Public Thesis defense - IRMP

Aspects of Bottom-up Hidden Sector Models by Philipp KLOSE

Pour l’obtention du grade de Docteur en sciences

The standard model (SM) of particle physics is an extremely successful description of nature, but it is known to be incomplete. For instance, it cannot accommodate finite masses for the SM neutrinos or explain the origin of the primordial baryon asymmetry (BAU) in the early universe. One way to address these and other problems is to postulate the existence of new but hidden particles. This thesis studies such “hidden sectors” by following two complementary approaches:

First, an effective theory approach is used to construct electroweak (EW) and GeV scale portal effective theories (PETs) that couple the SM to a generic light hidden mediator of spin 0, 1/2, or 1, thereby capturing a large class of viable hidden sector models. The EW scale PETs encompass all available portal operators of dimension d ≤ 5, and the corresponding GeV scale PETs encompass all portal operators of dimension d ≤ 7 that contribute at leading order after the heavy SM degrees of freedom are integrated out. These PETs are used to derive a leading order PET chiral perturbation theory (ChPT) Lagrangian that describes hidden sector induced light meson transitions such as K^+→π^+ Sin fixed target experiments such as NA62 or SHiP.

Second, the type-I seesaw model is examined in detail. This hidden sector model couples the SM to n ≥ 2 sterile neutrinos that can also help explain the BAU via the mechanism of leptogenesis. The present work studies the impact of thermal and spectator effects in high-scale leptogenesis with two hierarchical sterile neutrinos, for which the sterile neutrino coupling to the early universe SM plasma is characterized by the washout strength K. For a vanishing initial abundance of sterile neutrinos and K≪1 or K≫1, it is found that thermal and spectator effects result in a sign-flip and strong relative enhancement of the final BAU. Much lighter sterile neutrinos may be detected by searching for lepton number violating (LNV) decays in collider experiments. However, such LNV decays may be suppressed relative to lepton number conserving (LNC) decays in models without ‘t Hooft fine-tuning. The present work shows that the available parameter space for models without fine-tuning can be split into three regions: (a) In which LNV decays are unsuppressed, (b) In which LNV decays are suppressed, (c) In which LNV decays may or may not be suppressed, irrespective of fine-tuning considerations.

Jury members :

• Prof. Jean-Marc Gérard (UCLouvain), supervisor
• Prof. Marco Drewes (UCLouvain), supervisor
• Dr. Chiara Arina (UCLouvain), supervisor
• Prof. Vincent Lemaître (UCLouvain), chairperson
• Prof. Eduardo Cortina-Gil (UCLouvain), secretary
• Prof. Björn Garbrecht (TUM, Germany)
• Dr. Christoph Weniger (University of Amsterdam, NL)

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