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New avenues towards solving the dark matter puzzle

Periodic Reporting for period 4 - NewAve (New avenues towards solving the dark matter puzzle)

Reporting period: 2019-12-01 to 2020-05-31

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. To identify the nature of dark matter is evidently of fundamental importance and one of the top priorities in science today. The project NewAve aimed at exploring new avenues towards solving the dark matter puzzle as part of the global efforts in this direction, with a particular focus on research questions centred around (i) theoretical dark matter model building, (ii) the study of new collider signatures, (iii) developing new techniques for the comparison and interpretation of direct detection experiments and (iv) identifying astrophysical probes which constrain or give evidence for dark matter self-interactions.
Overall, while there is still no clear experimental signal for the elusive dark matter particle to date, a number of important steps towards identifying the dark matter of the Universe have been made in this project ranging from the exploration of novel collider observables to the simulation of astrophysical systems. To exploit the complementarity of different search strategies as well as theoretical ideas for dark matter, this project followed an inherently multi disciplinary approach and brought together different fields including elementary particle physics, astrophysics and cosmology.

An example which illustrates significant interdisciplinary developments concerns the study of dark matter self-interactions: Due to the complexity of astrophysical systems, numerical N-body simulations are the prime tool to study the effects of dark matter self-interactions in detail. A crucial question therefore is how to map the microphysics of self-interactions onto such a simulation. Currently these codes – which are mainly used within the astrophysics community – implicitly assume a specific type of DM self-interaction, where DM particles scatter isotropically. This form of interaction is relatively easy to implement as scatterings of this type have to be rare in order to satisfy observational constraints. From a particle physics perspective the scattering cross section however naturally depends on the scattering angle as well as the relative velocity of the dark matter particles and results from current N-body simulations cannot generally be applied. For a realistic treatment it is therefore very important to combine the expertise from particle as well as astrophysics to correctly implement different types of self-interactions so that firm conclusions on the properties of dark matter can be drawn from detailed astrophysical observations. In this project we presented the first N-body simulations that adapt the equations of smoothed particle hydrodynamics to capture the effect of dark matter self-interactions which are too frequent to be resolved explicitly. Our approach may be combined with explicit simulations of rare scatterings in order to simulate accurately the effects of arbitrary dark matter self-interactions in future cosmological simulations.
We also explored novel possible experimental signatures of dark sector particles in cosmology and direct dark matter searches as well as in high energy particle physics machines such as the LHC, in low energy colliders such as Belle II or in beam dump experiments such as NA62. The corresponding publications had a large impact on the community and most of the new search strategies have already been implemented by the respective experimental collaborations. As too many different aspects of the possible dark matter phenomenology have been explored in this project to describe here in detail, let me mention a couple of examples below.
Regarding searches for dark matter at particle physics experiments, we discussed a novel signature of dark matter production at the LHC resulting from the emission of an additional Higgs boson in the dark sector. The presence of such a dark Higgs boson is motivated simultaneously by the need to generate the masses of the particles in the dark sector and the possibility to relax constraints from the dark matter relic abundance by opening up a new annihilation channel. If the dark Higgs boson decays into Standard Model states via a small mixing with the Standard Model Higgs boson, one obtains characteristic large-radius jets in association with missing transverse momentum that can be used to efficiently discriminate signal from backgrounds. Both the ATLAS and CMS groups now perform the corresponding search and first experimental results are imminent.
Another example of a new signature which has been studied in this project concerns light dark sector particles (which could play the role of dark matter mediators). We showed that the NA62 experiment at CERN could probe unexplored parameter space by running in 'dump mode' for a few days and discussed opportunities for future experiments such as SHiP. Also here the experimental NA62 collaboration started to perform these searches as suggested by us.
Yet another example concerns light axion-like particles (ALPs). We performed a detailed calculation of the expected sensitivity of the Belle II experiment in Japan, which can search for visibly and invisibly decaying ALPs, as well as long-lived ALPs. This analysis has been done in a collaboration with experimentalists from the Belle II experiment. The Belle II sensitivity is found to be substantially better than previously estimated, covering wide ranges of relevant parameter space. In particular, Belle II can explore an interesting class of dark matter models, in which ALPs mediate the interactions between the Standard Model and dark matter. In these models, the relic abundance can be set via resonant freeze-out, leading to a highly predictive scenario consistent with all existing constraints but testable with single-photon searches at Belle II in the near future. Here the first experimental results have been made public very recently.

Overall these and other significant results have had a substantial impact on the community and have led for example to a number of new searches for dark sector particles in large experimental research facilities or have impacted the design of completely new experiments.
The corresponding results have also been well presented at a number of conferences and workshops, as well as in a large number of outreach events.
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 novel ideas for the simulation of dark matter self-interactions which are too frequent to be resolved explicitly in the context of numerical N-body simulations.