Beyond the Standard Model

Abstract

'BSM physics' is a phrase used in several ways. It can refer to physical phenomena established experimentally but not accommodated by the Standard Model, in particular dark matter and neutrino oscillations (technically also anything that has to do with gravity, since gravity is not part of the Standard Model). 'Beyond the Standard Model' can also refer to possible deeper explanations of phenomena that are accommodated by the Standard Model but only with ad hoc parameterizations, such as Yukawa couplings and the strong CP angle. More generally, BSM can be taken to refer to any possible extension of the Standard Model, whether or not the extension solves any particular set of puzzles left unresolved in the SM. In this general sense one sees reference to the BSM 'theory space' of all possible SM extensions, this being a parameter space of coupling constants for new interactions, new charges or other quantum numbers, and parameters describing possible new degrees of freedom or new symmetries. Despite decades of model-building it seems unlikely that we have mapped out most of, or even the most interesting parts of, this theory space. Indeed we do not even know what is the dimensionality of this parameter space, or what fraction of it is already ruled out by experiment. Since Nature is only implementing at most one point in this BSM theory space (at least in our neighborhood of space and time), it might seem an impossible task to map back from a finite number of experimental discoveries and measurements to a unique BSM explanation. Fortunately for theorists the inevitable limitations of experiments themselves, in terms of resolutions, rates, and energy scales, means that in practice there are only a finite number of BSM model 'equivalence classes' competing at any given time to explain any given set of results. BSM phenomenology is a two-way street: not only do experimental results test or constrain BSM models, they also suggest - to those who get close enough to listen - new directions for BSM model building. Contrary to popular shorthand jargon, supersymmetry (SUSY) is not a BSM model: it is a symmetry principle characterizing a BSM framework with an infinite number of models. Indeed we do not even know the full dimensionality of the SUSY parameter space, since this presumably includes as-yet-unexplored SUSY-breaking mechanisms and combinations of SUSY with other BSM principles. The SUSY framework plays an important role in BSM physics partly because it includes examples of models that are 'complete' in the same sense as the Standard Model, i.e. in principle the model predicts consequences for any observable, from cosmology to b physics to precision electroweak data to LHC collisions. Complete models, in addition to being more explanatory and making connections between diverse phenomena, are also much more experimentally constrained than strawman scenarios that focus more narrowly. One sometimes hears: 'Anything that is discovered at the LHC will be called supersymmetry.' There is truth behind this joke in the sense that the SUSY framework incorporates a vast number of possible signatures accessible to TeV colliders. This is not to say that the SUSY framework is not testable, but we are warned that one should pay attention to other promising frameworks, and should be prepared to make experimental distinctions between them. Since there is no formal classification of BSM frameworks I have invented my own. At the highest level there are six parent frameworks: (1) Terascale supersymmetry; (2) PNGB Higgs; (3) New strong dynamics; (4) Warped extra dimensions; (5) Flat extra dimensions; and (6) Hidden valleys. Here is the briefest possible survey of each framework, with the basic idea, the generic new phenomena, and the energy regime over which the framework purports to make comprehensive predictions.