A new model for dark matter

(Nanowerk News) Dark matter remains one of the greatest mysteries of modern physics. It is clear that it must exist, because without dark matter, for example, the motion of galaxies cannot be explained. But it has never been possible to detect dark matter directly in an experiment. Currently, there are many proposals for new experiments: They aim to detect dark matter directly via its scattering from the constituents of the atomic nuclei of a detection medium, i.e. protons and neutrons.
A team of authors including Gilly Elor, a postdoctoral researcher at the PRISMA+ Cluster of Excellence at Johannes Gutenberg University-Mainz, Germany, and Robert McGehee and Aaron Pierce of the University of Michigan in Ann Arbor, USA, has now proposed a new candidate for dark matter – and named it HYPER, for "HighlY Interactive ParticlE Relics." The twist: In the HYPER model, some time after the formation of dark matter in the early universe, the strength of its interaction with normal matter increases abruptly – which on the one hand makes it potentially detectable today and at the same time can explain the abundance of dark matter.
The researchers now present the HYPER Dark Matter model and the phase transition it contains for the first time in the journal Physical Review Letters ("Maximizing Direct Detection with Highly Interactive Particle Relic Dark Matter").
The HYPER model covers almost the complete parameter range of planned experiments for the direct search for dark matter
The HYPER model covers almost the complete parameter range of planned experiments for the direct search for dark matter. (Image: Gilly Elor) (click on image to enlarge)

The new diversity in the dark matter sector

After the search for heavy dark matter particles, so-called WIMPs, has so far not led to success, the research community is looking for alternative dark matter particles, especially lighter ones. At the same time one generically expects phase transitions in the dark sector, after all there are several in the visible sector. But previous studies have tended to neglect them. "There has not been a consistent dark matter model for the mass range that some planned experiments hope to access," says Gilly Elor. "However, our HYPER model illustrates that a phase transition can actually help make the dark matter more easily detectable."
The challenge for a suitable model: if dark matter interacts too strongly with normal matter, its (precisely known) amount formed in the early universe would be too small, contradicting astrophysical observations. However, if it is produced in just the right amount, the interaction would conversely be too weak to detect dark matter in present-day experiments.
"Our central idea, which underlies the HYPER model, is that the interaction changes abruptly once. So we can have the best of both worlds: the right amount of dark matter and a large interaction so we might detect it," Robert McGehee explains. And this is how the researchers envision it: In particle physics, an interaction is usually mediated by a specific particle, a so-called mediator – and so is the interaction of dark matter with normal matter. Both the formation of dark matter and its detection function via this mediator, with the strength of the interaction depending on its mass: The larger the mass, the weaker the interaction.
The mediator must first be heavy enough so that the correct amount of dark matter is formed and later light enough so that dark matter is detectable at all. The solution: there was a phase transition after the formation of dark matter, during which the mass of the mediator suddenly decreased. "Thus, on the one hand, the amount of dark matter is kept constant, and on the other hand, the interaction is boosted or strengthened in such a way that dark matter should be directly detectable," Aaron Pierce reports.

New model covers almost the full parameter range of planned experiments

Even more: "The HYPER model of dark matter is able to cover almost the entire range that the new experiments make accessible," complements Gilly Elor.
Specifically, the research team first considered the maximum cross section of the mediator-mediated interaction with the protons and neutrons of an atomic nucleus to be consistent with astrophysical observations and certain particle-physics decays. The next step was to consider whether there was a model for dark matter that exhibited this interaction.
"And here we came up with the idea of the phase transition," the authors describe in the current article. "We calculated the amount of dark matter that exists in the universe and then simulated the phase transition in our calculations." There are a great many constraints to consider, such as a constant amount of dark matter. "Here we have to systematically check and include very many scenarios, for example asking the question whether it is really certain that our mediator does not suddenly lead to the formation of new dark matter, which of course must not be", says Gilly Elor. "But in the end, we verified that our HYPER model works!"
Source: University of Mainz
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