1. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 1 Spin Dance 2000 Measurement Joe Grames Hall A Workshop December 10, 2002

2. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 2 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 Jefferson Lab Spin Dance 2000 Collaboration

3. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 3 Overview Present and proposed experiments at the worlds 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 Labs polarimeters Spin Dance 2000 measurement

4. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 4 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 N= ·(P b ·P t ·A) 2 P b PbPb 2 Polarimeter Analyzing Power P b ~ 70% A tot ~ 40% Target: 1 m Au = A tot · P b = N + + N - N + - N -

5. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 5 Why is A tot difficult to know? Precise knowledge of the analyzing power is limited. A tot 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

6. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 6 Jlab Electron Polarimeters

7. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 7 The Role Spin Rotation Plays spin = ·· bend 2 ( g-2 ) x z

8. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 8 A spin dance example… The measurable component of the beam polarization depends upon the polarimeter, and its design. P meas sin( Wien + ) The Wien angle varies the measurable component of the beam polarization.

9. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 9 Source Strained GaAs photocathode ( = 850 nm, P b >75 %) Accelerator 5.7 GeV, 5 pass recirculation The Experiment

10. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 10 Spin Dance 2000 Results P meas sin( Wien + )

11. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 11 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. Relative Analyzing Powers Compared P meas normalized to Mott for reference

12. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 12 Spin Based Energy Measurements E = 2mec22mec2 g e - 2 ·

13. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 13 Hall B 35 MeV shift? = -2.9° or = -0.22°

14. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 14 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. Conclusions

15. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 15 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. One last remark...