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Two-photon Exchange as a Laboratory for Nucleon Structure Studies

Two-photon Exchange as a Laboratory for Nucleon Structure Studies. The entire response of the nucleon is contained in the two-photon box. A y : Measure the target SSA using quasi-elastic scattering from vertically polarized 3 He. E05-015 ,

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Two-photon Exchange as a Laboratory for Nucleon Structure Studies

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  1. Two-photon Exchange as a Laboratory for Nucleon Structure Studies • The entire response of the nucleon is contained in the two-photon box. • Ay: Measure the target SSA using quasi-elastic scattering from vertically polarized 3He. • E05-015, Ayn can only be non-zero through two-photon exchange. GPD’s predict Ayn ~ -2% at Q2=1 GeV2. • E08-005, Ayn ~ 50% expected at Q2 ~ 0.2 GeV2 due to nuclear FSI. γ γ T. Averett, J. P. Chen, D. Higinbotham, X. Jiang, V. Sulkosky The Hall A and Polarized 3He Collaborations, Thesis students: Yawei Zhang, Ellie Long

  2. Data Quality (from Yawei Zhang) Q2=0.13 GeV2 Pion rejector1 Pion rejector2 Cerenkov ADC channels Q2=0.46 GeV2 Pion rejector1 Pion rejector2 Cerenkov ADC channels Q2=0.97 GeV2 Pion rejector1 Pion rejector2 Cerenkov ADC channels

  3. Raw asymmetries are consistent between LHRS and RHRS Ayraw = 0.108% +/-0.012% Ayraw = -0.107% +/-0.012% Left arm Right arm Q2=0.96 (GeV/c)2

  4. E07-013 Ay in the Deep-Inelastic Region • Inclusive DIS: Measure the target SSA using deep-inelastic scattering from vertically polarized 3He (neutron). • AyDIS vanishes in the Born approximation. Two-photon exchange produces a small, but non-zero asymmetry in the constituent quark and other partonic models • Unlike QE, the DIS asymmetry is expected to be ~10-4 (Afanasev, Strikman, Weiss, PRD) due to contributions beyond leading-twist. • Proton measurement at SLAC found zero asymmetry with uncertainty of 10-2. • JLab E07-013 measured 2 points at Q2=1-3 GeV2 to the 10-3 level; a factor of 10 improvement in uncertainty. • Large backgrounds and small number of photoelectrons in BigBite cerenkov detector prevented having a clean trigger and higher statistics. T. Averett, R. Gilman, T. Holmstrom, X. Jiang The Hall A and Polarized 3He Collaborations, Thesis student: Joseph Katich—William and Mary

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