the hit on PREX detector, cut on PREX detector’s ADC values
• However, for some runs, PREX detectors didn’t perform well� Geometric Cut
• Geometric cut: VDC projection on PREX detector and cut on the size of
the detector
• By average, Q2 by ADC cut is ~0.3% higher
Preliminary Q2 Results
0.0085
0.0087
0.0089
0.0091
0.0093
0.0095
0.0097
0.0099
0.0101
0 5 10 15 20 25
Q2
Run
Q2RHRS (whole)
LHRS (whole)
500 kHz
Thin Pb
150 kHz
• Q2 deviates in opposite direction can be due to the beam shifted by 0.5 mm
• Due to the low beam current, the fast feedback was turned off.
Date Target LHRS Run Q2 RHRS Run Q2
1 4/3/2010 Pb/D #3 27257 0.009292 - -
2 4/18/2010 Pb/D #3 27421 0.009361 6714 0.008775
3 4/18/2010 Pb/D #3 27422 0.009378 6715 0.008773
4 4/18/2010 Pb/D #3 27423 0.009345 6716 0.008771
5 4/18/2010 Pb/D #3 27424 0.010110 6717 0.009383
6 4/18/2010 Pb/D #3 27425 0.010120 6718 0.009380
7 4/18/2010 Pb/D #3 27426 0.009318 6719 0.008765
8 4/18/2010 Pb/D #3 27427 0.009358 6720 0.008772
9 4/25/2010 Pb/D #3 27506 0.009403 6740 0.008849
10 4/25/2010 Pb/D #3 27507 0.009324 6741 0.008799
11 4/25/2010 Pb/D #3 27508 0.009296 6742 0.008786
12 4/25/2010 Pb/D #3 27511 0.009336 6745 0.008770
13 5/7/2010 Pb/D #2 27610 0.009405 - -
14 5/19/2010 Pb/D #2 27697 0.009413 6823 0.008721
15 5/19/2010 Pb/D#2 - - 6824 0.008734
16 5/19/2010 Pb/D #2 27698 0.009340 6825 0.008725
17 5/19/2010 Pb/D #3 27700 0.009364 6826 0.008784
18 5/19/2010 Pb/D #3 27701 0.009371 6827 0.008787
19 5/19/2010 Pb/D #3 - - 6828 0.008782
20 6/4/2010 Pb/D #3 27722 0.009328 6839 0.008749
21 6/4/2010 Pb/D #3 27723 0.009334 6840 0.008747
22 6/12/2010 Thin Pb 27824 0.009260 6906 0.008796
23 6/12/2010 Thin Pb 27825 0.009250 6907 0.008793
24 6/12/2010 Thin Pb 27826 0.009242 6908 0.008795
25 6/18/2010 Pb/D #1 - - 6965 0.008828
Future Plans• GEM Analysis
• Explanation of the 1% fluctuation in Q2
• Table of errors