Phenomenological analysis of particle dark matter models (Doctoral thesis)

Καραμήτρος, Δημήτριος


Nowadays, there is irrefutable evidence for the existence of dark matter (DM), probably in the form of electrically neutral, stable particles. However, despite the increasing experimental and theoretical efforts, the exact nature of the dark matter particle(s) remains unknown. In this thesis, we present three models predicting a dark matter particle. In each model, focusing on the most natural cases, we study the phenomenological implications of the DM candidate particle as well as its parters. In the frst chapter, we briefly discuss the Standard Model (SM) of particle physics, cosmology and dark matter. We start by demonstrating the form of the SM gauge invariance. Then we introduce its particle content and explain the origin of the various masses via the Higgs mechanism. Afterwards, we discuss the basic formulation of the Standard Cosmological Model. We introduce the Friedman-Roberson-Walker metric, and discuss the expansion and thermal history of the Universe. Finally, we discuss the dark matter problem. We present the evidence for the existence of dark matter as well as the general characteristics of the hypothetical corresponding particle. Then, we present two mechanisms that explain the evolution of the DM number density. Closing this chapter, we brie y discuss various experimental and observational efforts that focus on the detection of DM signatures. In the second chapter, we present the simplest fermionic DM model, which consists of one Majorana gauge singlet (S). Since there are no possible interactions between the DM particle and the SM at the renormalizable level (due to a Z2 symmetry), we introduced the only d = 5 non-renormalizable operator available, describing the interaction between the DM particle and the Higgs boson. Then, we examine the production of the relic abundance for the S-particle, via the freeze-out and freeze-in mechanism, in two distinct mass regions. In the third chapter, we study an extension of the SM that consists of a fermionic dark sector. This dark sector is composed of two Weyl iso-doublets with opposite hypercharge and a Majorana iso-triplet. Under the assumption of a Z2 symmetry, these particles always interact in pairs, therefore the lightest neutral particle is a DM candidate. We show that the Yukawa terms of this dark sector are symmetric under a custodial symmetry, which is responsible for the suppression of the DM-nucleon cross section, since the tree-level interactions between the DM particle and the Higgs as well as the Z-boson vanish. Focusing on this symmetric limit, the lightest neutral fermion of this dark sector can naturally explain the observed DM density, with a mass at the electroweak scale. Furthermore, we show that the new charged fermions are responsible for a suppression of the branching ratio of the Higgs boson to two photons. Finally, we show that the DM particle of this model can be produced and detected at the LHC in the near future. In Chapter 4, we consider a dark sector consisting on a pair of Weyl SU(2)L-doublets with opposite hypercharges. Treating this dark sector as a low-energy limit of a UVOutline complete model with a cutoff energy at the TeV scale, we perform a detail phenomenological analysis, including both renormalizable and non-renormalizable d = 5 interactions between the dark sector particles and the SM. We find that the DM particle can have a mass near the electroweak scale, while evading all experimental and observational constraints, provided that sizeable dipole interactions between the dark sector particles and the gauge bosons are present. We, then, discuss potential detection of the DM particle at missing energy searches at the LHC. We show that the cross sections for the current mono-X searches are suppressed, with the mono-jet channel being the most promising probe for future detection. Next, in the fifth chapter, we introduce a minimal model that predicts a DM particle with a naturally obtained sub-GeV mass. This model is a Peccei-Quinn (PQ)-symmetric two-Higgs doublet model, which consists of two Weyl fermionic SU(2)L-doublets and a majorana gauge singlet. The iso-doublets are neutral under the PQ-symmetry and assumed to be decoupled throughout the history of the Universe. The DM particle is the gauge singlet, which is charged under the PQ-symmetry, and therefore massless at tree level. The mass of the DM particle is generated predominantly at one-loop level, by a soft PQ-braking term in the scalar potential. We show that the mass generation is compatible with DM production via the freeze-in mechanism at the early Universe, without relying on parameter fine tuning. In closing, we summarise the results of this thesis and discuss possible future directions of the work presented.
Institution and School/Department of submitter: Πανεπιστήμιο Ιωαννίνων. Σχολή Θετικών Επιστημών. Τμήμα Φυσικής
Subject classification: Σκοτεινή ύλη (Αστρονομία)
Dark matter (Astronomy)
Keywords: Cosmology,Dark matter
URI: http://olympias.lib.uoi.gr/jspui/handle/123456789/28145
Appears in Collections:Διδακτορικές Διατριβές

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