Transcript Spin Dance 2000 Measurement Joe Grames Hall A Workshop December 10, 2002 Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for.

Spin Dance 2000 Measurement
Joe Grames
Hall A Workshop
December 10, 2002
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Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
Jefferson Lab Spin Dance 2000 Collaboration
Jefferson Lab - E. Chudakov, H. Fenker, A. Freyberger, J. Grames, J. Hansknecht,
J. Mitchell, M. Poelker, C. Sinclair, M. Steigerwald, M. Tiefenback
CEA Saclay, DSM/DAPNIA/SPHN - C. Cavata, S. Escoffier, F. Marie, T. Pussieux, P. Vernin
Florida International University - R. Nasseripour, B. Raue
Karkov Institute of Physics and Technology - V. Gorbenko
Massachusetts Institute of Technology - D. Higinbotham, R. Suleiman
North Carolina Ag. and Tech. State University - S. Danagoulian
Old Dominion University - V. Dharmawardane
University of Virginia - R. Fatemi, K. Joo, M. Zeier
Universitaet Bonn - T. Reichelt
Vrije Universiteit - B. Zihlmann
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Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
Overview
Present and proposed experiments at the world’s polarized electron
accelerators invest considerable resources to design, construct, and
operate polarimeters to measure the beam polarization.
While experiments using polarized targets or recoil polarimetry
do not generally require high precision electron polarimeters, this
is not the case with parity violation experiments.
Some of the planned parity experiments desire absolute knowledge
of the beam polarization at the 1% level.
• Some discussion of Jefferson Lab’s polarimeters
• Spin Dance 2000 measurement
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Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
Polarimeter Analyzing Power
e = Atot · Pb =
N=
dPb
Pb
N+ - NN+ + N-
2
Pb ~ 70%
Atot ~ 40%
Target: 1mm Au
-1
·(Pb·Pt ·A) 2
What are desirable (necessary) features of electron polarimeters?
• Large total analyzing power
• Designs with reduced sensitivity to major systematics
• High luminosity to rapidly achieve small statistical uncertainty
• Non-invasive continuous measurement does not disrupt experiment
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Thomas Jefferson National Accelerator Facility
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Why is Atot difficult to know?
Precise knowledge of the analyzing power is limited.
Atot is not a directly measured quantity:
• measurement requires difficult double-scattering experiments
• the analyzing power is determined by theory and simulation
Factors that affect knowledge of the total analyzing power
• inferred target polarization
• detector acceptance
• multiple scattering
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Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
Jlab Electron Polarimeters
Polarimeter
Target Ptarget
I ave
Px Py Pz
ZA
5 mA x
Hall A Moller

e
> 99% 100 mA
~8%
2 mA x
Hall B Moller
e
~8%
5 nA
Hall C Moller
e
~8%
2 mA
Injector Mott
Hall A Compton
x
x
x
x
x
x
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Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
The Role Spin Rotation Plays
jspin =  ·
( g-2 )
· qbend
2
x
z
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Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
A “spin dance” example…
The measurable component of
the beam polarization depends
upon the polarimeter, and its
design.
The Wien angle varies the
measurable component of the
beam polarization.
Pmeas sin(hWien + f)
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Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
The Experiment
Source
Strained GaAs photocathode (l= 850 nm, Pb >75 %)
Accelerator
5.7 GeV, 5 pass recirculation
Polarimeter
Injector Mott
Hall A Compton
Hall A Moller
Hall B Moller
Hall C Moller
Thomas Jefferson National Accelerator Facility
I ave
Px P y
2 mA x x
70 mA
1 mA x
10 nA
x
1 mA
Pz
x
x
x
x
Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Spin Dance 2000 Results
Pmeas sin(hWien + f)
Polarimeter
f (deg)
Hall A Compton
10984.2  0.8
Hall A Moller
10983.9  0.7
Hall B Moller
10500.4  0.6
Hall C Moller
10023.0  0.7
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Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
Relative Analyzing Powers Compared
• Only statistical uncertainties used to reveal systematic uncertainty.
• Polarimeters of 3 types (Mott, Moller, Compton) agree.
• Uncertainty in Wien angle induces < 0.2% relative effect.
Pmeas normalized to Mott for reference
Thomas Jefferson National Accelerator Facility
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Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
Spin Based Energy Measurements
2mec2
DY
E = g -2 ·
DQ
e
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Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
Hall B 35 MeV shift?
DY = -2.9°
or
DQ = -0.22°
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Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
Conclusions
Jefferson Lab has started down the path of developing high
precision polarimetry, which presently can be inferred only
by intercomparison of different polarimeters with different
systematics using a beam known to have the same polarization
for all the polarimeters.
The Spin Dance 2000 experiment:
• First high precision comparison between Mott, Moller, and Compton
• All five polarimeters do not agree within quoted systematic uncertainty
• Three different polarimetry techniques (Mott, Moller, Compton) agree
• Revealing systematic differences is a first step toward understanding them
Often, the polarimeter is viewed only as the tool, but to
reach the 1% mark it must continue to be the experiment.
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Thomas Jefferson National Accelerator Facility
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One last remark...
Part of a letter by L.H. Thomas to Goudsmit (25 March 1926). Reproduced from a
transparency shown by Goudsmit during his 1971 lecture. The original is
presumably in the Goudsmit archive kept by the AIP Center for History of Physics.
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Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy