At its center, a supernova remnant contains a neutron star, thought to contain a soup of quark matter. A new study suggests the soup may lack strange quarks.

Image: (X-ray) NASA/CXC/University of Amsterdam/N. Rea et al.; (Optical) DSS

Weird new form of nuclear matter might lie just beyond experimenters’ grasp

Within a neutron star—the remains of an exploded, middle-weight star—pressures climb a billion billion times higher than in the sun’s core. For decades, some theoretical physicists have speculated that under those conditions, a bizarre type of matter might emerge: a soup of the subatomic particles called quarks. Now, a new analysis indicates the recipe for that soup, called cold quark matter, needs revision. If correct, it suggests that particle accelerators on Earth might be able to produce stable bits of the quark matter. It also would put the kibosh on hypothetical particles called strangelets, which fearmongers once claimed could destroy the world.

“It’s a speculative argument, but there is nothing obviously wrong with it,” says Robert Pisarski, a nuclear theorist at Brookhaven National Laboratory in Upton, New York, who was not involved in the work.

Atomic nuclei consist of protons and neutrons, which themselves consist of trios of up and down quarks—two of the particles’ six “flavors”—bound tightly by the strong nuclear force. Since the 1970s, some theorists have predicted that under extreme pressures like those in the hearts of neutron stars, quarks might break free of their strong-force chains to create a soup of cold quark matter. They also predicted that the soup’s ingredients would differ from those of protons and neutrons. Their calculations suggested that to minimize its energy, quark matter should include a third flavor of quarks known as strange quarks.

Even though strange quarks emerge only fleetingly, usually in collisions at particle accelerators, calculations suggested that such strange quark matter might have a lower energy than ordinary nuclear matter has. That means that specks of strange quark matter, or strangelets, could be stable and that, in principle, ordinary nuclei could change into them. That transformation would require simultaneous conversions of up and down quarks to strange quarks—something unlikely to happen spontaneously in the age of the universe. But strangelets generated in cosmic rays or lingering from violent astrophysical events might survive indefinitely. Scientists have searched for them in many ways, so far unsuccessfully.

Now, Bob Holdom, a nuclear theorist at the University of Toronto in Canada, and his colleagues say they have banished strange quark matter with better estimates of how, through quantum effects, quarks change the energy of the vacuum of space itself, a key component of quark matter’s total energy. “Our model allows us to see how the vacuum energy depends on the flavor of the quark,” Holdom says. Mixing in strange quarks incurs a bigger energy penalty than previously thought, so high that cold quark matter should consist of just up and down quarks, the researchers report in a paper in press at Physical Review Letters.

Atomic nuclei clearly don’t readily convert into up-down quark matter either. The team calculates that for masses below about 300 times that of the proton, ordinary nuclei are stable because effects akin to surface tension increase quark matter’s energy. However, if experimenters striving to make new superheavy elements could push up in mass just a bit beyond the heaviest nucleus spotted yet—oganesson, with an atomic mass of 294—then they might make stable “nuggets” of up-down quark matter, the theorists predict.

Is this the end for strange quark matter? Probably not, Pisarski says. The theory of quarks is so mathematically intractable that, like everybody else, Holdom had to resort to approximate models, he says. Laura Paulucci, an astrophysicist at the Federal University of ABC in São Paulo, Brazil, adds that the analysis also doesn’t quite rule out strange quark matter in neutron stars, where the density should be significantly higher than the theorists assume. “I’m not sure the theory they’re using is adequate” for the conditions in neutron stars, she says.

Still, the study may be good news for scientists hunting stable quark matter, says Evan Finch, an experimenter at Southern Connecticut State University in New Haven who has searched for strangelets by running moon dust through a mass spectrometer and looking for particles with odd charge-to-mass ratios. “There’s a suggestion that maybe we’ve been looking in the wrong place” for quark matter, he says. “I’m a bit skeptical, but it’s fun.”

The new picture of cold quark matter could also dispel a far-fetched threat to the world. Opponents of the Large Hadron Collider in Switzerland had argued that the atom smasher might produce negatively charged strangelets that would gobble up positively charged atomic nuclei in a runaway process. Physicists had countered that if such a catastrophe were possible, strangelets from space would have long since consumed the planet. Up-down quark matter would definitively rule out the doomsday scenario: It should be positively charged and repel atomic nuclei.

Still, a nugget of the stuff could be useful, Holdom says. Bombard it with neutrons and it would convert them to quark matter while generating energy, he predicts.