Licentiate Thesis: Phenomenological Studeis in Cosmoparticle Physics - Expansion Histories in Non-Einstein Gravity and Supersymmetric Dark Matter at the LHC

by Sara Rydbeck (DESY)

Wednesday 28 Oct 2009, 13:15 → 15:15 Europe/Stockholm

FB52

As the Big Bang model has become established by a range of observations, the fields of cosmology and particle physics have become intertwined. Evidence coming from observations of galaxies, nucleosynthesis, supernovae and the cosmic microwave background forces us to consider the new phenomena of dark matter and dark energy. This interpretation is based on our understanding of gravity, while the standard model of particle physics deals with 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 would not need to be a new contribution to the energy content at all. Examples from two classes of models are considered. One is the hypothesis that our world is confined to a brane residing in infinite size extra dimensions, to which only gravity can escape, as proposed by Dvali, Gabadadze and Porrati (DGP). The other is an f(R) low curvature modification to the Einstein-Hilbert action. The expansion histories in these 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. We show the difficulty of distinguishing between DGP and Einstein gravity with a cosmological constant due to systematic uncertainties in the data, while we find the 1/R model for cosmological expansion to be ruled out using its prediction for the thickness of the last scattering surface. The second 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, in direct detection experiments underground or indirect detection using gamma-ray or cosmic ray telescopes, the uncertainties due to astrophysics and the unknown distribution of the dark matter are large. Particle accelerators are our way of imitating the conditions of the early universe in the lab, where we can hope to produce yet unknown heavy particle states in high energy collisions and in a more controlled environment determine their properties. We study the prospects of discovering minimally supersymmetric models of weakly interacting dark matter at the Large Hadron Collider using early data. Using a signature of isolated leptons but no missing energy, we find an important model property for early discovery to be a mass spectrum where sleptons are light compared to squarks.