Two-Photon Exchange and Electromagnetic Proton Form Factors
  The presence of $2\gamma$-exchange in electron proton elastic scattering is discussed. From C-invariance and crossing symmetry, $2\gamma$ contribution induces a specific dependence of the reduced cross section on the variable $\epsilon$. No evidence of such dependence exists in the available experimental data. 
Egle Tomasi-Gustafsson