Search for long-lived heavy neutral leptons in semileptonic final states

Korte inhoud:This thesis presents the experimental search for long-lived heavy neutral leptons (HNL) with the Compact Muon Solenoid (CMS) detector at the Large Hadron Collider (LHC). The search is performed with proton-proton collision data at a center-of-mass energy of 13 TeV, which were collected during the data-taking period from 2016-2018 and which amount to an integrated luminosity of 138 fb-1. A heavy neutral lepton is added to the SM through the type 1 seesaw mechanism. It is produced through coupling with the SM neutrinos and its mass is considered as a free parameter. Within a certain mass-coupling range, the HNL becomes naturally long-lived, leading to discernible decay distances within the CMS detector volume. The presented experimental search focuses on this mass range and employs several analysis techniques that leverage the distinct signature of long-lived heavy neutral leptons. A dedicated machine learning algorithm is used to extract HNL events effectively. Events are selected where the hypothetical HNL is produced through mixing with a SM neutrino in W boson decays and where the HNL decays semileptonically into a single charged lepton and hadronic decay products from a virtual W boson. The search distinguishes between the potential Dirac or Majorana nature of the HNL by separating lepton-number-conserving and lepton-number-violating final states. The obtained results are consistent with the SM background predictions. As a consequence, exclusion limits at 95% confidence level are set on the existence of a heavy neutral lepton in a mass range of 1 to 20 GeV for effective couplings between 1e-7 and 1e−2. Only HNL couplings to the first and second lepton generation are considered. These are among the strongest limits on heavy neutral leptons presently available.

Jaar van publicatie:2023

Toegankelijkheid:Open