Phenomenology of pseudo Nambu-Goldstone dark matter

The microscopic nature of dark matter (DM) is one of the key open problems in fundamental physics. Over the last few years, the theoretical proposal that DM may be a pseudo Nambu-Goldstone boson (pNGB) particle has risen to prominence, because it explains by elegant symmetry arguments the lack of signals in direct detection experiments. This Thesis discusses several aspects of pNGB DM. After a general introduction to particle DM in Chapter 1, the most salient phenomenological features of pNGB DM are presented in Chapter 2. Chapter 3 focuses on the prospects to probe pNGB DM at a future muon collider (MC), through its pair-production in vector boson fusion mediated by an off-shell Higgs. This process yields large missing mass, plus a muon and an anti-muon at large rapidities. To record such muons, a dedicated forward detector would need to be installed at the MC. Here, a first estimate of the required coverage and resolution is performed, based on the pNGB DM signal as well as the related process of Higgs decay to invisible particles. Chapters 4 and 5 review the existing literature on pNGB DM models, considering first extended composite Higgs scenarios where the Higgs and DM arise as “sibling” pNGBs (Chapter 4), then discussing simple scalar extensions of the Standard Model that give rise to pNGB DM (Chapter 5). Conclusions are drawn in Chapter 6.