Theoretical Developments Beyond the Standard Model

Abstract

The technical hierarchy problem still remains a guiding principle for particle physics beyond the Standard Model. Low energy supersymmetry remains the only perturbatively calculable solution to the problem. It can contain a suitable dark matter candidate, which may be produced at future colliders. If enough properties of the minimal supersymmetric standard model (MSSM) are measured, a prediction of the relic density can be made, providing useful cosmological information. Universal extra dimensions (UED) is a concrete “straw man” to the MSSM, giving very similar signatures in colliders. Spin-dependent observables are necessary in order to distinguish UED from the MSSM. Some authors have questioned whether the hierarchy problem should be used as a guiding principle and have suggested split supersymmetry as an example of a model that does not solve it. There have also been suggestions which postpone the hierarchy problem to a higher energy scale, in little Higgs models for example. A T-parity symmetry helps the model to satisfy precision electroweak constraints. If one dispenses with the Higgs altogether, models with a tower of heavy WW'/Z' bosons can postpone the onset of perturbative unitarity violation, with an associated relaxation in the effects of precision electroweak constraints. In the UED, MSSM and T-parity little Higgs models, a parity symmetry introduced for seperate phenomenological reasons provides a stable particle which can constitute the dark matter.