The fundamental particles of matter

Ultimately, the whole point of particle physics is to figure out what the world is made of at the most fundamental level. It turns out it’s made of some weird stuff. About 2,600 years ago Anaximenes of Miletus claimed it was made of air, fire, earth and water, but I’m not sure what that was about since he apparently just made it up. About 25 centuries later Mendeleev came up with a better answer, which of course you guys are all familiar with: the periodic table. It’s kind of a monster, with well over a hundred chemical elements, and they’re not exactly arranged in a simple way, which accounts for most of why I was never very good at chemistry.

The fact that there are so many elements, and the fact that their properties seem to have repeating patterns, are both strong suggestions that they have a substructure; that is, they’re not fundamental, but rather made up of smaller particles. Why? Well, consider this: the periodic table may have a lot of elements, but it’s still a big step forward, because it showed us we could make the entire world out of a (relatively) few things: we didn’t need tree atoms for trees and book atoms for books and people atoms for people; we could make everything out of chemical elements. You can make the trillions of things around you with just a hundred elements. Not a bad start. But a hundred is still a lot, so people started looking for ways to make the chemical elements out of much fewer constituents.

The repeating systematics of the periodic table also suggest the elements are made of smaller particles: consider, if they really were fundamental, why would they be anything like one another? The fact that their properties repeat seems to mean that the elements are not themselves simple, but that they’re made of simple things, which is why we see patterns at all.

Okay, so nowadays we know that the elements in Mendeleev’s table are indeed built up of something more fundamental: electrons, protons, and neutrons. The protons and neutrons are crammed together inside the nucleus, while the electrons orbit way far away in the orbitals. Actually, while we’re on the subject, let me give you a puzzle to mull over: we know that it’s the electromagnetic force that binds the electrons in an atom; that is, electrons are electrically attracted to the protons in the nucleus, which is why they stick around. But why in the world do protons stay so smashed up against each other? They repel each other electrically, and yet not only do they hang close to each other, they wedge themselves in about a hundred thousand times closer than the nearest electron! What gives? I’ll answer this question next week, but in the meantime let it simmer a little.

Getting back to the story, we hit some bad news: as it happens, the neutron and proton were not alone. In fact, they just turned out to be among the lightest in a huge spectrum of particles called hadrons. The last I checked the Particle Data Group webpage, they had listed about 200 different kinds of hadrons. At one point the problem got so bad that a prominent physicist joked that they should start giving the Nobel prize to whoever didn’t discover a new particle that year. So it seemed we were right back where we started: a huge proliferation of “fundamental” particles needed to explain the universe.

I suppose that the advantage of being back where you started is that you know which way to go. In a straightforward replay of the discovery that atoms were made up of smaller particles, people figured out that the protons and neutrons and all the other hadrons were actually made up of smaller particles themselves: the quarks. And that’s where we are now: quarks, as near as we know, are not made up of anything smaller, but are true fundamental components of the universe.

By the way, perhaps you’re wondering whatever happened to the electron in this story. Well, people did find a couple extra particles that were like electrons in some ways, and they called these particles leptons. However, unlike the hadrons, we didn’t get a huge mess of them, and there is no evidence that they are made of something smaller, so we currently believe that leptons, like quarks, are truly fundamental. Here’s a table showing where things currently stand with the elementary particles of matter:

There are a few things you probably noticed about this table. For one thing, the names of the quarks are pretty ridiculous, and to some degree that’s the fault of a dude named Murray Gell-Mann, but it’s probably too late to do anything about it now.

Perhaps more importantly, all the particles are grouped into pairs of quarks and leptons, sometimes called “generations”. Each generation is basically a carbon-copy of the others, except for the masses of the particles: each one has two quarks, a charged lepton, and a neutral lepton (the neutrinos). Moreover, the particles are very similar across generations; for example, the muon is exactly like the electron, just heavier, and the tau is just a very heavy copy of the muon.

Maybe this repeating pattern of generations makes you a little antsy. After all, wasn’t it patterns just like this that convinced us atoms were made of protons, neutrons, and electrons? And then that protons and neutrons were made of quarks? How do we know that quarks and leptons aren’t made of still smaller particles?

Well, we don’t! All we can say at the moment is that there isn’t any evidence (yet) that quarks and leptons are made of smaller particles. We’ve looked at them pretty closely (down to about 10^-16 m, or a ten-thousandth of a billionth of a millimeter), but we’ve never seen any suggestion that they are composite.

But maybe three generations, while irritating, aren’t nearly as bad as the hundred atoms of the periodic table, or the two hundred hadrons found later. Right now we’ve got twelve “fundamental” particles in the table, which is sort of right at the edge: it’s a lot, but not quite so many that you figure they’ve got to be made of something smaller.

Ah, but that’s a good point: do we know there are only three generations of particles? Couldn’t there be a fourth generation out there waiting to be discovered? Well, yes and no. We can’t say for certain that there are no additional generations, but we can say that if there are, they’d have to be very different from the first three. The reasons are fairly technical, so I won’t go into them now, but we might revisit this in a future blog. Still, a lot of people are intrigued by the idea of finding additional “fundamental” particles, and many people here at CERN are going to be looking for that very thing when the LHC turns on, so we’ll just have to wait and see what happens.

For now, let's talk a little more about hadrons: https://mywebspace.wisc.edu/mweinberg/web/constituentsOfMatter.pdf