A RETHINK could reveal the secrets of dark matter. The mysterious particles that make up most of the unseen mass of the universe have evaded physicists for decades. But it seems they may have been looking for the wrong thing.
A new hypothetical particle, related to the fundamental quarks found inside protons and neutrons, has popped out of a model originally developed to solve a separate mystery in physics. This could be the missing dark matter. What’s more, the Large Hadron Collider (LHC) – the particle smasher at CERN near Geneva, Switzerland – could find evidence for it in a few years.
Advertisement
Astronomers think that dark matter exists because of rapidly rotating galaxies, which would have long since disintegrated without the gravitational pull of an invisible mass.
The favoured candidate for dark matter is a hypothetical particle called the neutralino. This comes from supersymmetry, a theory that solves some niggling problems with the standard model of particle physics by doubling the number of particles.
The neutralino is neutrally charged and only meaningfully interacts with other particles gravitationally – exactly the properties of dark matter. But if the simplest models of supersymmetry are correct, we should probably have seen some evidence of it at the LHC. Yet there hasn’t been even the faintest hint.
“Neutralino dark matter looks contrived or challenged,” says Mark Wise of the California Institute of Technology in Pasadena.
Now Bartosz Fornal and Tim Tait of the University of California, Irvine, have stumbled on another possibility while trying to solve an unrelated puzzle: why the proton, one of the particles inside atoms, is stable.
Many particles in the standard model decay into others, but some don’t because of the laws of conservation. The electron, for example, is the lightest charged particle and cannot decay to something lighter without destroying charge.
No existing conservation law prevents a proton from decaying, but physicists are yet to see it happen. Current estimates say the proton must live for at least 1034 years – but our universe is only about 1010 years old.
Fornal and Tait wondered whether an unknown conservation law could be preventing proton decay, and turned to a property called the baryon number.
“The baryon number seems to be conserved, though we don’t really understand why,” says Tait. For example, the conservation of charge is related to the electromagnetic force. So to give the baryon number a similar status, Fornal and Tait decided to associate it with a new force.
The pair hypothesised this force by first positing a superforce that exists only at the high energies present instants after the big bang. As the universe cooled, the unified force split into two distinct forces: one for the baryon number and the other being the strong nuclear force, which binds quarks together to create protons and neutrons.
There was a surprise in store. To make the unification work, the pair had to add new particles – the properties of which were predicted by the model. “When you do that, the theory automatically contains dark matter,” says Tait. “You try to solve a different problem and you end up finding dark matter staring you in the face.”
The new model requires quarks to have heavier partners, and the lightest of these has the right properties to be dark matter. For instance, these particles would have been made in about the right amount in the early universe to explain the dark matter we see today ().
“It’s exactly the kind of cool, risky idea that I think is most interesting,” says LHC researcher Daniel Whiteson, also of UC Irvine. He is already making plans to verify the model’s predictions. Although the LHC is not powerful enough to create the dark matter particles hypothesised by the model, it could, within the next few years, create particles related to the new unified force.
“Some ideas just feel like they are on the right track; this is one of those,” says Wise. “However, nature will do what nature does and it’s up to experiment to be the final judge.”
Image credit: Denis Balibouse, Reuters
This article appeared in print under the headline “Hunt is on for quark dark matter”
