Classic Edition: Subatomic Botany

Since I found myself talking about particle physics yesterday, and since I find myself in the middle of a seasonal allergy flare-up that's sapping my bloggy motivation, I thought I would dust off and re-post some old articles about particle physics. These date back to 2003, but I think they still stand up reasonably well.

This is the first of four Classic Edition articles, covering the different types of particles making up matter in the universe.

A few days ago, I linked to a news story about this paper in Physical Review Letters describing the discovery of a new type of subatomic particle. The abstract of the paper is, of course, almost perfectly opaque, but you can get a better idea of what this is all about from news stories-- see, for example, the Jefferson Lab news release, or this story from Nature, which provides a fairly succinct summary of the whole thing:

A class of subatomic particle, consisting of five quarks rather than the normal two or three, has been discovered by physicists in Japan. Theorists had expected combinations of four or more to exist, but experiments over the past 30 years had failed to detect them.

A nice collection of references has been put up on the Web by a member of one of the collaborations involved, including an explanation for physicists, with actual data and stuff.

This is a big story, but a tough one to explain in general terms. I'm going to attempt it anyway, as much for my own benefit as anything else-- my understanding of particle physics barely passes the undergraduate level, and attempting to write a coherent explanation of this experiment will force me to get a slightly better grip on the material. The best way to learn a subject is to try to teach it to others, and all that.

The biggest problem faced in trying to explain the importance of a new type of subatomic particle is the fact that we've already got so many of the damned things. Enrico Fermi famously remarked "If I could remember the names of all these particles, I'd be a botanist," and approached from the top down, there's an unmistakable similarity. You've got the familiar protons, neutrons, and electrons, plus a dizzying array of other particles-- muons, pions, kaons, etas, lambdas, sigmas, W's, Z's, and a bunch of others. The textbook I used for modern physics last fall lists 37 distinct particle types, and that doesn't count anti-particles. These dozens of particles are then divided into various groups (mesons, hadrons, bosons, leptons) using a rather mysterious set of rules.

On top of that, all these things decay into other forms with frightening rapidity, so that a neutron can decay into a proton, an electron, and a neutrino, while a pion turns into a muon and a neutrino, and a muon into an electron and a couple of neutrinos. There are rules about what reactions are allowed (kaons can decay into pions, but pions can't become protons), which things interact with what other things, and the whole structure seems frighteningly arbitrary. It's a huge mess.

What ties this all together for physicists is a picture called the "Standard Model," which is appealing because it allows everything to be tied together into a single spiffy chart. (This particular version is a little out of date, but it has the virtue of being more compact than the big poster version...) Clears everything up, doesn't it?

The Standard Model says that the entire universe is made up of three classes of particles: quarks, leptons, and bosons (which serve as force carriers). Everything else is some combination of these things. For the moment, the force bosons (the rightmost column of the simple chart) don't really matter-- we'll come back to those later on.

Leptons are probably the easiest group of particles to explain, as they're the only kind of fundamental particles we see directly. Electrons are leptons, and so are neutrinos. There are two other electron-like leptons: muons and tau particles. They behave sort of like heavy electrons, only they're unstable, and quickly decay into other particles (muons into electrons and neutrinos, taus into muons and neutrinos).

For arcane reasons, there are also three types of neutrinos ("electron," "muon," and "tau" neutrinos), all of which have extremely small masses, and hardly interact with anything. They've been the subject of much theoretical and experimental interest, and have even inspired the odd bit of doggerel. The six leptons appear in the bottom two rows of the chart linked above.

The third group of fundamental particles are the quarks (the top two rows of the chart linked above). In some sense, these are probably the most important particles in the Standard Model, as they're the ingredients making up protons and neutrons. Quarks are never seen alone, but always come clumped together in the form of other particles-- the ridiculous array of subatomic particles alluded to above are mostly different combinations of quarks.

There are six types of quarks, which matches the six types of leptons, and they're usually grouped into three pairs with related names. The most common are "up" and "down"-- an "up" quark has two thirds of a (positive) electron charge, while a "down" quark has one-third of a (negative) electron charge. Two "up"s and a "down" get you a proton (+1 charge), while two "down"s and an "up" make a neutron (no electric charge).

The other two pairs of quarks-- "strange" and "charm", and "top" and "bottom"-- are unstable, decaying into combinations of "up" and "down" quarks and anti-quarks (all the particles come in both matter and anti-matter varieties, so a proton is two "up"s and a "down," while an anti-proton is two "anti-up"s and an "anti-down") in very short order. A "strange" quark, for example, will decay into an "up", an "anti-up", and a "down."

Once you've got the quark model, all the baffling classifications of particle physics start to make sense. The particle classifications are based on how many quarks a particle contains-- "leptons" are particles which aren't quarks at all, while "hadrons" (the source of many a giggle-inducing typo) are particles made up of some combination of quarks. "Hadrons" are further divided into "baryons" and "mesons"-- loosely speaking, "baryons" are particles made up of three quarks (protons and neutrons are baryons), while "mesons" contain only two quarks (one quark, and one anti-quark, actually).

Putting together the six quarks and six leptons, a hierarchy of "generations" is quickly established. Ordinary matter is made up of "generation I" particles: "up" and "down" quarks, and electrons. The other two "generations" are unstable, and decay into "first generation" particles. Tau particles (generation III) can decay into muons (generation II) and thence into electrons (generation I), which are stable. "Top" quarks (generation III) can decay into "strange" and "charm" quarks (generation II), which decay into "up" and "down" quarks (generation I), which are stable. Under ordinary circumstances, there's no way to move up a generation.

All of this together explains the various decays-- what you end up with depends on what you start with. A pion, which starts out with two quarks ("up" and "anti-down", or "down" and "anti-up") can't turn into a proton, which has three quarks. If you start with something containing a "strange" quark (a kaon, say), it will decay into something involving "up" and "down" quarks, but something that starts out with only "up" and "down" quarks can never turn into a kaon.

You can also change a quark from one type to its complement (within the same generation) by emitting some leptons (for example, a neutron decays into a proton when one of the "down" quarks turns into an "up," plus an electron and a neutrino), which accounts for the rest of the rules regarding particle decays. In general, particles will keep decaying until there's nothing left but electrons, neutrinos, and up and down quarks.

Got that? It sounds horrifically complicated, but believe me, it's better than what went before. I've glossed over some of the rules governing decay possibilities, but that's as clear an outline of the Standard Model as I can manage. If you want another take on the whole thing, I recommend checking out The Particle Adventure, which goes over the all this stuff in slightly more detail, with fancy graphics.

So how does the new experiment fit into all this?

To this point, all the hadrons that have been observed have contained either two or three quarks. There are theories to explain why single quarks are never seen, but there's never been any obvious reason why particles containing four or five quarks can't exist. People have been looking for four- and five-quark particles for a long time now (thirty-odd years), but they turn out to be hard to find. The new "pentaquark" is the first such object observed, and consists of two "up"s, two "down"s and an "anti-strange" (there's still some debate, though, about whether it's really a single particle, or just a baryon and a meson orbiting one another at a very small separation).

Of course, a reasonable question to ask would be "How did they make this widget in the first place, starting with ordinary matter, containing only 'up' and 'down' quarks and electrons?" The answer to that question takes us into really strange territory, but will have to wait for another post.