Nuggets of New Physics

Phys. Rev. Focus 20, 18
If there are building blocks inside quarks, they might be detectable in the form of superdense objects in space.
D. Aguilar/Harvard-Smithsonian Center for Astrophysics
Death of a star. A large star explodes in a supernova, sterilizing an Earth-like planet, in this artist’s rendering. Some supernovae emit bursts of gamma rays that could be used to search for preon nuggets, proposed superdense objects composed of sub-quark particles.

Most physicists agree that quarks are the fundamental building blocks of all matter. But some researchers believe quarks may contain constituents called preons that are well beyond the detection limits of current and most future particle colliders. So in the December Physical Review D, a team describes two strategies for detecting them in space, in the form of preon “nuggets” that might also be a form of dark matter. Although many researchers are skeptical that preons exist, the team receives high marks for looking beyond conventional particle physics in a new way.

Preons were originally proposed as quark constituents over three decades ago, but in 2005, Fredrik Sandin and Johan Hansson of the Luleå University of Technology in Sweden came up with the concept of preon “stars” or “nuggets” in space [1]. These objects, made entirely of preons, but not in the form of quarks, would be somewhere between the size of a pea and a football, with a mass comparable to the Moon. Their density would be in the range between a neutron star–the densest ordinary form of matter–and a black hole. The team believes preon nuggets could have been created in the early moments of the universe and lingered as a form of dark matter that primarily interacts with normal matter though gravity.

Sandin and Hansson have now calculated exactly how observers might find preon nuggets, using two different techniques. Hansson says too many theorists who attempt to look beyond conventional particle physics don’t propose experiments. “There are zillions of [untestable] theories,” he says. “Preon theories, in contrast, are testable today.”

The idea behind the team’s work is that a preon nugget should have much less mass than a neutron star or black hole but still have a density higher than the densest ordinary matter. The first detection technique they discuss is so-called gravitational femtolensing, where the spectrum of gamma rays from a distant source would be distorted by a relatively nearby preon nugget. The effect comes from interference between two slightly offset images of the source created by the nugget’s light-bending gravity. The team calculates the wiggles expected in the gamma-ray spectrum. They assume a nugget mass between 1015 and 1023 kilograms, the range that has not yet been ruled out by previous searches for dark matter. They also found possible preon nugget signatures in some unexplained wiggles in real gamma-ray burst data.

Next Sandin and Hansson look at gravitational waves, the as-yet-undetected ripples in spacetime that are thought to be continually bombarding Earth from events in the cosmos. If some pairs of preon nuggets are in close binary orbits, then at least some fraction should eventually merge and emit high frequency gravity waves. The team calculates the frequency and amplitude of such waves for various nugget masses and concludes that the European Gravitational Wave Observatory–currently in the planning stage–would have a chance of detecting merging preon nuggets. They also note that the odds are better with a higher frequency detector, such as the 100 megahertz tabletop model currently being tested at the University of Birmingham in England. Preon nuggets might occasionally slam into the Earth or Moon unnoticed, so the researchers are also considering studies of terrestrial and lunar seismic records as a possible third approach.

Dan Hooper of the Fermi National Accelerator Lab outside Chicago says that while there is no evidence that quarks or leptons are made of component particles, he is impressed by Sandin and Hansson’s sense of experimentation. Francis Halzen of the University of Wisconsin, Madison, agrees: “It’s refreshing to read a paper that dares thinking outside the [conventional] box of the particle physicists.”

–Bruce Dorminey

Bruce Dorminey, a science journalist who covers astronomy and astrophysics, is author of (Springer, 2001).


  1. J. Hansson and F. Sandin, “Preon Stars: A New Class Of Cosmic Compact Objects,” Phys. Lett. B 616, 1 (2005)

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