Heavy neutral leptons (HNLs) are particles beyond the Standard Model (SM), typically introduced to explain the small observed neutrino masses. In this talk, we discuss the theory foundations of HNL extensions of the SM, as well as the present status of experimental searches for them at the LHC. Beyond an introduction to HNL interactions and properties, the theory part will focus on a comparison of benchmark models for collider searches, and on the theory expectation for HNL-induced lepton number violation in collider processes. In the experimental part, we will focus on three analyses published recently by the ATLAS and CMS collaborations which combine different signatures to probe HNL masses from 2 GeV up to 20 TeV.
Stefan Antusch received his Diploma and Ph.D. at the Technische Universität München. He then worked as postdoc at the University of Southampton (from 2003 to 2005) and at the Universidad Autonoma de Madrid (from 2005 to 2007). From 2007 to 2011 he led a Max Planck Research Group at the Max-Planck Institut für Physik (Werner-Heisenberg-Institut) in Munich. In 2011 he became Professor of Theoretical Physics at the University of Basel. His research area is theoretical elementary particle physics beyond the Standard Model and its connection to early universe cosmology.
Matthias Saimpert has been a member of the ATLAS collaboration for 10 years. He received the ATLAS thesis award for his Ph.D. from CEA Université Paris-Saclay documenting his work on di-photon production at the LHC and fast electronics for picosecond timing detectors. Matthias then worked as a DESY fellow in Hamburg, Germany on b-tagging performance and dark matter searches. During his time at DESY, Matthias convened the ATLAS b-tagging calibration working group and also contributed to the first prospects for the LUXE experiment at DESY Hamburg. Matthias became CERN research fellow in 2019. He worked on the level-1 central trigger of ATLAS and led phenomenological & experimental efforts to probe Heavy Neutral Leptons beyond the TeV range at the LHC. He was hired as permanent physicist at CEA Saclay in 2021 and convened the ATLAS ‘lepton + x’ Exotic working group in 2022-2023 while continuing his work on HNL searches at high mass. At Saclay, Matthias also leads locally the ATLAS inner tracker pixel upgrade project and is involved in the measurement of the CP properties of the Higgs in the ttH channel.