The properties of dark matter - its microphysical form, and its cosmological origin and history - are one of the most important mysteries in fundamental physics. So far, evidence for matter beyond the Standard Model comes entirely from gravitational effects. However, other experiments are reaching the point where the 'simplest' models of dark matter are coming into tension with data, and may be strongly constrained by medium-term observations. This, along with theoretical considerations, motivates the exploration of other possibilities for the history and composition of dark matter, especially those with the possibility of new, generic observational signatures. In this thesis, we explore some different classes of new dark matter models, focussing on regimes in which they may display approximately model-independent phenomenology. Firstly, we look at a class of dark matter models featuring large-number, stable composite states, and investigate how these may be synthesised in the early universe. As the example of Standard Model nuclear physics and Big Bang Nucleosynthesis demonstrates, the properties of small-number composite states in strongly-coupled theories may be complicated, and sensitive to the precise details of the theory. However, it may reasonably be expected that the properties of large enough composite states will obey simple geometrical scaling laws. In this case, if large enough states are synthesised in the early universe, the overall results of the synthesis process may become broadly independent of the detailed parameters of the model, and of initial conditions. We model 'dark nucleosynthesis' in such a regime, and find that the late-time number distribution takes on one of two characteristic forms, in both cases with weak dependence on small-number initial conditions and behaviour. Following on from this, we consider the scattering phenomenology that would result from dark matter being made up of such large composite states. This includes the coherent enhancement of scattering rates - for example, at direct detection experiments - compared to e.g. collider production processes. The spatially extended nature of composite dark matter states could also lead to characteristic momentum-dependent form factors in scattering processes, which may be identifiable in direct detection experiments. In addition, inelastic interactions between dark matter states may be important in astrophysical settings. Illustrating the effects of dark-sector energy injections, we present calculations for dark matter halo modifications through velocity kicks. As an example application, we discuss a different class of asymmetric dark matter models, in which late-time decays of part of the dark matter can re-populate a symmetric component, giving annihilation signals in galactic halos. The velocity kicks arising from the decay process may modify the spatial profile of such signals, to the extent to eliminating them almost completely from low-escape-velocity systems such as dwarf galaxies.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:714079 |
| Date | January 2015 |
| Creators | Lasenby, Robert |
| Contributors | March-Russell, John |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://ora.ox.ac.uk/objects/uuid:ac7e21bc-c79b-49cc-9303-6b3fb5783e75 |
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