Majorana Dark Matter with a Coloured Mediator: Collider vs Direct and Indirect Searches

We investigate the signatures at the Large Hadron Collider of a minimal model where the dark matter particle is a Majorana fermion that couples to the Standard Model via one or several coloured mediators. We emphasize the importance of the production channel of coloured scalars through the exchange of a dark matter particle in the t-channel, and perform a dedicated analysis of searches for jets and missing energy for this model. We find that the collider constraints are highly competitive compared to direct detection, and can even be considerably stronger over a wide range of parameters. We also discuss the complementarity with searches for spectral features at gamma-ray telescopes and comment on the possibility of several coloured mediators, which is further constrained by flavour observables.Comment: 32 pages, 9 figure

Early search for supersymmetric dark matter models at the LHC without missing energy

We investigate early discovery signals for supersymmetry at the Large Hadron Collider without using information about missing transverse energy. Instead we use cuts on the number of jets and isolated leptons (electrons and/or muons). We work with minimal supersymmetric extensions of the standard model, and focus on phenomenological models that give a relic density of dark matter compatible with the WMAP measurements. An important model property for early discovery is the presence of light sleptons, and we find that for an integrated luminosity of only 200--300 pb$^{-1}$ at a center-of-mass energy of 10 TeV models with gluino masses up to $\sim 700$ GeV can be tested.Comment: 28 pages, 12 figures; published versio

Expansion history and f(R) modified gravity

We attempt to fit cosmological data using $f(R)$ modified Lagrangians containing inverse powers of the Ricci scalar varied with respect to the metric. While we can fit the supernova data well, we confirm the $a\propto t^{1/2}$ behaviour at medium to high redshifts reported elsewhere and argue that the easiest way to show that this class of models are inconsistent with the data is by considering the thickness of the last scattering surface. For the best fit parameters to the supernova data, the simplest 1/R model gives rise to a last scattering surface of thickness $\Delta z\sim 530$, inconsistent with observations.Comment: accepted in JCAP, presentation clarified, results and conclusions unchange

Testing the DGP model with ESSENCE

We use the recent supernova data set from the ESSENCE collaboration combined with data from the Supernova Legacy Survey and nearby supernovae to test the DGP brane world model and its generalisations. Combination of this data with a flatness prior and the position of the peak of the CMB disfavours the DGP model slightly. Inclusion of the baryon acoustic peak from the Sloan Digital Sky Survey increase the tension of the DGP model with the data, although it is not clear how self consistent this procedure would be without a re-analysis of the survey data in the framework of the DGP cosmology. Generalisations of the DGP model are tested and constraints on relevant parameters obtained.Comment: Minor corrections, clarifications and references added. Published in JCA

Phenomenological Studies in Cosmoparticle Physics : Expansion Histories in non-Einstein Gravity and Dark Matter at the Large Hadron Collider

As the Big Bang model has become established, the fields of cosmology and particle physics have become intertwined. A range of observations forces us to consider the phenomena of dark matter and dark energy. This interpretation is based on our understanding of gravity, while the standard model of particle physics describes the other fundamental forces in nature and fails to explain the dark components. This thesis includes two different types of studies where hypotheses of physics beyond the standard models of particle physics and cosmology are faced with what observations and experiments can tell us. The first one deals with the possibility that our theory of gravity is what has to be modified at large distances to explain the dark energy, which then need not be a contribution to the energy content at all. The expansion histories in two such frameworks are tested with data from type Ia supernovae and measurements of the baryon acoustic peak in the galaxy distribution as well as in the cosmic microwave background. The second type of study concerns the possibility of establishing the particle nature of dark matter through interactions other than gravitational. While there are ways of doing this using astrophysical observations, the uncertainties due to astrophysics and the unknown distribution of the dark matter are large. High energy particle colliders provide a way of imitating the conditions of the early universe in the laboratory, where we can hope to produce yet unknown heavy particle states and in a more controlled environment determine their properties. We study the prospects for discovering two types of weakly interacting dark matter candidates at the CERN Large Hadron Collider.At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript

Social Media Marketing : A Quantitative Study on the Basis of a Social Network Perspective

I studien studeras hur komplexiteten kring sociala medier kan studeras, samt på vilket sätt sociala nätverk är relaterade till nätverken i sociala medier och hur det påverkar konsumenterna.  Syftet med studien är att undersöka de sociala medierna i förhållande till de sociala nätverken med fokus på konsumentbeteende och därigenom skapa en generell modell som förklarar problematiken.  Studien bygger på en litteraturstudie som i första hand sammanställts av artiklar med fokus på ämnesområdena sociala nätverk, elektroniska sociala nätverk och sociala medier. En enkätundersökning gjordes för att sammanställa konsumenters åsikter om ämnesområdena, i syfte att tillslut utmynna i en analys och diskussion.  Det går att konstatera att sociala medier har vidgat det sociala nätverket för den enskilde konsumenten. Detta resulterar i att den enskilde konsumenten nu kan komma i kontakt med fler aktörer och andra konsumenter som i många fall kan vara okända. Trots att de är okända tenderar konsumenter att lita på den information som erhålls i den utsträckningen att det kan påverka köpbeslutet. The study examines the complexity of how social media can be studied, and in what way the social networks relate to networks in social media and how it affects the consumers. The purpose with the study is to examine the relation between social media and social networks, with a focus on consumer behavior and thereby create a general model that explains the problem.  The study is based on a literature review that is primarily compiled by articles with focus on issues surrounding social networks, electronic social networks and social media. A survey was done to compile consumers' opinions about the area for the topics, in order to eventually culminate in an analysis and discussion.  It is possible to conclude that social media has expanded the social network for the individual consumer. The result is that individual consumers now can get in touch with more influencing factors and other customers that in many cases may be unknown. Even though they are unknown, consumers tend to rely on the information obtained to the extent that it can affect the purchase decision.

Status of the Inert Doublet Model and the Role of multileptons at the LHC

A possible feature of the inert doublet model (IDM) is to provide a dark matter candidate together with an alteration of both direct and indirect collider constraints that allow for a heavy Higgs boson. We study the IDM in light of recent results from Higgs searches at the Large Hadron Collider (LHC) in combination with dark matter direct detection limits from the XENON experiment. We ask under what conditions the IDM can still accommodate a heavy Higgs boson. We find that IDM scenarios with a Higgs boson in the mass range 160-600 GeV are ruled out only when all experimental constraints are combined. For models explaining only a fraction of the dark matter the limits are weakened, and IDMs with a heavy Higgs are allowed. We discuss the prospects for future detection of such IDM scenarios in the four-lepton plus missing energy channel at the LHC. This signal can show up in the first year of running at \sqrt{s} = 14 TeV, and we present detector-level studies for a few benchmark models.Comment: Matches published version apart from: A note added on 126 GeV SM scalar mass including a table of viable benchmark models, and a typo corrected in Eq. 3. (23 pages, 10 figures and 7 tables