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Title: Dijet production and multiscale QCD at HERA
Author: Lightwood, Matthew Stephen
ISNI:       0000 0001 3609 9485
Awarding Body: University of London
Current Institution: University College London (University of London)
Date of Award: 2005
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This thesis describes the analysis of dijet production in photoproduction and DIS, using the ZEUS detector at HERA. At ZEUS, positrons and protons were collided with a centre of mass energy of /i = 300 GeV during the 1996-1997 running period and y/s = 318 GeV thereafter. Dijet production is studied for the all-flavours case and for the case when a charm quark is demanded. In the all-flavours analysis, the dependence of dijet production on the virtuality of the exchanged photon, Q2, was studied by measuring dijet cross sections in the range 0.0 < Q2 < 2000 GeV2, using data from the 1996-1997 running period corresponding to an integrated luminosity of 38.6 pb_1. Dijet cross sections were measured in the photon-proton centre-of-mass frame for jets with transverse energy E1 > 7.5 and 6.5 GeV and pseudorapidity in the range 3.0 < rfet < 0.0. The variable x BS, a measure of the fractional photon momentum entering the hard process, was used to enhance the sensitivity of the measurements to the photon structure. The Q2 dependence of the ratio of low to high x BS events was measured. Next-to-leading-order QCD predictions were found to generally underestimate the low x BS contribution relative to that of high x BS. Leading-logarithmic parton-shower Monte-Carlo predictions which use a partonic structure for the photon, falling smoothly with increasing Q2 provide a qualitative description of the data. The all-flavours dijet analysis was repeated with the demand that a D*(2010) meson was identified in the decay channel D* -> D0tts > (Kit)its with pr{D*) > 3.0 GeV and t7(D*) < 1.5. The charm data were collected from the running period 1996-2000, corresponding to an integrated luminosity of 120.4 pb-1, and cover the range of photon virtualities 0.0 < Q2 < 5000 GeV2. The measurements were compared to next-to-leading-order QCD predictions which were generally found to give a good description of the data. Leading order pQCD models with parton shower simulation which include a parameterisation of the virtual photon were found to give a good description of the DIS data. Such models were found to be consistent with the photoproduction data only when the non-perturbative vector meson like component of the resolved photon is removed. When no parameterisation of the hadronic component of the virtual photon was used DGLAP evolution was found to lie below the data, and CCFM evolution generally closer to the data.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available