This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Researchers at the DarkSide collaboration have unveiled results from a seven-year experiment that tested whether dark matter particles could exist as composites of smaller, elementary particles. While the search turned up no direct evidence of this "ultraheavy nuclear" dark matter, results from the DarkSide-50 detector could lay the groundwork for future experiments, helping physicists work out whether dark matter has an internal structure.
The research has been published in Physical Review D. To date, two of the most prevalent theories explaining the nature of dark matter are axions and weakly interacting massive particles (WIMPs), neither of which interacts with radiation or regular matter except via gravity. While the two theories differ widely, they share the assumption that dark matter exists entirely as elementary particles, which can't be broken down further—unlike an atomic nucleus, which separates into constituent protons and neutrons.
Yet as direct detection of these particles continues to elude physicists, researchers including Jocelyn Monroe at the University of Oxford have considered an alternative idea. "About a decade ago, my collaborators and I explored the possibility that dark matter might instead have its own nuclear physics," she describes. Under this description, "dark particles bind together in the early universe to form very large 'dark nuclei," potentially containing billions or vastly more constituents." Building on these initial theories, Monroe's team considered how composite ultraheavy dark matter particles would appear in experimental detectors, in ways comparable to the instruments built to detect axions and WIMPs.
The scope of the work soon grew to include an international network of physicists, which became the DarkSide collaboration. From 2013 to 2020, the collaboration operated DarkSide-50, a detector built around a cylindrical tank of liquid argon, housed at INFN's Laboratori Nazionali del Gran Sasso in Italy—the world's largest underground research facility. Like other dark matter detectors, DarkSide-50 was designed to pick up faint flashes of light predicted to occur when dark matter particles scatter off atomic nuclei in the liquid, causing a nucleus to recoil and deposit energy.
For elementary axions or WIMPs, this interaction would produce a single recoil. "The key distinction is that the analysis takes their internal structure into account," Monroe explains. "The dark matter has a finite size and a corresponding form factor, and a single object can potentially scatter several times as it passes through the detector." In their latest study, the DarkSide collaboration presents an exhaustive analysis of data collected during DarkSide-50's run.
In a familiar result, the experiment found no evidence of composite dark matter interacting with argon nuclei. Nonetheless, the results let the team rule out a wide range of proposed properties for ultraheavy dark matter, sharpening the target for future experiments. "An interesting feature of the result is that the sensitivity depends on the properties of the constituents—the 'dark nucleons'—showing that the internal structure of the dark matter can have observable consequences," Monroe adds.
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