Dark matter is the mysterious substance that makes up roughly a quarter of the Universe.
There is strong indirect evidence for its existence from measurements of cosmic primordial radiation, anomalies in the radial dependence of galactic rotational curves and gravitational lensing.
Despite its apparently pivotal role in the Universe the physical origin of dark matter remains unknown.
Theoretical physicists suspect that it is made of unseen particles that neither reflect nor absorb light, but are able to exert gravity.
Since the search for one of leading candidates, called weakly interacting massive particles (WIMPs), has not yet led to success, the research community is looking for alternative candidates, 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,” said Dr. Gilly Elor, a postdoctoral researcher at the Johannes Gutenberg University of Mainz.
“However, our HYPER (HighlY Interactive ParticlE Relics) 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,” said Dr. Robert McGehee, a researcher at the University of Michigan.
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,” said Dr. Aaron Pierce, also from the University of Michigan.
“The HYPER model of dark matter is able to cover almost the entire range that the new experiments make accessible,” Dr. Elor said.
Specifically, the 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,” Dr. McGehee said.
“We then calculated the amount of dark matter that exists in the Universe and then simulated the phase transition using our calculations.”
There are a great many constraints to consider, such as a constant amount of dark matter.
“Here, we have to systematically consider 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,” Dr. Elor said.
“But in the end, we were convinced that our HYPER model works.”
Sources:
Gilly Elor et al. 2023. Maximizing Direct Detection with Highly Interactive Particle Relic Dark Matter. Phys. Rev. Lett 130 (3): 031803; doi: 10.1103/PhysRevLett.130.031803
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