Two-Photon Effects in Lepton-AntiLepton Pair Photoproduction from a Nucleon Target using Real Photons

We consider the production of a lepton-antilepton pair by real photons off a hadronic target. The interference of one and two photon exchange amplitudes leads to a charge asymmetry term that may be calculated explicitly in the large-t limit in terms of hadronic distribution amplitudes. A rather compact expression emerges for the leading order asymmetry at fixed angle in the centre-of-mass of the lepton pair. The magnitude appears sizeable and is approximately independent of the pair mass in the asymptotic limit. Measurement of the nucleon’s electric and magnetic form factors has traditionally been based upon the well-known Rosenbluth formula[1] which assumes that scattering occurs through the exchange of a single photon. A recent extraction of GpE/G p M in the Q 2 range from 0.5 to 5.6 GeV 2 has used the polarization-transfer technique exploited at Jefferson Laboratory[2]. This method, which does not use the Rosenbluth separation, revealed a large discrepancy with previously published form factors. Subsequently, a close scrutiny was made of two-photon exchange effects in elastic electron-proton scattering. In this process a virtual photon knocks the incident proton into