VSAT Two-Photon Physics.

Generally, VSAT can deal with types of gamma-gamma (two-photon) analysis: no tag, single tag, double tag. Word tagging (which give a name to VSAT - Very Small Angles Tagger) means a hit in one VSAT modules. These types can be explained like: no tag event should have no hits in VSAT, single tag - one hit in one module, double tag means two hits in two VSAT modules.

The Single Electrons events are the main background process for one tag analysis. Also Bhabha and False Bhabha events represent the backroung processes for double tag analysis.

The two-photon collision events have main physcical types: hadronic events (there gamma-gamma collision create a number of hadrons which made a visible tracks in others DELPHI detectors) and leptonic events (there gamma-gamma collision makes a pair of leptons). The leptonic process has around 10 times higher cross sections then hadronic but it is more difficult to study.

More detailed review of the two-photon events.