Global fits of sub-GeV dark matter models

We have seen enormous success in building the Standard Model of particle physics for the visible sector using the frameworks of gauge theories and quantum field theory. It has been challenging to translate this success to dark matter, because unlike the visible sector, dark matter has so far only been unambiguously observed through its gravitational consequences.
This allows for a degeneracy in models that can fit data from current dark matter searches.
Due to the lack of complementary dark matter signals in experiments that can break the degeneracy, the current strategy to find the dark matter model closest to the truth is by means of exclusion. Formally, this is done through rigorous statistical fits of dark matter models against all available experimental data – called global fits – and through model comparisons.
In the first part of the thesis, we extend and employ the code GAMBIT to perform global fits of a well motivated class of dark matter models. In the second part, we present GAMBIT UTILISeD, a tool developed to interactively visualise global fit results obtained using GAMBIT.
We perform global fits of models with thermal sub-GeV dark matter candidates coupled to a dark photon mediator that undergoes kinetic mixing with the standard model photon. ... mehrDirect detection bounds on dark matter scattering are relatively weaker for sub-GeV dark matter compared to the traditional GeV-TeV scale weakly interacting massive particles (WIMPs).
Sub-GeV dark matter is thus a particularly well-motivated alternative to traditional WIMPs.
However, for s-wave annihilating fermionic sub-GeV dark matter, the strong constraints from indirect detection and the cosmic microwave background (CMB) can significantly restrict the model parameter space that reproduces the correct dark matter relic abundance. We explore different ways in which these constraints can be evaded for fermionic dark matter and also study the p-wave annihilating complex scalar dark matter candidate, which is free from the CMB constraint. These models additionally face new bounds from laboratory experiments and we thus perform global fits to determine their current status. We also study an extension of this model class which includes a dark Higgs that gives rise to the dark matter and dark
photon masses. The first-order phase transition of such a dark Higgs in a sub-GeV dark sector can produce nano-Hertz gravitational waves. This is particularly interesting in the context of the nano-Hertz stochastic gravitational wave background (SGWB), recently observed by various pulsar timing arrays. The statistical fit of the leading astrophysical explanation of this signal – gravitational waves from merging super-massive black hole (SMBH) binaries – leaves room for contribution from exotic sources, such as the first-order phase transition in our sub-GeV dark sector. We thus study the viability of this model to explain the gravitational
wave background (in addition to the contribution from SMBH binaries ) and simultaneously reproduce the correct dark matter relic abundance, while satisfying constraints imposed by laboratory, astrophysical and cosmological data.