Isolated photon production in deep inelastic scattering at HERA.
PhD thesis, University of Glasgow.
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This thesis presents measurements of isolated photon production in deep inelastic ep scattering made with the ZEUS detector at HERA using an integrated luminosity of 320 pb-1. Measurements were made in the isolated-photon transverse-energy and pseudorapidity ranges 4 < E_T < 15 GeV and -0.7 < eta < 0.9 for exchanged photon virtualities, Q^2, in the range 10 < Q^2 < 350 GeV^2 and for invariant masses of the hadronic system W_X > 5 GeV. Differential cross sections are presented for inclusive isolated photon production as functions of Q^2, x, E_T and eta. Leading-logarithm parton-shower Monte Carlo simulations and perturbative QCD predictions give a reasonable description of the data over most of the kinematic range.
To increase the precision of the measurement, a study was undertaken to improve the Monte Carlo description of electromagnetic showering in the ZEUS calorimeter. This utilised isolated electromagnetic clusters from ZEUS data to which Monte Carlo event samples with varying parameters for electromagnetic showering were fitted. These improvements have been adopted in the standard ZEUS detector simulation and were also shown to improve electron identification.
For the main isolated photon analysis a range of methodologies for photon clustering and identification,
isolation and signal extraction were investigated and the optimal methods were chosen. As a result of these refinements and the improved electromagnetic shower modelling, the systematic errors decreased significantly, typically by a factor of three when compared to the same phase space in the previous ZEUS publication.
In addition the luminosity was increased threefold and the phase space was expanded. These two improvements brought about a factor three reduction in statistical uncertainty.
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