How do we know all this?

Testing the theory

I've completely swept under the rug something of extreme importance. That thing I've swept is the question of how we actually know about all this stuff that I'm claiming is true. Well, the way we've learned this stuff is the same way you learn about almost anything in science. That is, you do experiments. And this is where the real science is. All I've been talking about so far are the results of science. The real science is about how you learn.

The whole idea of science is this. It's not enough to simply have a really neat idea about how something works. That idea has to undergo rigorous testing through good experiments. So, for example, the reason that most scientists accept Einstein's theory of relativity is not because it's a really neat idea, and not because it was put forth by EINSTEIN, but because his ideas have been confirmed by experiments. An example may help.

The "really good idea" in particle physics, or one of them anyway, is largely the work of three physicists named Weinberg, Salam, and Glashow. These guys came up with a theory describing the weak force, and its very intricate relation with electromagnetism. There are many consequences to this theory, many of which have been verified, and none of which have been contradicted so far. I'll focus on one.

One feature of the Weinberg-Salam-Glashow model (also known as the Glashow-Salam-Weinberg model, the Glashow-Weinberg-Salam model, etc.) is that there must be an even number of quarks. Why that is, is a long story. Basically, the math just doesn't work out with an odd number of quarks. (Well, I guess it wasn't so long.) On the other hand, for many years physicists only knew about five quarks: up, down, charm, strange, and bottom. So here lies a way to test the theory. The theory predicts that there must be a sixth quark. So if you find a sixth quark, that's an indication that the theory may be true. If you can't find a sixth quark and there are only five, then the theory is wrong. So you look for a sixth quark.

Making the top quark

This leads us to the question: "How does one find a quark?" It's not easy. You can't just look at a piece of wood under a microscope and see quarks. They're way to small for that. And furthermore, even if you had a microscope capable of seeing something so small (never mind the quantum physics implications of that) there just aren't any top quarks (the name given for the sixth quark) lying around in a piece of wood. The top quark is highly unstable and decays immediately after it's formed. So there's no sense looking for top quarks, you have to make them.

Particles that are too unstable to exist in ordinary matter can be created in collisions between stable particles in a particle accelerator. The heavier the particle you're trying to create, the faster you have to collide things. (E = mc2, you know. The more m you want, the more E you need.) The top quark was believed to be a very heavy particle, and so the particle accelerator for the job was the Tevatron at Fermilab. This is a machine that collides protons with anti-protons at speeds very very very very (keep going) close to the speed of light. The theory predicts that in very high energy collisions between protons and anti-protons, one will, from time to time, produce a top quark.

Finding the top quark

Of course, making the top quark is just one part of the problem. The other part is finding out that you've made one. And for that, you have to do the math. Remember, the top quark is unstable, and decays immediately after it's formed. In fact, it decays into other particles that are also unstable. And some of these particles are also unstable. So you can't observe most of this stuff directly. The best you can do is to wrap a big particle detector around the place where your protons and anti-protons collide, and see what comes out. Of course, this only helps if you know what is supposed to come out, in the event that you actually produce a top quark. And this is where the math comes in. (Don't worry, we won't do the math here.) What you need to calculate is how often you can expect to produce a top quark, and what you can expect a top quark to decay into. Then you look at what you find in your detector, and you see how often you find something consistent with what a top quark should decay into.

This picture, which I've shamelessly stolen from the Fermilab website illustrates one example of what a top quark might decay into.

One type of top quark decay

Here's what's happening in this picture. The proton and anti-proton collide, annihilate, and produce a top quark (above) and an anti-top quark (below). The top quark decays into a b quark and a W+. The W+ decays into a u and d quark. Now remember, quarks never occurs on their own, but always as parts of other particles. So as the quarks pull away from each other, they either have to snap back on each other, or break apart, producing other quarks in the process. The quarks in this decay are moving so fast that the breakup produces a whole spray of particles (such as protons and pions and neutrons) called a "jet." That's what's denoted by the little spray of lines coming out of the quarks.

The top anti-quark in this process decays into a b anti-quark and a W-. The b produces another jet, while the W- decays into a muon and an anti-neutrino. So what you do is you see if your detector picks up any collisions that result in a muon and some jets. (The neutrino will escape undetected. As for the jets, some may overlap so you won't necessarily see four.)

Of course, it's not quite this easy. There are other ways to get a muon and jets in a proton/anti-proton collision. So what you really need to figure out is whether your experiment gives you more "events" with a muon and jets than what you would expect to get if there wasn't a t quark. (More, by the right amount, I might add.) This, incidentally, is what makes the decay shown in this picture better than most other decays. A muon won't turn up in most of the more ordinary processes. Anyway, if you do get more events, then you look at the energies of all the decay products in these events, and that will tell you the mass of the t quark. This all took place a few years ago at Fermilab, and so we have further experimental evidence supporting the Weinberg-Salam-Glashow model.

I've simplified things a bit, but I think this gives you an accurate flavor for how particle physics is done. You start with a theory. You examine the theory in detail, looking for a way to test it in a particle accelerator, and then you collide the particles, analyze what comes out of the collision, and see if the results match your theory. There are other kinds of experiments you can do in particle physics, but most of the big results of the past 30 or more years have come from particle accelerators.

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