Published on November 23, 2007
Slide1: Spin Dance 2000 Measurement Joe Grames Hall A Workshop December 10, 2002 Slide2: 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 Slide3: 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 Slide4: 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 Polarimeter Analyzing Power Slide5: 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 Slide6: Jlab Electron Polarimeters Slide7: The Role Spin Rotation Plays Slide8: A “spin dance” example… The measurable component of the beam polarization depends upon the polarimeter, and its design. Pmeas sin(hWien + f) The Wien angle varies the measurable component of the beam polarization. Slide9: Source Strained GaAs photocathode (l = 850 nm, Pb >75 %) Accelerator 5.7 GeV, 5 pass recirculation The Experiment Slide10: Spin Dance 2000 Results Pmeas sin(hWien + f) Slide11: 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 Pmeas normalized to Mott for reference Slide12: Spin Based Energy Measurements Slide13: Hall B 35 MeV shift? DY = -2.9° or DQ = -0.22° Slide14: 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 Slide15: 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...