A new model for dark matter
Phase transition in the early universe changes strength of interaction between dark and normal matter
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.
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. 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," said Dr. 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 is that 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.
"The central idea underlying our 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," explained Robert McGehee. The researchers envision that – in particle physics – an interaction is usually mediated by a specific particle, a so-called mediator. 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.
New model covers almost the full parameter range of planned experiments
And even more: "The HYPER model of dark matter is able to cover almost the entire range that the new experiments make accessible," added Dr. Gilly Elor.
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 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", said Elor. "In the end we were able to verify that our HYPER model works."
G. Elor, R. McGehee, A. Pierce, Maximizing Direct Detection with Highly Interactive Particle Relic Dark Matter, Physical Review Letters 130: 031803, 20 January 2023,