Study of two-photon processes at low $Q^2$ using the Vsat calorimeter in the DELPHI experiment

The study is based on data collected at the LEP II accelerator during the second period of the Large Electron-Positron collider operations (1996&ndash;2000). During that time the DELPHI (DEtector with Lepton, Photon and Hadron Identification) experiment collected about 90 Gb of data on tape corresponding to 687&ndash;691 <italic> pb</italic><super>&minus;1</super> of luminosity. The data obtained by the VSAT (Very Small Angle Tagger) electromagnetic calorimeter was stored and used for luminosity measurements and for gamma-gamma physics analysis. The analysis was concentrated on single and double tag events. Three Monte Carlo sets were generated for years 1998&ndash;2000 using) three different generators (PYTHIA, PHOJET and TWOGAM). Gamma-gamma physics at such high energies (189&ndash;206 GeV in the c.m.s.) and small polar angles (3&ndash;12 mrad) is unique. Background and beam conditions influenced the analysis and were studied in order to obtain a &ldquo;clean&rdquo; signal with as little background as possible. The total hadronic cross-sections for different invariant mass were estimated. The upgrade of the VSAT data-taking software and some hardware problems due to aging were also discussed.