Two photon exchange and transverse spin asymmetries in the A4 experiment

The A4 collaboration at the MAMI facilities has measured the transverse spin asymmetry in the elastic and quasi-elastic scattering of transverse polarized electron on proton and deuteron, respectively, at backward angles and at a four momentum transfer of Q2 = 0.10 (GeV/c)2, Q2 = 0.23 (GeV/c)2 and Q2 = 0.35 (GeV/c)2. This beam transverse spin asymmetry is sensitive to the imaginary part of the two photon exchange amplitude in the elastic-nucleon scattering. These measurements are compared with the model calculation of the imaginary part of the two photon exchange amplitude that includes all the intermediate hadronic states using the unitary isobar model for pion photo- and electroproduction on the nucleon to evaluate the transition amplitudes, called MAID.