Supersymmetry

The Standard Model of the electroweak and strong interactions is remarkably successful in explaining experimental observations at the presently accessible energies. However, the model has serious theoretical shortcomings, including 19 arbitrary parameters, the ad hoc introduction of a negative Higgs mass squared term to break the electroweak symmetry, and a fine-tuning problem in higher-order corrections in perturbation theory to the Higgs boson mass.

The existence of a supersymmetry, with equal numbers of boson and fermion states connected by the symmetry, would naturally explain the spontaneous symmetry breaking and solve the fine-tuning problem through additional loops that cancel quadratic divergences. A natural cancellation of the divergences requires that supersymmetric particles exist below about 1 TeV, though less stringent bounds up to 20 TeV are possible for first and second generation states. Thus collider energies at the TeV scale make possible the search for the particles of supersymmetry and to thereby verify or reject the supersymmetry hypothesis.

The experimental signatures of sparticles depend on the way that the supersymmetry breaking is communicated from the hidden sector of the theory to the physical sector, with gravity-mediated models and gauge-mediated models leading to different decay patterns of the sparticles and consequently to very different signals. We are making extensive phenomenological studies of the collider signals of supersymmetry and their Standard Model backgrounds that are important to define the criteria of experimental searches.