Following the discovery of a discrepancy between nucleon formfactors extracted from polarization transfer methods and the traditional (single photon) Rosen-bluth formula, two-photon exchange processes have come under scrutiny We consider here the production of a lepton-antilepton pair by real photons oft 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-* 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.
展开▼
