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When electrons and positrons interact in LEP they radiate photons at small angles relative to the beam axis. This is the source of two-photon collisions, producing a hadronic state X according to . While for a given beam energy the kinematics of an annihilation process is fixed, the continuos spectra of the photons will allow simultaneous measurements at different invariant masses and for different momentum transfers.

The electron-photon vertices are pure QED processes and therefore exactly calculable. The main interest lies instead in studying the photon-hadron coupling, i.e. the coupling between photons and quarks. For two-photon events with high momentum transfer this coupling is best described by the Quark Parton Model (QPM), where the photons have a point-like coupling to a quark which subsequently fragments into hadrons. For smaller momentum transfers the Vector meson Dominance Model (VDM) works better. Here the photons transform into virtual vector mesons, which then interact through the strong force, showing characteristics of soft hadron interactions. It has recently been suggested that events with medium multi-jet production can be described by a QCD-based model of the hard scattering of the photon's hadronic constituents, thus implicitly needing a gluon component in the photon. Smooth transistions between these different models are expected.

It is possible to use forward electromagnetic calorimeters for tagging one or both of the outgoing electrons for two-photon reactions. In DELPHI the Small Angle Tagger (SAT) and Very Small Angle Tagger (VSAT) are used for this purpose. These taggers are situated in the forward and the very forward regions of DELPHI where the cross-section for two-photon events is highest. Real events are compared with Monte Carlo predictions generated using the above QPM, VDM and QCD models. Initial results for the one-tagged events are promising, showing good agreement between experiment and theory.

By studying double tag events, where both the electrons are measured, it is possible to fully determine the momentum carried by the photons. Thereby getting a precise measurement of the invariant mass of the hadronic system. Recent analysis of double tag events in the VSAT has measured the cross section of two-photon events at invariant mass energies up to 35 GeV at LEP1.

The SAT was replaced in 1994 by a new calorimeter, the Small angle TIle Calorimeter (STIC). The use of STIC and VSAT as taggers for two-photon events will be important at LEP200 where this type of collisions will dominate. In 1998 the radius of the beampipe will be decreased by 1 cm, giving the VSAT access to a higher accepted cross section at smaller polar angles.

People Involved

• Göran Jarlskog
• Sverker Almehed
• Per Jonsson
• Nikolai Zimin