Searches for supersymmetric long-lived particles with displaced-vertex signatures and studies of the electronic noise in the Tile Calorimeter at the ATLAS experiment

by Filip Backman (Stockholm University)

Thursday 1 Jun 2023, 14:00 → 17:00 Europe/Stockholm

Sal FD5 (AlbaNova Main Building)

The Standard Model of particle physics is successful at describing nature on the smallest scales, but it has known flaws.These can be remedied by extended theories, such as supersymmetry. A quantum number in the supersymmetry framework is R-parity. This quantity is generally conserved, but can be violated.This thesis presents two searches for particles beyond the Standard Model using the ATLAS detector at the Large
Hadron Collider. The particles are produced through an R-parity violating supersymmetric interaction. They have long enough lifetime to fly a macroscopic distance from the interaction point but still decay inside the inner detector of ATLAS, giving rise to unconventional signatures. The searches are conducted using data from proton-proton collisions with a centre-of-mass energy of 13 TeV produced during Run 2 of the LHC. The decay of long-lived particles inside the inner detector is indicated by displaced vertices, which are points in space where multiple charged-particle tracks emerge.

This thesis presents in detail how background due to stable particles interacting with dense detector material, creating displaced vertices, is suppressed. The tool developed to suppress this background is in the form of a detailed map of the material. The map is used to veto displaced vertices that appear in regions with dense detector material.The material map is produced mainly using secondary vertices reconstructed in data, which are complemented by a simulation model of the detector. The region covered by the veto spans a radius of 300 mm from the beam pipe and ±300 mm longitudinally from the nominal interaction point.
The first analysis searches for long-lived stop squarks decaying into a quark and a muon. The second analysis searches for long-lived neutralinos decaying into three quarks. This analysis considers two models, one where the neutralino is produced via the strong interaction through a promptly decaying gluino and one where the neutralino is produced via electroweak interactions. The data used in the analyses were recorded during Run 2 of the LHC, which corresponds to the years 2015-2018. The observed data in both searches are consistent with the background-only hypothesis and the results are used to set new limits to the proposed particles. Countless ATLAS analyses rely on the accurate measurement of hadron energies that the Tile Calorimeter provides. In this thesis, measurements of the electronic noise in the Tile Calorimeter for the years 2016-2019 are presented. Cells that provide unreliable information need to be masked to avoid inaccurate measurements of the energy and new tools used to find these problematic cells are presented in this thesis.
Stockholm 2023
http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-216506
ISBN 978-91-8014-304-2
ISBN 978-91-8014-305-9