West, Benjamin John; (2001) Charm and the virtual photon at HERA and a global tracking trigger for ZEUS. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Previous measurements at ZEUS have demonstrated suppression of photon structure like effects due to both the virtuahty of the photon, Q2, and the presence of charm. In this thesis aspects of these two measurements have been combined in order to determine whether these two suppressions are independent. The measurement was made with the ZEUS detector at HERA in the kinematic region 0 Q2 5. 103 GeV2 using dijet events containing a D* meson. Events having two or more jets with large transverse energies were selected using the longitudinally invariant kT algorithm in the laboratory frame. The dijet cross section was measured as a function of the fractional momentum of the photon participating in the dijet production, xobsy, and of Q2. The ratio of low to high xobsy cross sections was found not to change significantly wdth Q2. This is in marked contrast to previous measurements which did not require a D*, demonstrating for the first time that the observed suppressions of the low xobsy cross section due to non-zero photon virtuality and due to charm are not independent. The ratio was also compared to the predictions of leading-order pQCD. Calculations which included either a resolved virtual photon in the DGLAP evolution scheme or used CCFM evolution gave a better description of the data than a DGLAP calculation with no photon structure. During the 2000-2001 shutdown both the HERA accelerator and ZEUS detector were upgraded. In order to take advantage of these improvements a new global tracking trigger, combining information from the Central Tracking Detector and the newly installed Micro Vertex Detector at the Second Level Trigger, has been developed. The algorithm is described and its performance evaluated. The event z vertex resolution is two orders of magnitude better than that for the present algorithm. This will enable future measurements of the cross section ratio to be made with much greater precision.

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