If you’ve been following the ongoing hunt for the elusive dark matter, you probably noticed the recent article in Physics World about a new paper in Physical Review Letters proposing a hypothetical “X” particle as an alternative candidate to Weakly Interacting Massive Particles (WIMPs).
As an added bonus, an “X” particle could solve another perplexing cosmological mystery: why is there more matter than antimatter in the universe? (It’s known as “baryon asymmetry” among physicists.)
Back when our universe was still in its earliest birthing throes, matter and antimatter were colliding and annihilating each other out of existence constantly. This process slowed down as our universe gradually cooled, but there should have been equal parts matter and antimatter — and there weren’t.
Instead, there were slightly more matter particles than antimatter. Every bit of matter in our observable universe, from galaxies to dust mites and everything in between, exists because matter won that long-ago war of attrition. And physicists have no idea why that asymmetry should have existed in the first place.
Nature does seem to have a slight preference for matter, still: a violation of so-called charge-parity (CP) symmetry that has been experimentally verified many times in the lab. Decaying subatomic particles are slightly more likely to generate matter than antimatter. But it’s just a slight preference, and the effect is far too small to account for the observed imbalance between matter and antimatter.
So how might this asymmetry have come about? Brookhaven’s Hooman Davoudiasl and colleagues at TRIUMF and the University of British Columbia suggest that the presence of their mysterious “X” particle in the earliest moments of our universe could hold the key both to the nature of dark matter, and to baryon asymmetry.
Maybe there is a lot more antimatter out there than we realize, because we just can’t see it. Maybe antimatter — made up of “X” particles — accounts for a substantial fraction of the mysterious dark matter.
What we’re talking about here is a sort of “yin/yang” phenomenon, to borrow an analogy from the Physics World article. Davoudiasl et al propose a hypothetical X particle/anti-X particle pair in which the X particle decays to normal matter and the anti-X particle decays to antimatter. They call this process “hylogenesis.”
Specifically, the X particle can decay either to a neutron, or to two hypothetical “hidden” particles dubbed “Y” and “theta” (no points to the physicists for originality in nomenclature). Meanwhile, the anti-X particle decays either to an antineutron or to two “hidden” antiparticles: anti-Y and anti-theta.
We still see a version of CP violation. The X particle decays more often to neutrons than to “hidden” particle pairs. But this effect is balanced out by the anti-X particle, which tends to decay more often to “hidden” antiparticle pairs than to antineutrons.
There may even be a means of detecting the hypothetical anti-Y and anti-theta particle pairs (themselves the products of a decayed anti-X particle). Unlike WIMPs, for example, this pair does not annihilate (cancel each other out). But they could cause a proton to decay — a very big deal, if it turns out to be correct, since proton decay isn’t allowed under the current Standard Model of particle physics.
If an anti-Y particle collided with a proton, said proton would turn into a theta particle and a kaon. And that kaon could be detected by any machine looking for proton decays, namely Japan’s SuperKamiokande detector (pictured top).
I contacted Fermilab’s Matt Buckley — a particle physicist who is also quoted in the Physics World article — for a more in-depth reaction to this intriguing new hypothesis. He clarified a couple of vital points.
First, while technically, this new “X” factor scenario results in a net matter/antimatter asymmetry of zero — that is, the number of baryons that make up all known matter in the universe is balanced out by all the unseen dark matter — the question is actually a bit more nuanced than that.
I am among those who routinely phrase the baryon asymmetry question as “Why is there more matter than antimatter?” (I did it in the lede of this post, in fact.) That’s because it’s tough to sum up the inherent complexities in just a sentence or two, and usually the simpler description suffices. But not this time.
According to Buckley, the more correct question is “Why are there more visible nucleons than anti-nucleons?” That’s because it’s still necessary to break the symmetry between matter and antimatter, even with the hylogenesis scenario.
Hylogenesis is simply “a unique way of doing that, which makes very interesting and specific predictions about dark matter”: namely, very low mass (just a few GeV, compared to 1 GeV for a single proton), and the ability to annihilate with nucleons, giving rise to a unique, potentially detectable signature.
“For us, made of baryons, to exist, the mirror of charge-parity symmetry needs to be broken,” Buckley explains. “In the standard view, where dark matter isn’t the antimatter, the Universe is left with only one half of the broken mirror. In the hylogensis paper, the Universe has both halves of the mirror, but that mirror is still fundamentally broken.”
Second, I asked Buckley what all of this might mean for the current leading candidate for dark matter, WIMPs — the subject of numerous experimental searches around the world. The short version: “WIMPs aren’t the only game in town.” But they are still very much in the game.
Buckley cites some of the recent conflicting results from dark matter experiments like CoGeNT and DAMA/LIBRA, on the one hand, and FERMI on the other, that may help determine the fate of Davoudiasl et al‘s hylogenesis scenario, noting that the CoGeNT and DAMA/LIBRA results, in particular, are still a bit, well, contentious. (There are also at least two other experiments looking for dark matter, CDMS and XENON10.)
If the FERMI results turn out to be correct, that’s bad news for hylogenesis. But if the CoGent and DAMA results are borne out — calling for a mass of 7 or 8 GeV, much lighter than theoretical predictions for the other leading candidates, which call for a mass around 100 GeV — then the dark matter particle “looks somewhat different from what we expected,” Buckley says, at which point hylogenesis and similar daring new models would likely take the place of the current WIMP models.
Buckley is careful to point out that just because all those dark matter detector experiments aren’t seeing WIMPs at the target range (100 GeV), it doesn’t constitute hard evidence that this new hylogenesis scenario is correct, either. However, he says, “It’s evidence against a naive WIMP story, so it behooves us to start thinking of new ideas. Hylogenesis is one of those new ideas.” As is so often the case with the Big Questions in physics, we’ll have to wait and see.