In this study, we investigate the feasibility of Feebly Interacting Massive particles (FIMP) as possible candidates to constitute the observed Dark Matter abundance in the universe. FIMPs are particles that couple very feebly with known particles in the Standard Model (SM). As such, they do not attain thermal equilibrium with the baryon abundant environment in the early universe before Nucleosynthesis. In contrast to a freeze-out mechanism common for Weakly Interacting Massive Particles (WIMP) as DM candidates, FIMPs are produced by the so-called freeze-in mechanism that we will describe in this study. The purpose of this study is to investigate how the Coleman Weinberg (C-W) mechanism affects the FIMP [freeze-in] mechanism. We specifically consider a minimal extension of SM in which an Electroweak Singlet-scalar ($S$) couples only to the Higgs-boson ($H$); This is called the Higgs-portal mechanism. We study the C-W effective potentials for the Higgs and Dark-scalar singlet and their implications on FIMP mechanism.Using these, we focus on the High-temperature production of the DM with just the $HH\mapsto SS$ to compute the reaction rates, comparing Bose-Einstein statistics ($\Gamma_{HH\mapsto S S}^{B-E}$) to Maxwell-Boltzmann statistics $\Gamma_{HH\mapsto S S}^{M-B}$. We employ only $\Gamma_{HH\mapsto S S}^{B-E}$ to compute DM relic abundance ($Y$) at several Dark-scalar masses ($m_S$) as a function of coupling $k$, establishing that Higgs-Dark scalar coupling $k$ $\mapsto$ $k_{DM}$ corresponding to actual DM abundance lies in between ${10}^{-8.7}$ and ${10}^{-8}$, i.e. ${10}^{-8.7}<k_{DM}<="" div=""> / I denna studie undersöker vi mycket svagt interagerande massiva partiklar (FIMP) som möjliga kandidater till den observerade mängden av mörk materia i universum. FIMP:er är partiklar som interagerar väldigt svagt med de kända partiklarna i Standarmodellen (SM). Som sådana uppnår de ej termisk jämvikt med den baryonrika omgivningen i det tidiga universumet innan nukleosyntes sker. I kontrast med en frys-ut mekanism vanligt för svagt interagerade massiva partiklar (WIMPS) som DM kandidater, så produceras FIMP:er av en så kallad frys-in mekanism som vi skall beskriva i denna studie. Syftet med studien är att undersöka hur Coleman Weinberg (C-W) mekanismen påverkar frys-in mekanismen för FIMP:er. Vi tar särskilt hänsyn till en minimal utvidgning av SM i vilken en Elektrosvag singlettskalär ($S$) interagerar med endast Higgsbosonen. Denna mekanism kallas för Higgs-portalmekanism. Vi studerar de C-W effektiva pontetialerna för higgs och mörk-skalärsingletten och deras implikationer för FIMPmekanismen. När vi använder dessa, så fokuserar vi på högtemperatursproduktion av DM med endast $HH\mapsto SS$ för att beräkna reaktionshastigheter, och jämför Bose-Einstein statistik ($\Gamma_{HH\mapsto S S}^{B-E}$) med Maxwell-Boltzmann statistik $\Gamma_{HH\mapsto S S}^{M-B}$. Vi använder endast $\Gamma_{HH\mapsto S S}^{B-E}$ för att beräkna kvarvarande DM mängd ($Y$) för flera mörk-skalärmassor ($m_S$) som en funktion av interaktion $k$, vilket slår fast att Higgs-Mörk skalär interaktionen $k$ $\mapsto$ $k_{DM}$ som korresponderar mot den faktiska DM mängden ligger mellan ${10}^{-8.7}$ and ${10}^{-8}$, i.e. ${10}^{-8.7}<k_{DM}<="" div="">

This thesis investigates the possibility that the gauge boson in a certain hypothetical SU(2) gauged sector can constitute all the non-baryonic dark matter. The gauge bosons acquire mass from the Higgs mechanism as in the Standard Model and scatter elastically among themselves non-gravitationally. It is expected that this self interaction of the dark gauge bosons would resolve the various discrepancies between the ΛCDM model and astrophysical observations on small (e.g. galactic or galaxy cluster) scales. Parameter space within the domain of validity of perturbation theory satisfying the constraints of dark matter abundance, the elastic self-scattering momentum transfer cross-section suggested by recent astrophysical observations, and consideration of the Big-Bang nucleosynthesis was found to be non-empty in the “forbidden” regime where the mass of the dark Higgs boson is greater than the mass of the dark gauge boson. / Graduate

This thesis presents the search for the dark sector process h -> Zd Zd -> 4l in events collected by the ATLAS detector at the Large Hadron Collider in 2015--2018. In this theorized process, the Standard Model Higgs boson (h) decays to four leptons via two intermediate Beyond-the-Standard-Model particles each called Zd. This process arises from interactions of the Standard Model with a dark sector. A dark sector consists of one or more new particles that have limited or zero interaction with the Standard Model, such as the new vector boson Zd (dark photon). It could have a rich and interesting phenomenology like the visible sector (the Standard Model) and could naturally address many outstanding problems in particle physics. For example, it could contain a particle candidate for dark matter. In particular, Higgs decays to Beyond-the-Standard-Model particles are well-motivated theoretically and are not tightly constrained; current measurements of Standard Model Higgs properties permit the fraction of such decays to be as high as approximately 30%. The results of this search do not show evidence for the existence of the h -> Zd Zd -> 4l process and are therefore interpreted in terms of upper limits on the branching ratio B(h -> Zd Zd) and the effective Higgs mixing parameter kappa^prime. / Graduate