MSc defense by Mads M. Hyttel and Edis Tireli

Title: Exploring a common solution to Dark Matter, Neutrino oscillations and Baryogenesis of the Universe Displaced Heavy Neutral Lepton searches at the Large Hadron Collider

What is Dark matter? Why is there an asymmetry between matter and anti-matter? How do neutrinos get mass? Despite its overwhelming success in describing the results of high-energy experiments at subatomic scales, the Standard Model of particle physics cannot answer these questions. Scientists seek therefore extensions of the Standard Model, that can play a pivotal role in our understanding of the universe at micro- and macro-scales.

Heavy neutral leptons (or HNLs) provide one such extension. These hypothetical particles are akin to the Standard Model neutrinos — neutral fermions, interacting with W±, Z0 and Higgs bosons. However, HNL phenomenology is distinct, owing to two facts: (1) their interaction strength is much weaker than that of neutrinos, and (2) they are much heavier. HNLs can be long-lived, traveling macroscopic distances before their decay. This determine possible strategies of their experimental tests.

This thesis focuses on HNL searches at the ATLAS experiment (part of the Large Hadron Collider at CERN). Our work differs from the previous searches in a number of ways. First, we consider HNL production not only from W± bosons, but also in decays of mesons containing charm or beauty quarks (B±, Bc±, D±). The amount of such mesons is much larger than that of W -bosons, which boosts HNL production for masses below that of mesons. Second, unlike all the previous searches we focus on utilising the Muon Spectrometer of the ATLAS experiment. The overall instrumented volume of the Muon Spectrometer is several m3 which allows us to probe for the existence of particles with the proper decay distance of the order of meters and even more. Specifically, we do not rely on tagging of prompt leptons or jets but rather limit our decay patterns to the processes with displaced μ+μ− pair in the final state. We expect that as the quest for long-lived particles continues, it will become possible to have more advanced triggers for displaced muons and/or retain highly displaced muon events. We also investigate HNL-mediated processess with three muons in the final states, two of which are displaced. The displacement in excess of 2 cm is expected to reduce the background of Standard Model processes to a negligible level, making the proposed searches essentially background-free.

Our results demonstrate that such a displaced muons analysis can push the exclusion limits both to smaller mixing angles and to higher HNL masses. The main improvement comes in case of the HNLs produced in W decays where the limits can improve by as much as the order of magnitude with forthcoming LHC runs. For the heavy-flavour production channel, low transverse momenta of the resulting particles greatly impair the efficiency of event selection and the resulting searches are less sensitive than those with parent W-bosons.

In order to explore the data efficiently, we have developed a dedicated analysis pipeline that combines Monte Carlo generators (pythia, MadGraph) and post-processing tools. Our pipeline provides a versatile interface for dissecting the data by quickly changing data selection criteria. A significant part of this thesis is devoted to the validation of this suite of tools.
As a demonstration of the power of our tool, we also perform the sensitivity estimates for HNLs with the proper distance in hundreds of meters that can potentially be detected by the new experiment SND@LHC. We confirm that such searches will not improve over the existing bounds owing to the small decay volume of the SND@LHC experiment.
This thesis should be regarded as feasibility studies for the HNL searches with the ATLAS Muon Spectrometer. Our results demonstrate potential in such searches in reaching deeply into otherwise unreachable part of the parameter space and therefore warrant further careful investigations, including proper detector response and background evaluation.