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Could X particle solve two puzzles?

Super-Kamiokande

Technicians check the photomultiplier tubes that ring an underground cylindrical stainless steel tank known as the Super-Kamiokande detector. The detector is on watch for faint glimmers of radiation from exotic particles zipping through the earth.

Can one particle explain both dark matter and the mysterious origins of matter and antimatter? Some physicists think so. They're calling the as-yet-only-theoretical object the "X particle."

Physicists from Canada's TRIUMF particle-physics facility, the University of British Columbia and Brookhaven National Laboratory laid out their ideas on the X particle in a paper published last month by Physical Review Letters -- and since then, the ideas have been picked up by PhysicsWorld magazine as well as Discovery News. (You can read a full draft of the paper on the arxiv.org website.)


The concept addresses two of the deep mysteries in modern physics:

  • Dark matter: Observations of distant galaxies and galaxy clusters suggest that the matter we can see accounts for about a fifth  of their gravitational mass. The other four-fifths is thought to exist in the form of exotic matter than can be detected only by its gravitational effect. So what is that stuff?
  • Matter vs. antimatter: Theory dictates that equal amounts of matter and antimatter must have existed at the beginning of the universe -- and yet, we see lots of matter and virtually no antimatter in the universe today. What happened to the antimatter, and why did matter win out?

The physicists suggest that X particles and anti-X particles -- each with about 1,000 times as much mass as a proton -- existed in the early universe. Such particles would show a "yin-yang" pattern of decay. Theoretically, the X particles would decay into detectable neutrons, or a pair of hidden particles called Y and Φ (the Greek letter phi). The anti-X particles would decay into antineutrons, or pairs of anti-Y and anti-Φ particles. But the X's would be more likely to decay into neutrons, while the anti-X's would be more likely to produce hidden anti-Y's and anti-Φ's.

"When almost all particles with an available antiparticle annihilated one another in the early universe, these discrepancies left a chunk of visible matter and a heavier chunk of dark antimatter to form the cosmos," PhysicsWorld's Kate McAlpine wrote.

The researchers suggest that the existence of the anti-Y and anti-Φ particles could be confirmed by their interactions with protons. Such interactions "could be on the boundary of detectability" at facilities such as the Super-Kamiokande underground particle detector in Japan, said UBC's Kris Sigurdson.

This is by no means the only hypothesis that's been offered to explain the nature of dark matter and the roots of the matter-antimatter balance. One of the main experiments at Europe's Large Hadron Collider, LHCb, is designed to study the decay of B-mesons and anti-B-mesons to see if additional data can help unravel the antimatter mystery. The LHC may also identify exotic particles (neutralinos, maybe?) that account for the dark matter.

I asked SLAC particle physicist Helen Quinn, co-author of the book "The Mystery of the Missing Antimatter," to take a quick look at the X particle concept. "It's very speculative," she told me, "and this is one of the things that particle physicists do all the time."

For now, the X factor is merely one of several hypotheses that might or might not explain one or both of the great mysteries. The truth is out there, and one day physicists will figure out which hypothesis serves as the best explanation for dark matter and/or antimatter. In the meantime, Quinn told me, "there's an awful lot of space out there in which to build models."

"Time will tell," she said.

That's a saying that could be applied to lots of the things that come up in physics -- or life in general.

More on dark matter and antimatter:


In addition to Sigurdson, authors of the paper in Physical Review Letters, "Unified Origin for Baryonic Visible Matter and Antibaryonic Dark Matter," include Hooman Davoudiasi, David E. Morrissey and Sean Tulin.

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