# Theoretical and experimental approaches to the dark energy and the cosmological constant problem.

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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