Searches for Massive Highly Ionising Particles at the ATLAS Experiment and in Polar Volcanic Rocks, and Performance Studies of the First Level ATLAS Trigger System

Sammanfattning: The Standard Model (SM) of particle physics describes the elementary particles and their interactions. Despite passing a number of high precision falsification tests, it is argued that the SM suffers from a number of shortcomings. Many Beyond the Standard Model (BSM) theories have therefore been postulated. Exotic highly ionising particles such as magnetic monopoles and Highly Electrically Charged Objects (HECOs), with masses at or above the TeV-scale, are predicted in many of these theories. Monopoles arise naturally in grand unification theories. Proposed candidates for HECOs are Q-balls, strangelets and micro-black hole remnants.The Large Hadron Collider (LHC) at CERN is the world's largest and most powerful particle accelerator, colliding protons at centre-of-mass energies up to 13 TeV. One of the main purposes of the LHC is to search for particles beyond the SM. The research presented in this thesis comprises a search for magnetic monopoles and HECOs at one of the largest of the LHC detectors, the ATLAS detector. In addition, studies were made on the performance of the ATLAS trigger system, which is responsible for making the initial online selection of interesting proton-proton events.The search for monopoles and HECOs at ATLAS was conducted using a customized trigger and selection variables optimized for the non-standard particle signature in ATLAS. The dataset corresponds to an integrated luminosity of 7.0 fb^{-1} and the centre-of-mass energy was 8 TeV. No events were observed and upper limits on production cross-sections were set for monopoles and HECOs of mass 200-2500 GeV and charge in the range $0.5-2.0$ g_D, where g_D is the Dirac charge, and 10 - 60 e, respectively.Magnetic monopoles were also sought in polar volcanic rock using a SQUID magnetometer at ETH, Zürich. No candidates were found leading to limits on the monopole density in polar igneous rocks of 9.8 ' 10^{-5}/gram.