This work consists of two parts. In the first part we construct the complete
extension of the Minimal Supersymmetric Standard Model by higher dimensional
effective operators and study its phenomenology. These operators encapsulate
the effects on LHC physics of possible new degrees of freedom at the multiTeV
scale. The effective analysis includes the case where the multiTeV physics is
the supersymmetry breaking sector itself. Beyond the new effective couplings,
the analysis suggests an interpretation of the 'little hierarchy problem' as an
indication of new physics at multiTeV scale. In the second part we explore the
power of constrained superfields in extended supersymmetry. It is known that in
N = 2 supersymmetry the gauge kinetic function cannot depend on hypermultiplet
scalars. However, it is also known that the low energy effective action of a
D-brane in an N = 2 supersymmetric bulk includes the DBI action, where the
gauge kinetic function does depend on the dilaton. We show how the
nonlinearization of the second SUSY (imposed by the presence of the D-brane)
opens this possibility, by constructing the global N = 1 linear + 1 nonlinear
invariant coupling of a hypermultiplet with a gauge multiplet. The constructed
theory enjoys interesting features, including a novel super-Higgs mechanism
without gravity.Comment: Ph.D. thesis. Advisor: Prof. Ignatios Antoniadis. 236 pages, 9
figure