Study of Two-Photon processes at low Q2 using the VSAT calorimeter in the DELPHI experiment

Abstract: The study is based on data collected at the LEP II accelerator during the second period of the Large Electron-Positron collider operations (1996-2000). During that time the DELPHI (DEtector with Lepton, Photon and Hadron Identification) experiment collected about 90 Gb of data on tape corresponding to 687-691 pb-1 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 1998-2000 using three different generators (PYTHIA, PHOJET and TWOGAM). Gamma-gamma physics at such high energies (189-206 GeV in the c.m.s.) and small polar angles (3-12 mrad) is unique. Background and beam conditions influenced the analysis and were studied in order to obtain a "clean" 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. The articles in the Appendices are connected with VSAT data-taking and the two-photon physics analysis. Appendix A, is the VSAT status report with estimation of VSAT performance for higher energies. The situation with the VSAT detector hardware and software upgrades are explained in the article. Appendix B, was written during the summer of 1999. The main subject of the article is the background condition in VSAT, STIC and other calorimeters. Some effective methods of removing background are proposed for each of the DELPHI calorimeters. Appendix C, describes problems of the long-time radiation damage and the short periods of high background which caused an increase of the bias currents in the VSAT modules. Appendix D, summarizes all beam parameters changes, position shifts of the VSAT modules observed by geometrical surveys and consequences of that during LEP II period. Appendix E, is devoted to the two-photons physics analysis of the DELPHI - VSAT data. The final analysis of two-photon events in double and single tag modes is presented.