Theoretical and Experimental Approaches to the Dark Energy and the Cosmological Constant Problem

The cosmological constant problem is one of the most pressing problems of physics at this time. In this dissertation the problem and a set of widely-discussed theoretical solutions to this problem are reviewed. It is shown that a recently developed Lorentz gauge theory of gravity can provide a natural solution. In this theory presented here, the metric is not dynamical and it is shown that the Schwartzschild metric is an exact solution. Also, it is proven that the de Sitter space is an exact vacuum solution and as a result the theory is able to explain the expansion of the universe with no need for dark energy. Renormalizability of the theory is studied as well. It is also shown that, under a certain condition, the theory is power-counting renormalizable. Supersymmetry provides an alternative solution to the cosmological problem as well. The idea behind supersymmetry is reviewed and an experimental search for supersymmetry is presented. The experimental search discussed in this dissertation is based on all-hadronic events with large missing transverse momentum produced in proton-proton collisions at √ s = 13 TeV. The data sample, corresponding to an integrated luminosity of 2.3 fb−1 , was collected with the CMS detector at the CERN LHC in 2015. The data are examined in search regions defined with jet multiplicity, tagged bottom quark jet multiplicity, missing transverse momentum, and the scalar sum of jet transverse momenta. The observed numbers of events in all search regions are found to be consistent with the expectations from standard model processes. Exclusion limits are presented for simplified supersymmetric models for pair production of gluinos, supersymmetric partners of gluons. Depending on the assumed gluino decay mechanism, and for a massless, weakly interacting, lightest neutralino, lower limits on the gluino mass from 1440 to 1600 GeV are obtained, significantly extending previous limits.