Medium Energy (MEP) Group - Polarized 3He Target Experiement
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THeoretical Background:
The goal of this experiment is to acquire a unique set of values for the four parameters ( γ1, γ2, γ3, and γ4) that desribe the spin polarizability of the nucleon. The spin-polarizability of a nucleon is considered analogous to the Faraday effect in solids, where a material will rotate the direction of polarization of polarized light.
Two combinations of these parameters ( γ0 = γ1 - γ2 - 2γ4 and γπ = γ1 + γ2 + 2γ4 ) have already been measured; therefore, only two additional measurements need to be made to unique determine all the spin-polarizabliity parameters. To see where these parameters come from one can look at the Compton scattering cross-section at low energies :
where ω and ω' are the incident and scattered photon energies, M is the nucleon mass and T is the scattering amplitude. This scattering amplitude is written in terms of six functions (A1, A2, A3, A4, A5 and A6) that are combined according to their dependence on photon and target polarizations:
These six functions contain the nuclear and spin polarizablities. The first two terms do not contain any of the spin-polarizablity parameters; however, they are present in the last four terms at third order:
where Q is the charge of the nucleon and ? is the anomalous magnetic moment.
As stated before, two combinations of these polarizabilities have already been measured, therefore only two additional measurements need to be done to get a unique determination of the all the spin-polarizablilites. However, to do this one will require a circularly polarized photon source and polarized target . Fortunately, both will be available at HIGS in the near future.