Resonance Formation in Two-photon Collisions

Two-photon collisions at electron positron storage rings are a good laboratory to investigate the properties of meson resonances and play a crucial role in glueball searches. A resonant state R can be formed by the collision of two photons via the reaction e+e− → e+e−R (fig. 1). The outgoing electron and positron are usually scattered at very small angles and are not detected (no-tag mode). In this case the two photons are quasi real and the resonant state R must be neutral and unflavoured with C=1 and J6=1. If one of the two photons is highly virtual, the outgoing electron or positron can be detected at low angle and the spin of the resonant state is allowed to be one (single-tag mode). In both cases the two outgoing particles carry nearly the full beam energy and the mass of the resonant state is much smaller than the e+e− centre of mass energy. This fact allows a clean separation between the twophoton and the annihilation process by using a cut in the visible energy. Since there are no particles produced other than R, the reconstruction of the final state can be performed in a very clean experimental environment. The cross section for this process is given by the convolution of the QED calculable luminosity function L, giving the flux of the virtual photons, with