Analysis of Isolated Photons in Photoproduction in PYTHIA

Abstract

Collision of particles at high energies at accelerators is the main source of data used to obtain deeper understanding of the fundamental interactions and the structure of the matter. Processes of isolated photon production have provided many tests of theoretical descriptions of the universe on scales smaller than the proton. This work is dedicated to the analysis of the large amount of collision data that has been accumulated at ZEUS in 2004-2007 period and new methods of processing isolated photons that have been proposed. The authors develop software algorithms that allow obtaining the signal of isolated photons from the data collected on the ZEUS detector at electron-proton collider HERA, calculating the differential cross sections, and comparing the measured data with PYTHIA Monte Carlo predictions. Taking into account the features of the ZEUS detector, the photon signal is separated from the background events and the number of isolated photons is calculated. Computational mathematical and numerical methods have been used to simulate the interaction of particles in the detector. Monte Carlo predictions for differential cross sections as functions of the pseudorapidity and transverse energy of the photon η^g, E_T^g and the jet η^jet, E_T^jet, and the fraction of the photon momentum х_g^meas carried by the interacting parton have been calculated and compared with the experimental data. The results of the study are compared with the previous studies and show for the first time that all isolated photon HERA measurements are consistent with each other. New results show improved uncertainties. The formation of isolated inclusive photons and photons with the accompanying jet was measured in photoproduction with ZEUS detector at HERA collider using the integrated luminosity of 374 ± 7 pb^-1. For the first time, more complex Monte-Carlo simulation models of isolated photons for ZEUS detector were generated and applied, and the description of the photon signal was improved. It has been found that PYTHIA describes the shape of the cross section as a function of η^g well enough, but does not fully reproduce the shape of E_T^g, E_T^jet, and the middle region of х_g^meas, while η^jet is described not very well. The reason for this discrepancy can be the lack of corrections of higher orders in the predictions for cross sections of direct photons. Scaling of the cross sections obtained with PYTHIA improves the description of E_T^g and η^g. The unsatisfactory description of η^jet indicates that further studies are required.