What’s the Matter with Antimatter?

Why are we here? Ok this is a uselessly vague question; I’ll rephrase. Under what mechanism did the stars, planets, life, and all that come about?

Ask a particle physicist this, and they may be tempted to drag you to the nearest blackboard, write down four lines of maths, and stare at you expectantly as if you’re meant to understand what on earth it means. These four lines represent the standard model of particle physics, which is our most up-to-date attempt to mathematically describe the fundamental constituents of matter, and the forces with which they interact. By matter, I mean what makes up everything around us- planets, stars, life, all composed of atoms, which in turn are composed of smaller particles like electrons and quarks. These are the particles that the standard model governs.

SM.JPG

Fig.1: The Standard Model

The standard model has been highly successful at predicting the outcome of experiments, for example in the Large Hadron Collider (LHC) at CERN, and in fact it has withstood basically every test that has been thrown at it. It explains the nature of matter on a very fundamental level. But it’s actually pretty useless at explaining how all that matter got here in the first place.

So what do these equations have to say about how matter came into existence? I hope you’ll agree with me that electrons definitely exist. How is an electron created? The standard model says it can come into existence during for example, photon decay [ref. fig 2]. But in this process necessarily another particle, a positron must also appear at that same place and time. The positron is the electron’s antiparticle, meaning it has the same mass as the electron but oppositely charged. Such an event is not the only way electrons can be created, but whatever the process, the electron must always emerge with it’s oppositely charged sibling. The universe seems to run a two for one deal on fundamental particles.

eegamma.JPG

Fig.2: The only way to get your hands on an electron.

It seems like if the standard model is the true description of reality, it would imply that there are just as many positrons in the universe as electrons. We know there are plenty of electrons present today; each atom has a bunch of electrons orbiting them. But positrons are a rather rare spectacle; they appear in cosmic rays but not many other places. We have arrived at a paradox.

Perhaps there is some way the positrons could have been destroyed but the electrons survived. The only way you can destroy a positron, according to the standard model, is to annihilate it with an electron, e.g. the process in fig. 2, but in reverse.

The only way to create or destroy electrons or positrons is via processes similar to the above. This is not just the case for electrons; the same applies to all particles in the standard model that make up matter.

While a proton is made up of three bounded quarks, there can exist anti-protons consisting of three antiquarks. With anti-protons, anti-neutrons and positrons, anti-atoms can form, which, just like atoms, can bind in all the ways necessary to end up with anti-stars, anti-planets, anti-life, the whole shebang. It’s all referred to as antimatter, which is a little misleading since it behaves rather a lot like matter, just with charges reversed.

If we had the same number of antiprotons as protons and antineutrons as neutrons, we would end up with the same number of anti-atoms, anti-stars and anti-planets.

Where is all of this antimatter? In 1998 an experiment was sent into space (the Alpha Magnetic Spectrometer, or AMS) to compare the density of helium and anti-helium in cosmic rays. It detected at least three million helium atoms per anti-helium.

Perhaps matter and antimatter have become separated somehow. Perhaps we just live in some huge region of space containing matter, and there are antimatter regions elsewhere in the universe.

Could we detect the presence of an antimatter region, say, by finding an anti-galaxy? To detect normal galaxies, we rely on nuclear fusion in stars, to emit light that can be picked up by our telescopes.

Since anti-fusion in anti-stars would emit the same frequency of radiation as normal fusion, we can’t really tell whether or not a galaxy is made of matter or antimatter. However, we know that galaxy collisions are reasonably common. The collision between galaxy and anti-galaxy would result in essentially annihilation between each particle-antiparticle pair, which come into contact. The reverse of figure 2 on a grand scale.

This would be a truly awesome event, making the black hole merger recently detected via gravitational waves look like a sneeze. The energy emitted would be E = mc2 where m is the combined mass of the two galaxies. In a back-of-the-envelope calculation, one would simply add the masses of the two galaxies and multiply by c2 to find an energy output 1,000,000,000,000,000 times more than your average supernova. Suffice to say we definitely would have noticed this.

Regardless of how you theorise how matter and antimatter have been distributed, the inevitable annihilations of objects and anti-objects would create a lot more radiation coming from the skies than we observe. It seems matter rules our universe.

Is it curtains for the standard model? Not quite. There have been a number of results from experiments throughout the years that may account for the matter/antimatter imbalance.  The first of which was concerning a strange particle called the Kaon. Similar to a proton made of three quarks bound together, a Kaon consists of only two quarks. It’s possible for the Kaon to transform into an anti-Kaon spontaneously. It was originally thought that the probability of such a transition was the same as that of the reverse, an anti-Kaon turning into a Kaon, so overall the matter/antimatter balance would be preserved.

In 1964 it was discovered that in fact the two probabilities are different; the particles prefer to be Kaons rather than anti-Kaons. The technical expression for phenomena like these is “CP-violation” if you want to do some hardcore technical reading. These types of processes, combined with some extra conditions about thermodynamics in the early universe, could be enough to explain the imbalance we see.

For some time Kaons seemed to be the only particle which could behave in such a CP-violating way. However in 1983, CP-violation made a comeback when it was found that a whole new family of particles, called the B mesons, was capable of a plethora of such processes.

The imbalance caused by Kaons and B mesons is still nowhere near enough to explain the matter-dominated universe. The race to find more of these processes, and understand the mysterious underlying mechanisms that cause them, is still happening today. Experiments around the world like the LHCb detector at CERN are dedicated to this goal. In tandem, theoretical physicists are searching for extensions to the standard model that induce imbalance of the extent we need.

It looks like the search may have only just begun, and could lead us to some new and profound realisations about why the universe is the way it is.

I still don’t get what this antimatter stuff is

The AMS experiment

More on CP violation

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