Master thesis defense by Alexander Pedersen Lind

Titl: A Study of Diffractive Scattering with the ATLAS and ALFA Experiment

This thesis presents a study of diffractive scattering of high-energy protons. Hadronic diffraction is not well-understood and many alternative approaches exist. The Monte Carlo event generator Pythia 8 follows a Pomeron-based approach, where the invariant mass of the diffractive system and the squared momentum transfer of the system is set up according to a phenomenological Pomeron flux parameterization, which is parameterized by the super-critical Pomeron trajectory with an intercept parameter and a slope parameter that can be tuned by the user. 

A fast detector response simulation of the ALFA and ATLAS detectors has been developed in the Rivet toolkit for the purposes of this thesis. The developed simulation framework is able to handle the beam transport of diffractively scattered protons as well as impose the kinematic acceptance of the ALFA detector.

The simulation framework has been used in a phenomenological study of single diffraction, in order to study observables most sensitive to the Pomeron flux parameters. Using this information, a statistical fit procedure to determine the two Pomeron flux parameters has been developed and tested on several generated test samples. 

Data recorded at a center-of-mass energy of 13 TeV and with a beam optics of 90 m at the ATLAS experiment in the Run 2 period has been analysed. Several structures in the measured single diffractive distributions are observed in data which are not present in the simulation. An investigation into the possible background sources of these structures is presented.