These supremely stable particles could explain dark matter.
After decades of poking around in the math behind the glue holding the innards of all matter together, physicists have found a strange hypothetical particle, one that has never appeared in any experiment. Called a sexaquark, the oddball is made up of a funky arrangement of six quarks of various flavors.
Besides being a cool-sounding character, the sexaquark could eventually explain the ever-maddening mystery of dark matter. And physicists have found that if the sexaquark has a particular mass, the particle could live forever.
Quarks of nature
Almost everything you know and love is made of tiny particles known as quarks. There are six of them, given the names, for various nerdy reasons, of up, down, top, bottom, strange and charm. The up and down varieties are the lightest of the bunch, which makes them by far the most common. (In particle physics, the heavier you are, the more likely you are to decay into smaller, stabler things.)
The protons and neutrons inside your body are all composed of trios of quarks; two ups and a down make a proton, and two downs and an up make a neutron. Indeed, due to the complicated nature of the strong force, quarks really enjoy hanging out in groups of three, and that is also by far the stablest and most common configuration.
Occasionally in our particle colliders, we create particles each consisting of a pair of quarks; these conglomerations are unstable and quickly decay into something else. Sometimes, when we try really hard, we can glue five quarks together and make them play nicely with each other — briefly — before they, too, decay into something else.
And to date, those are all the combinations of quarks that we’ve been able to manufacture.
However, there may be something stranger.
The forge of the elements
After decades of poking around the mathematical corners of the strong nuclear force, physicists found a strange combination that has yet to appear in our experiments: an arrangement of six quarks, consisting of two ups, two downs and two stranges: the sexaquark.
Theories don’t predict a mass for the sexaquark; this value would depend on the precise arrangement and interaction of the individual quarks inside that particle, so it’s up to the experimental physicists to suss it out. And as for the sexaquark’s stability? Calculations suggest that if its mass falls below a certain threshold, it would be absolutely stable forever, meaning it wouldn’t ever decay. And if the mass is a little bigger than that, but still below a certain threshold, then the particle would decay, but over such long timescales that it might as well be stable forever.
So if it’s stable, why haven’t we ever seen it?
Curiously, the range of stable masses for the sexaquark falls below the threshold of what many particle collider experiments can create; these tools were designed to study much rarer, much heavier, much more fleeting particles. In other words, the sexaquark may be hiding in plain sight, having simply flown under the radar all these years.
But particle colliders aren’t the only place to make sexaquarks. The earliest moments of the Big Bang were a frenetic hotbed of nuclear energies, with temperatures and pressures high enough to forge helium and hydrogen out of a raw soup of quarks. And that forge may have also flooded our cosmos with sexaquarks, along with all the more-familiar subatomic characters.
Preliminary calculations suggest that if the sexaquark is a real thing within the right range of masses, it could have been produced in ridiculous abundance in the early universe. And it could have survived that youthful inferno. In fact, sexaquarks may still exist, not really interacting with anything, not really decaying into anything else — just existing, creating extra gravitational pulls wherever they collect, due to their mass.
An invisible particle that’s flooding the universe and that interacts only through gravity? Bingo. That’s dark matter.
A light in the dark
In order for the sexaquark to make up dark matter, it has to actually exist. That is currently a subject of debate, because the object has never been spotted in a particle collider experiment. But like we saw earlier, the sexaquark’s relatively light mass may mean it’s been able to slip by unnoticed, simply because we haven’t been looking for it.
But that’s beginning to change. The BaBar Detector at the SLAC National Accelerator Laboratory in California is really good at producing lots of combinations of quarks, including some really heavy ones that decay into stabler and more reasonable arrangements. BaBar should also produce a bumper crop of sexaquarks, if they exist.
A paper published Jan. 2 to the arXiv database has reported the latest result: no sign of the sexaquark. But that finding is certain to a confidence level of only 90%. That means that if the more massive and less stable combinations of quarks do decay into stable sexaquarks, they do so very rarely, at a rate of only 1 decay in every 10 million.
Does this rule out the sexquark as a dark matter candidate? Not quite. It could be that the conditions of the early universe allowed enough sexaquarks to be made that they could account for the amount of dark matter that we estimate is in the universe. But the new result does make it challenging to use the sexaquark to explain dark matter.
Nice try, sexaquark, but no cigar — at least, not yet.
Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of Your Place in the Universe.