Die Beobachtung von Neutrino-Oszillationen hat gezeigt, dass Neutrinos eine von Null verschiedene Masse haben. Dieses Phänomen wird nicht durch das Standardmodell der Teilchenphysik beschrieben, aber eine mögliche Erklärung für dieses Dilemma ist die Existenz von schweren neutralen Leptonen in Form von rechtshändigen Neutrinos. Abhängig von ihrer Masse und Kopplung zu den Neutrinos des Standardmodells könnten diese Teilchen auch eine wichtige Rolle bei der Lösung weiterer unerklärter Beobachtungen wie Dunkler Materie und der Baryonenasymmetrie des Universums spielen. Diese Arbeit präsentiert die erste Suche nach schweren neutralen Leptonen mit dem IceCube Neutrino-Observatorium. Das standardmäßige Drei-Flavor-Neutrino-Modell wird erweitert, indem ein vierter Massenzustand im GeV-Bereich hinzugefügt wird und eine Mischung mit dem Tau-Neutrino durch den Parameter \ut4 erlaubt wird. Es werden drei Massenwerte für schwere neutrale Leptonen, $m_4$, von \SI{0.3}{\gev}, \SI{0.6}{\gev} und \SI{1.0}{\gev} getestet, wobei zehn Jahre Daten aus den Jahren 2011 bis 2021 verwendet werden. Für keine der drei getesteten Massen wird ein signifikantes Signal von schweren neutralen Leptonen gemessen. Die resultierenden Einschränkungen für den Mischungsparameter sind \ut4$ < 0.19\;(m_4 = \SI{0.3}{\gev})$, \ut4$ < 0.36\;(m_4 = \SI{0.6}{\gev})$ und \ut4$ < 0.40\;(m_4 = \SI{1.0}{\gev})$ im \SI{90}{\percent} - Konfidenzniveau. Diese erste Analyse legt die grundlegende Basis für zukünftige Suchen nach schweren neutralen Leptonen in IceCube. / The observation of neutrino oscillations has established that neutrinos have non-zero masses. This phenomenon is not explained by the standard model of particle physics, but one viable explanation to this dilemma is the existence of heavy neutral leptons in the form of right-handed neutrinos. Depending on their mass and coupling to standard model neutrinos, these particles could also play an important role in solving additional unexplained observations such as dark matter and the baryon asymmetry of the universe. This work presents the first search for heavy neutral leptons with the IceCube Neutrino Observatory. The standard three flavor neutrino model is extended by adding a fourth GeV-scale mass state and allowing mixing with the tau neutrino through the parameter \ut4. Three heavy neutral lepton mass values, $m_4$, of \SI{0.3}{\gev}, \SI{0.6}{\gev}, and \SI{1.0}{\gev} are tested using ten years of data, collected between 2011 and 2021. No significant signal of heavy neutral leptons is observed for any of the tested masses. The resulting constraints for the mixing parameter are \ut4$ < 0.19\;(m_4 = \SI{0.3}{\gev})$, \ut4$ < 0.36\;(m_4 = \SI{0.6}{\gev})$, and \ut4$ < 0.40\;(m_4 = \SI{1.0}{\gev})$ at \SI{90}{\percent} confidence level. This first analysis lays the fundamental groundwork for future searches for heavy neutral leptons in IceCube.

Heavy Neutral Leptons (HNLs) are the heavier counterparts of the light neutrinos of the Standard Model of particle physics. HNLs can simultaneously solve several of the problems the Standard Model cannot yet resolve, one example being that they provide a candidate for Dark Matter. This thesis work aims to shed light on the nature of HNLs and study the displaced signature the particle gives rise to at colliders. The collider of interest is the Future Circular Collider that will be colliding electrons and positrons and the signal studied is the production of an HNL and a light neutrino from an intermediate Z boson, produced from the collision of an electron and a positron. The event generation was set up through MadGraph and PYTHIA and for the detector simulations DELPHES was used. Validation of three HNL samples were carried out in a standalone framework and in the FCC framework. The samples were validated by comparing theoretically calculated lifetimes with the lifetimes attained by simulation. Kinematic studies of the transverse momentum of the HNL and its decay particles showed correlation to the mass of the HNL. Reconstruction of the number of tracks created by the HNL decay was possible and the results of two track dominance were found to correlate with theory. For the vertexing study, the reconstruction of the production vertex of the decay particles was possible where displaced vertices were observed, hence proving the possibility of implementing displaced signatures in the FCC framework for the very first time. The next step in this trajectory of the study would be to investigate vertex fitting of the reconstructed vertices in order to carry out tracking studies of the HNL. This work hence sets the foundation for further exploration of HNLs and provides stepping stones for the possibility of discovery of HNLs in the FCC-ee.

<p>The ability of the Mu2e experiment to probe, or discover BSM physics in direct CLFV μ+ and π+ decay modes is estimated.</p>