McKigney, Edward Allen; (1998) An investigation of singly and doubly tagged photon-photon scattering using the OPAL detector at LEP. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The calibration of the OPAL Far Forward Monitor (FFM) system is described. This system consists of a set of four small electro-magnetic calorimeters with an angular acceptance of approximately 5 to 15 mrad. The FFM system is used for the OPAL online luminosity measurement and to tag electrons at small angles. Doubly Tagged Two-Photon events are selected from a sample of Singly Tagged Two-Photon events measured with the OPAL detector. This data was taken during 1997 when LEP operated with a beam energy of 91.5 GeV. A measurement of the cross section for the process e+e- 'e+e- + Hadrons is made, with Q2 = 20.4 GeV2 and P2 = 0.27 GeV2. The measured cross section is 1.39 + 0.77(stat) + 0.18, 0.14 (syst) pb, which is consistent with theoretical expectations. With 350 pb-1 and the upgraded FFMs, a statistical error of 10% and a systematic error of less than 10% will be possible for this measurement.

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