Everyone, at some point in their lives, wonders why they are here. Existential questions don't stop at the personal level, though. Why is there a Universe, and why is it filled with matter? The last question is a puzzle that has gainfully occupied the minds of and employed physicists for many years. The time spent pondering such questions has not been wasted, as it turns out, as researchers from the LHCb detector report that one of the theoretical paths that allows matter to outnumber antimatter is open for business.

An overly simple reading of the Standard Model of physics predicts that matter will be produced at the same rate as antimatter. The antimatter and matter should, through simple statistics, collide and wipe each other out, leaving only energy. But that didn't happen. The substance we label matter was, somehow, produced in greater abundance than antimatter. In the beginnings of the Universe, antimatter was eliminated, leaving only matter.

A closer look at the Standard Model reveals that some imbalance is expected. But it also predicts a Universe with much less matter than we observe. And, experimentally, we've only observed the relevant matter/antimatter asymmetry for a particular class of particles, called mesons. That notably leaves out the particles that make up the Universe, called baryons. Luckily, baryon asymmetry is exactly what one of the LHC detectors, called LHCb, is designed to investigate.

Symmetry, asymmetry—I’m confused

Here is the deal: the most basic laws of physics, like conservation of energy and momentum, all come from symmetry rules. These rules carry over to the Standard Model but take on more abstract qualities. The quality that applies here is called charge-parity symmetry. To get an idea of what that involves, let's imagine that two particles are headed for a collision. They are both positively charged, and the two of them together have a wave function.

The wave function is a mathematical object that determines the change over time in the probability of finding that the particles possess certain properties. If we observe the wave function from a chosen position, it will spread out in space before us in a pattern. Now, I turn around and observe the wave function in the other direction, and again, it is spread out, forming some pattern in space. After recovering from the dizzying view, I can ask myself, are the two patterns identical or not? If they are identical, then the wave function has even parity; if they are not, it has odd parity.

How does this matter? When the two particles collide, the results of the collision—a cascade of new particles—are determined by the wave function and the charge.

Now, if I change the charges of the two particles so that they are both negative, the resulting decay paths will be different, so that is not a symmetry. But, if I change the charges and swap the parity (called CP symmetry), then I should get the same particle decay paths.

But if you look carefully enough, this symmetry is also violated. The violation comes about because a particle can decay by multiple paths to different end products. Those paths interfere differently when we compare a particle with its antiparticle, and this leads to just slightly more matter than antimatter.

So, short story: if there is only one path between a start and end point, CP symmetry holds, and matter and antimatter are symmetric. But, if there is more than one path to the end point, antimatter and matter do not behave identically, and the symmetry is broken. We've observed this in the decay of mesons.

Baryons do it, too

What the researchers at the LHCb have shown is that baryons also violate CP symmetry. Baryons differ from mesons in that they consist of three quarks, rather than a quark paired with an antiquark. They can be stable, and they make up most of the normal matter in the Universe. If the Standard Model is to predict the amount of matter in the Universe correctly, it is not enough that mesons violate CP symmetry—baryons should also do so.

Luckily, the LHCb produces huge numbers of a specific baryon and its antiparticle version. The researchers looked at how this particle decays into a proton and two mesons. Although I expect there are many other decay paths, the researchers focus on two paths that result in different types of meson pairs, because the symmetry breaking changes the balance between these two paths. For the anti-particle, it changes the balance in the opposite way.

By comparing the difference between the two balances, the change due to the broken symmetry becomes a bit more visible.

Particle physics results are dragged, kicking and screaming, out of the noise via careful statistical analysis; no discovery is complete until the chance of it being a fluke is below one in a million.* This result isn't there yet (it's at about the one-in-a-thousand level). So, it's promising, and, at the rate the LHC is generating data, the asymmetry will either be quickly strengthened or it will disappear entirely. However, given that the result for mesons is well and truly confirmed, it would be really strange for this result to turn out to be wrong.

Unfortunately, the early statistics aren't robust enough to answer the question of why there is so much matter in the Universe or even if the observed asymmetry falls within the bounds of the Standard Model. This is one of the reasons why the LHC was built, though, so expect more results to follow, with stronger statistics. Who knows, maybe the Standard Model will pass this test, too.

Nature Physics, 2017, DOI: 10.1038/NPHYS4021

* It is a well-known fact that one-in-a-million chances occur nine times out of ten.