Photon Emission in Hadronic Events at LEP

Smith, Michael Gordon (1994) Photon Emission in Hadronic Events at LEP. PhD thesis, University of Glasgow.

Abstract

From a sample of almost 1.2 million hadronic events collected with the ALEPH detector at LEP during 1990-92, events containing an energetic final state photon attributable to quark bremsstrahlung are selected after clustering all the constituent particles into jets and identifying photons with a fraction z > 0.7 of their associated jet's energy. Unlike previous analyses which used a two-step, isolation cone algorithm to select only isolated photons, this new 'democratic' algorithm allows the selection of photons within resolved hadronic jets, corresponding to emission at a much later stage of the hadronisation process. After verifying that the dominant backgrounds arising from non-prompt photons are reliably simulated by a parton shower Monte Carlo model, their contribution to the selected sample is estimated and subtracted statistically from the measured photon + n-jet rates. The quark-to-photon fragmentation function, Dq→gamma(z), is then extracted directly from the measured prompt photon production rate in events comprising one 'photon' jet and one other hadronic jet. Working with an O (alphaalphaS) matrix element calculation in the -MS renormalisation scheme, the unknown non-perturbative contribution to Dq→gamma(z) is determined at high z. This measurement provides a better description of quark bremsstrahlung than hitherto employed in high energy electron-positron collisions and may prove useful in describing inclusive prompt photon production at hadron-hadron colliders. Isolated photon production rates are measured as a function of the jet resolution parameter, ycut. Good agreement is found with predictions from the above O(alphaalphaS) matrix element calculation. Comparisons are also made with the rates predicted by the parton shower models JETSET, HERWIG and ARIADNE. A set of global event variables are calculated for the hadronic system in events comprising an energetic isolated photon and plotted as a function of the hadronic centre of mass energy. Comparisons are made with equivalent predictions from parton shower models where the events comprise an isolated photon and with events generated over a range of centre of mass energies.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: Ken Smith
• Keywords: High energy physics
• Date of Award: 1994
• Depositing User: Enlighten Team
• Unique ID: glathesis:1994-74973
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 27 Sep 2019 14:50
• Last Modified: 27 Sep 2019 14:50
• URI: https://theses.gla.ac.uk/id/eprint/74973