Search for decaying neutralinos at HERA and clustering optimisation for the ZEUS Micro Vertex Detector

Universität Hamburg, Diss., 2010; 124 pp., (2010). doi:10.3204/DESY-THESIS-2010-009
A search for gravitinos produced in supersymmetric events in electron proton collisions has been performed with data recorded with the ZEUS detector at HERA in the years 2003 to 2007. In R-parity violating supersymmetric models a neutralino can be produced by t-channel exchange of a selectron between the incoming electron and a quark of the proton. In the Gauge Mediated Supersymmetry Breaking model (GMSB), where the gravitino is the lightest supersymmetric particle, the neutralino in large regions of the parameter space dominantly decays into a gravitino and a photon, leading to events with an isolated high energetic photon and large missing transverse momentum. To separate signal and background, a multi-variate discriminant method was used. No evidence for supersymmetry was found, and limits were derived for the R-parity violating coupling strength and the masses of the selectron and the lightest neutralino. Moreover, the cluster position reconstruction system of the ZEUS experiment was improved by modifying and implementing an algorithm which uses estimated track angle and impact position from the pattern recognition phase of the tracking system to yield a better position resolution.
Published by Deutsches Elektronen-Synchrotron, DESY, Hamburg