Master thesis Defense by Mikkel Skaarup – University of Copenhagen

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Master thesis Defense by Mikkel Skaarup

Title: Study of diffractive pp interactions at the LHC using a tagged proton

This thesis is a study of diffractive processes in proton-proton (pp) collisions. To understand what these are and what to expect from them, a summary of the Standard Model and sections describing collider- and diffractive-physics are included. It is discussed that the experimental expectation is, for a single diffractive (SD) interaction, a proton scattered at very low angles on one side of the interaction point (IP) while the other proton breaks into a system of particles produced in the forward region. This is due to the colourless exchange of a diffractive interaction. A detailed description of the detectors used, ATLAS and ALFA, is also given.

A study of a single diffractive (SD) event selection at collision energies of $\sqrt{s}=8$ TeV is presented. The result of the selection is compared to a toy model Monte Carlo (MC) simulation using Pythia 8, and a rough agreement of this simple MC and data is seen. A selection of central diffraction (CD) is also discussed. As elastic events are background candidates to, especially, central diffractive events, a breif analysis of these is also presented, and they are used to remove background elastic events from the CD event selection. Diffractive parameters relevant to the selections are also discussed, such as the energy loss, $\xi$, and forward rapidity gap, $\Delta\eta_F$. It is seen that the SD selection provide energy loss and rapidity gap measurements that are in agreement with theory, but a more detailed study and simulations are needed for further conclusions. For CD events, a first look into event structures, such as the Bengtsson-Zerwas, K\"orner-Shierholtz-Willrodt and Nachtmann-Reiter angles, is presented as a first step towards measuring a triple-Pomeron vertex and the colour flow of CD interactions. As these are done using a naive approach, the result is preliminary.