1. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 1 Spin Dance Review Joe Grames JLab Parity Workshop October 26, 2001 Priorities for electron polarimeters Issues facing 1% polarimetry Spin Dance 2000 Results

2. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 2 What priorities must Jlab electron beam polarimetry serve? keV - MeV region for sources and/or nuclear parity experiments GeV for parity and nucleon spin structure experiments Future experiments are heading toward 1% absolute polarimetry Eager users (and the broader community!) What are desirable (necessary) features? Large, precisely known analyzing power Designs with reduced sensitivity to known systematics Techniques that do not disturb the experiment (non-invasive) High luminosity to rapidly achieve small statistical uncertainty N =·P b 2 ·P t 2 ·A 2 P b PbPb 2

3. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 3 The JLab Electron Polarimeters

4. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 4 Polarimeter analyzing power: A tot 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: Mott (Coulomb screening, finite nuclear size, multiple scattering) Compton (energy resolution, radiative background) Moller (detector acceptance, Levchuk effect, target polarization)

5. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 5 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 - 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

6. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 6 Polarized Source & Spin Precession The polarized electron beam is produced by photoemission from a strained GaAs crystal held at a potential of -100 kV using circularly polarized light of the appropriate wavelength. Spin precession accumulates in the transport arcs. At 6 GeV the total precession exceeds 10,000 deg. spin = ·· bend 2 ( g-2 )

7. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 7 Why call it a spin dance ?. P meas sin( Wien + ) Measuring the beam polarization at various Wien angles determines both the spin polarization and direction. To offset the spin precession in the accelerator we use a Wien filter near the electron gun to set ( Wien ) the beam polarization direction.

8. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 8 Source Strained GaAs photocathode ( = 850 nm, P b >75 %) Accelerator 5.7 GeV, 5 pass recirculation Spin Dance 2000 Experiment

9. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 9 Relative Analying Power Results P meas sin( Wien + )

10. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 10 Uncertainties are based on statistics and do not include any systematics. Polarimeters of 3 types (Mott, Moller, Compton) indicate agreement. Relative Analyzing Powers Compared P meas normalized to Mott for reference

11. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 11 Spin Based Energy Measurements Why consider measuring the beam energy by spin precession when precise (~10 -4 ) alternative methods exist and are quick (1-2 hours)? Answer 1: large cummulative spin precession (10,000 degrees) between the injector and end-stations yields 10 -4 to 10 -5 relative uncertainty and is sensitive to accelerator setup (injector energy, linac gradients, bend angles). Answer 2: small precession (500 degrees) between end-stations at final beam energy yields ~10 -3 relative uncertainty but requires only knowledge of end-station bend angles. This information comes essentially free at the cost necessary to intercompare the electron polarimeters. It provides two independent methods which can be used for cross-calibration and are sensitive to the accelerator where the others are not.

12. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 12 Energy Results Summarized +35 MeV -35 MeV

13. Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 13 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 Polarimeters often disagree at a level larger than quoted systematics, particularly beyond the level hoped for by users. A spin dance experiment aims to intercompare polarimeter analyzing powers by using a common test beam to improve polarimeter systematics. Spin Dance 2000 indicates agreement of independent polarimeter techniques (Mott, Moller, Compton), but also demonstrates systematic differences. Spin based energy measurements at Jlab are high precisision, are sensitive to accelerator parameters, and are a by-product of the polarimeter comparison. Often, the polarimeter is simply viewed as the tool. To reach the 1% absolute mark the polarimeter must be the experiment, not the tool. Conclusions