There is overwhelming evidence for the existence of dark matter over a very large range of astrophysical scales, ranging from galactic scales to the largest observable scales in the Universe. While the presence of dark matter has been inferred solely through gravitational...
There is overwhelming evidence for the existence of dark matter over a very large range of astrophysical scales, ranging from galactic scales to the largest observable scales in the Universe. While the presence of dark matter has been inferred solely through gravitational interactions, it is theoretically very well-motivated to assume that dark matter consists of new particles arising in physics beyond the Standard Model. An immediate question is therefore what do we know about the properties of the dark matter particles given the observational evidence.
Beyond the simple observation that the dark matter particle needs to be stable on cosmological time scales, it is also known how much dark matter there is: the most precise determination of the amount of dark matter comes from measurements of the cosmic microwave background, with the latest results from the PLANCK satellite determining the dark matter density with unprecedented precision, confirming that it is about five times more abundant than ordinary matter. This already entails a very crucial information: While it is unclear how dark matter interacts with ordinary matter, there is a strong preference for it to have additional interactions beyond the (well established) gravitational one in order to explain this observed relic abundance.
Another important piece of information comes from the comparison of N-body simulations with data, which indicates that dark matter should be cold (or at most warm), i.e. non-relativistic during the onset of structure formation, to match the observed large scale structure of the Universe. In these N-body simulations it is usually also assumed that dark matter particles are collisionless. While this standard paradigm is extremely successful on cosmological scales, there are well-known problems on galactic scales, for instance the observation that the dark matter profiles in dwarf galaxies have a core while the expectation from simulations with collisionless cold dark matter is a cuspy profile, or the overprediction of the number of small subhalos within larger systems. These problems might actually be hints that the dark matter rather than being collisionless is collisional with a sizeable self-interaction cross section.
To make progress towards the goal of identifying the dark matter there are two complementary ways to pursue. As dark matter is very likely a new, unknown particle, it should be theoretically described in the language of particle physics. Correspondingly a natural approach is to start from the particle physics perspective and find a theoretically consistent description of dark matter. This is highly non-trivial as all known facts not only about dark matter (e.g. the relic abundance and cross section bounds) but also everything else need to be simultaneously explained, i.e. this theoretical description must be consistent with every known experimental result. What is more the theoretical framework should ideally also solve other mysteries/shortcomings within particle physics, such as the naturalness problem of the Standard Model. The great potential of this very ambitious strategy is that many problems of particle physics are interconnected, and as all this has to be addressed within one consistent theoretical framework, there will undoubtedly be many non-trivial implications for the nature of dark matter.
The more direct approach is to look for dark matter experimentally and actually measure its non-gravitational properties. This is of course crucial to establishing the true nature of dark matter. Such searches are currently being performed with different techniques by a vast number of experiments. All these searches are motivated by the simple observation that dark matter should have some non gravitational interactions in order to explain the observed relic abundance. These interactions then imply that dark matter should be detectable in several different ways: direct detection experiments aim to measure dark matter parti
Overall there has been significant progress in all objectives and a total of 21 major publications until the end of the third reporting period. In the following we will describe the work aligned with the published results.
The first publication within this period is a study of an astrophysical system which may give evidence for dark matter self-interactions. Specifically self-interactions of dark matter particles can potentially lead to an observable separation between the DM halo and the stars of a galaxy moving through a region of large DM density. Such a separation has recently been observed in a galaxy falling into the core of the galaxy cluster Abell 3827. We estimate the DM self-interaction cross-section needed to reproduce the observed effects and find that the sensitivity of Abell 3827 has been significantly overestimated in a previous study. Interestingly we find cross sections which are marginally consistent with other astrophysical bounds, implying that the collisionality of dark matter should be testable in the near future. This work is part of objective 4 and has been published in MNRAS.
In a second publication we show that simplified models used to describe the interactions of dark matter with Standard Model particles do not in general respect gauge invariance and that perturbative unitarity may be violated in large regions of the parameter space. The modifications necessary to cure these inconsistencies may imply a much richer phenomenology and lead to stringent constraints on the model. We illustrate these observations by considering the simplified model of a fermionic dark matter particle and a vector mediator. Imposing gauge invariance then leads to strong constraints from dilepton resonance searches and electroweak precision tests. Furthermore, the new states required to restore perturbative unitarity can mix with Standard Model states and mediate interactions between the dark and the visible sector, leading to new experimental signatures such as invisible Higgs decays. The resulting constraints are typically stronger than the \'classic\' constraints on DM simplified models such as monojet searches and make it difficult to avoid thermal overproduction of dark matter. We also study the complementarity between collider searches and dark matter direct detection experiments. This work is part of objectives 1, 2 and 3 and has been published in JHEP.
In a third publication we study light pseudoscalar particles (mediators) at SHiP and NA62. The challenge in proton beam dumps is to reliably calculate the production of the new particles from the interactions of two composite objects, the proton and the target atoms. In this work we argue that Primakoff production of ALPs proceeds in a momentum range where production rates and angular distributions can be determined to sufficient precision using simple electromagnetic form factors. Reanalysing past proton beam dump experiments for this production channel, we derive novel constraints on the parameter space for ALPs. We show that the NA62 experiment at CERN could probe unexplored parameter space by running in \'dump mode\' for a few days and discuss opportunities for future experiments such as SHiP. This work is part of objectives 1 and 2 and has been published in JHEP.
In a fourth publication we show that contrary to naive expectations a diphoton excess at the LHC could be explained by an underlying vector resonance, which decays to a photon and a light scalar s, followed by a decay of the scalar into two photons: Zâ€²â†’Î³sâ†’3Î³. As the two photons from the scalar decay are highly boosted, the experimental signature is an apparent diphoton final state. In fact all the necessary ingredients are naturally present in Zâ€² models which are also interesting in the context of simplified dark matter models: Additional fermions with electroweak quantum numbers are required in order to render the theory anomaly free and naturally induce the required effective couplings, whil
Within this project there has been progress beyond the state of the art in many aspects, ranging from theoretical particle physics model building over the development of dedicated analysis strategies to test these models at colliders such as the LHC or Belle II to a novel effective description of small dark matter haloes, which can be used to capture the dominant effects of self-interacting dark matter on haloes that are not explicitly included in a simulation, but may be included in future cosmological N-body simulations.
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