Search for Heavy Majorana Neutrinos at the LHC

The origin of neutrino masses is one of the open questions of particle physics. This thesis presents work on the high-energy equivalent of neutrinoless double-beta decay. Two phenomenological studies investigate sensitivity to the $\mu^\pm \mu^\pm jj$ final state at the LHC. An extensive study based on a fast simulation of the ATLAS detector is presented. Results of this study are interpreted in the context of the Type I Seesaw model and as limits on the Weinberg operator. Assuming $300\,\mathrm{fb}^{-1}$ of data collected at $\sqrt{s} = 13\,\mathrm{TeV}$, LHC experiments could probe heavy Majorana neutrinos with masses exceeding $20\,\mathrm{TeV}$, while being more sensitive than previous searches for masses above approximately 700 GeV. LHC experiments could be significantly more sensitive than the NA-62 experiment towards the Weinberg operator, but would not be able to compete with indirect limits from neutrino oscillations. Following up on this, a search for the Type I Seesaw model and the Weinberg operator in the $\mu^\pm \mu^\pm jj$ final state with the ATLAS experiment is presented. In $140\,\mathrm{fb}^{-1}$ of data collected at $\sqrt{s} = 13\,\mathrm{TeV}$, no significant excess above the SM expectation is found. Upper limits at 95 \% confidence level are placed on the muon neutrino -- heavy neutrino mixing element $|V_{\mu N}|^2$ as a function of the heavy Majorana neutrino mass $m_N$ as well as on the effective Majorana mass $|m_{\mu\mu}|$. The LHC and its experiments will be upgraded to increase luminosity by approximately a factor of 10. For this, the ATLAS experiment will receive a new tracking detector, the Inner Tracker (ITk). New sensor modules are subject to an extensive quality control procedure, which includes thermal cycling. This thesis presents tests of the prototype of the thermal cycling setup used for endcap modules as well as work on the automation of the test setup.