Below you'll find a little introduction to two-photon physics - porbabely more than you ever wanted to know about it.
A photon can, within the bounds of the uncertainty principle, fluctuate into a charged fermion/ anti-fermion pair, to either of which the other photon can couple. This fermion pair can be leptons or quarks. In the latter case, we distinguish several cases:
While the cross-section of annihilation falls with the center-of-mass energy, the cross-section of the photon scattering rises logarithmically ( green curve)
When such a scattering process takes place, the electrons that emmited the photons get scattered out of their trajectory. If this scattering angle is large enough, the electron is "seen" inside the the detector, and is called a tag.
The momentum transfer or the virtuality of the photon is expressed as . The scaling variable tells us what fraction of the photon momentum was carried by the struck fermion inside the photon. P^2 is the virtuality of the target photon and is very small. W is the invariant mass of the hadrons coming from the interaction.
So, this is what a typical event looks like in the Opal detector: A tagged electron in the Forward Detector (right side) and some hadrons from the gamma-gamma collision.
The cross section of the process can be written in terms of the photon structure functions: