Most of the matter in the universe is undetectable in any way except for the gravity it exerts. While some think this mysterious “dark matter” must be made up of unknown particles, others suggest a familiar particle could be the key.
Dark matter has remained elusive, despite many searches to find something that could explain its effects on the universe. But some scientists are increasingly wondering whether dark matter could be explained by the existing laws of physics. One such idea posits that a particle called a hexaquark (also known as the sexaquark) could explain dark matter without requiring any fancy new theories.
Scientists have long had evidence that the universe contains more mass than what telescopes can see directly. This mass seems to form the scaffolding of the universe and produces anomalies, like galaxies rotating too fast or the gravity from clusters of galaxies warping the light passing through despite no visible objects to do the warping. A popular candidate emerged to explain these effects, a family of particles called weakly interacting massive particles (WIMPs). But after decades of searches for them, WIMPs remain unfound, and scientists are increasingly interested in other ideas.
Devising and hunting for new particles and new configurations of particles is the name of the game in particle physics. The centres of atoms are composed of protons and neutrons, which themselves are made from quarks. The six different quarks most commonly combine into sets of two or three, but physicists at particle colliders have started to discover larger configurations, like tetraquarks (particles with four quarks) and pentaquarks (particles with five). But why not a hexaquark, a particle with six quarks?
In fact, theoretical evidence has already demonstrated that some iterations of these six-quark configurations could be stable particles. And if so, suggested pioneering NYU physicist Glennys Farrar, such a particle could be the basis for dark matter. It’s possible that at higher energies, the particle could have condensed out of the primordial universe at the ratio of dark matter to regular matter that scientists see today and might still be present in the universe, according to a 2017 work of hers. The particle would be twice the mass of a proton and stable with tightly bound quarks (more specifically, two up quarks, two down quarks, and two strange quarks). Perhaps most excitingly, Farrar’s proposal even predicts the abundance of dark matter in the early universe without requiring fine-tuning – basically, if it were to exist, it should exist in the same abundance as theories of the universe predict, without human-inputted adjustments.
Farrar has even devised a way to find such a particle: The hexaquark dark matter would be captured in Earth’s gravitational field and might bind into the nuclei of particles in the Earth’s crust, she explained to Gizmodo. Scientists might look at a heavier flavor of oxygen, called oxygen-18, and compare whether the mass of any of these oxygen-18 nuclei differs from expectations. Some physicists have questioned whether such a hexaquark as Farrar described could truly exist, though she continues to push forward.
Another group of physicists think that evidence for the existence of hexaquarks has started to mount in experimental data. Back in 2011, physicists working on an experiment in Germany called WASA at the COSY particle accelerator observed a particle they increasingly think might be a hexaquark called the d*(2380), named for its mass, 2,380 million electron volts, a little more than twice the mass of the three-quark proton. Researchers behind the new study, published in Journal of Physics G: Nuclear and Particle Physics, calculated that such a particle could form a stable cloud called a Bose-Einstein condensate. Though present-day technology isn’t able to create one of these clouds of hexaquarks, experiments might be able to find signatures of their existence in space via an instantaneous burst of gamma radiation along with other particles caused by the collapse of such a cloud.
The York scientists plan to collaborate with physicists in Germany and the United States to test their theory. Most exciting is the fact that, if hexaquarks could explain dark matter, the mystery would be solved by a particle easily understood using the existing Standard Model of particle physics.
“Our first calculations indicate that condensates of [the d*(2380) particle] are a feasible new candidate for dark matter and this new possibility seems worthy of further, more detailed investigation,” Daniel Watts from the department of physics at the University of York said in a press release.
However, Farrar didn’t find their proposal convincing, since their specific hexaquark’s quark configuration would require a large, positive charge (a problem which they suggest would be overcome if each hexaquark were bound to an electron). To her, the hexaquark devised by the York team seemed seemed too fragile to persist as the abundant dark matter.
Regardless, it’s clear that physicists are pursuing new avenues to explain this mystery – including revisiting the theories we already know.
“Obviously, this would be wild, because it would demonstrate that dark matter, long presumed to be a beyond-the-Standard-Model idea, is in fact part of the Standard Model,” James Beacham, particle physicist with the ATLAS Experiment at the Large Hadron Collider at CERN, told Gizmodo in an email, commenting specifically on Farrar’s work.
Once again, the search for the true identity of dark matter continues.
Featured image: V. Springel, Max-Planck Institut für Astrophysik, Garching bei München