Presentation on theme: "A Measurement of the Target Single-Spin Asymmetry in Quasi-Elastic 3 He (e, e) Joe Katich for E05-015 and the Hall A Collaboration Two-Photon Physics World."— Presentation transcript:
A Measurement of the Target Single-Spin Asymmetry in Quasi-Elastic 3 He (e, e) Joe Katich for E05-015 and the Hall A Collaboration Two-Photon Physics World Data for A y Polarized 3 He Target E05-015 and Nucleon Modeling Why is A y Important?Expected Results for A y The study of nuclear structure relies on knowledge of form factors, previously measured using the Born Approximation (B.A.). This assumes one photon exchange, with multiple photon exchange neglected. As new precision data becomes available, the contribution of two-photon exchange can no longer be ignored. Two different methods of measuring p G E p /G M p, both using the B.A., give in conflicting results. Theory shows that taking two-photon exchange into account brings the results into agreement. E05-015 will make use of a new kind of hybrid target cell that uses both rubidium and potassium to polarize the 3 He using spin-exchange optical pumping. This allows for a much shorter polarization time, as well as record high polarizations of >50%! E05-015 will measure A y at Q 2 =0.5 & 1.0 GeV 2 For inclusive scattering, A y is exactly zero in the Born approximation. However, it receives a non-zero contribution from the interference between the real single- photon exchange amplitude and the imaginary part of the two-photon exchange amplitude ~90% E05-015 will consist of an unpolarized beam scattering from a 3 He target that is polarized normal to the electron scattering plane. By flipping the target polarization every 20 minutes we form the single-spin asymmetry (SSA), A y. The 2-photon exchange contains the entire response of the nucleon. The elastic response is well-known and gives a non-zero asymmetry. The inelastic response is not well-known. Polarized 3 He is effectively a polarized neutron target: ~8%~2% are complex functions of and Q 2 and contain information about nucleon structure. In the Born Approximation, the usual electric and magnetic form factors are recovered: e- n n The two main goals of A y are to: 1. Make a precise non-zero measurement of A y 2. Investigate the inelastic response of the nucleon and compare this to the predictions of existing nucleon models. are complex contributions to the nucleon form factors from multi-photon exchange. Hadronic physics relies heavily on models to accurately explain what is going on inside the nucleon. A measurement on A y would serve as an excellent tool to test the validity of several of these nucleon models. Generalized Parton Distributions (GPDs) GPD Handbag Diagram GPDs are used to model the nucleon response. In this, and other partonic models, it is assumed that only one quark interacts with the lepton…but how valid is this assumption? MAID Model The MAID model is a phenomenological fit to pion photoproduction data and is valid in the resonance region. The Jlab polarized 3 He target is being modified to accommodate the new hybrid cells and to allow for vertical polarization. Q 2 =1GeV 2 NIKHEF QE 3 He(e, e) at Q 2 =0.1 GeV 2 gave A y =-1.0+/- 5.4%
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