The mysterious X particle might solve the mysteries of matterS

Physics is still grappling with two basic questions about the nature of matter: why is there more matter than antimatter, and where and what is all the dark matter? Meet the hypothetical X particle, the potential answer to both questions.

To quickly review these two mysteries, we know from the observed gravitational effects on visible matter that there's a lot more matter out there that we can't observe, and this is dark matter. Some particle that doesn't readily interact with other particles is the most likely dark matter culprit, but we haven't found such a particle yet.

As for antimatter, we know that the universe is mostly matter, but there's no obvious reason why this should be. Matter and antimatter are on a more or less equal footing, and any inequality isn't enough to explain the current dominance of normal matter. This phenomenon is known as baryon asymmetry. If you want a more detailed overview of these questions, check out our comprehensive guides to particle physics here and here.

Physicist Hooman Davoudiasl and a team of colleagues from both the US and Canada have proposed a new particle that solves both mysteries in one fell swoop, as well as possessing a couple very intriguing properties. This hypothetical particle, dubbed the X particle, would have a mass about 1000 times that of a proton, and it would commonly decay into a smaller particle.

There are two possible decay scenarios: either it would decay into a neutron, or into a pair of additional hypothetical hidden particles, called the Y and Φ (theta) particles. Both of these hidden particles would only be a couple time times more massive than a proton. The same scenario would occur with its antimatter equivalent, the anti-X. This would decay either into an anti-neutron or an anti-Y and anti-Φ. And it's here that we get some very nifty theoretical physics.

The normal matter X particle would tend to decay into a neutron more often that it does a Y and Φ. The anti-X, on the other hand, prefers to decay into an anti-Y and anti-Φ instead of an anti-neutron. Now let's go back to the early universe and imagine most of the X and anti-X particles decay before they can meet and annihilate each other. The neutrons and anti-neutrons meet and annihilate, as do the pairs of hidden particles.

But once all the available partners are annihilated, there's going to be a surplus of neutrons and anti-Y and anti-Φ particles left over. This discrepancy leaves behind a chunk of visible matter that's bigger than the available antimatter, and a lot of hidden antimatter that can go on to become dark matter. And so the X particle solves both the dark matter mystery and the antimatter mystery.

Of course, all this skirts around a rather basic question - why should there be an X particle in the first place? Part of it is an attempt to build a framework around the latest experimental data, which suggests dark matter particles might be far less massive than previously thought and further implies that the explanations for baryon asymmetry and dark matter could actually be linked.

That said, the X particle could be responsible for something long thought forbidden by the standard model of physics: the decay of the proton. An anti-Y particle that collides with a proton could cause a virtual interaction that forces the proton to morph into a positively charged kaon particle, while the anti-Y particle changes into a Φ particle. This isn't impossible, and it actual offers the best chance of finding evidence for the X particle.

Detectors currently looking for evidence of proton decay predicted by other alternative models could spot a kaon produced in such a collision. The kaon would have far higher energies than those predicted by any other model, so an energetic kaon would provide some strong circumstantial support for the X particle. This is thought to be on the bounds of our current ability to detect particles, so it's possible we could test for it now or in the near future.

[Physics Review Letters]