Double-tagged interactions of photons with virtualities Q^2 between 10 GeV^2 and 200 GeV^2 are studied with the data collected by DELPHI at LEPII from 1998 to 2000, corresponding to an integrated luminosity of 550 pb^{-1}. The gam* gam* -> mu+mu- data agree with QED predictions. The cross-section of the reaction gam* gam* -> hadrons is measured and compared to the LO and NLO BFKL calculations.
Measured cross section for the process E+ E- --> E+ E- HADRONS.
Measured cross section for the process GAMMA* GAMMA* --> HADRONS.
Differential cross section for GAMMA* GAMMA* --> MU+ MU-.
Muon pair production in the process e+e- -> e+e-mu+mu- is studied using the data taken at LEP1 (sqrt(s) \simeq m_Z) with the DELPHI detector during the years 1992-1995. The corresponding integrated luminosity is 138.5 pb^{-1}. The QED predictions have been tested over the whole Q^2 range accessible at LEP1 (from several GeV^2/c^4 to several hundred GeV^2/c^4) by comparing experimental distributions with distributions resulting from Monte Carlo simulations using various generators. Selected events are used to extract the leptonic photon structure function F_2^\gamma. Azimuthal correlations are used to obtain information on additional structure functions, F_A^\gamma and F_B^\gamma, which originate from interference terms of the scattering amplitudes. The measured ratios F_A^\gamma/F_2^\gamma and F_B^\gamma/F_2^\gamma are significantly different from zero and consistent with QED predictions.
The measured QED photon structure function at Q**2 = 12.5 GeV for the combine SAT and STIC data.
The measured QED photon structure function at Q**2 = 120 GeV for the combine SAT and STIC data.
Ratio of the structure functions FA and FB to F2.