Painless Physics: A Particle Dialogue

by Bonnie Fleming
Lederman Fellow, MiniBooNE experiment

What are the first particles you learned about in physical science in 7th grade? Probably electrons, protons and neutrons.

The electron, the first subatomic particle to be observed, was discovered by J. J. Thompson in 1897. About 15 years later, the hydrogen’s nucleus was identified as a proton; in another 15 years, the neutron was discovered. Since then, scientists have split open the proton and neutron to find a treasure chest of new particles. But electrons, protons and neutrons remain the basics of matter in the universe, the constituents of every atom.

What are these three foundation particles, how do we define them, how do they fit into the theory of elementary particle physics, the Standard Model—and how do we use them to explore the subatomic world? All particles that make up matter fit into two classes: hadrons and leptons. The word hadron comes from the Greek hadros, meaning “robust.” Protons and neutrons, heavyweights in the Standard Model, are hadrons. The term lepton comes from the Greek leptos, meaning “thin.” Electrons are 2,000 times lighter than protons. Appropriately, they are members of the lepton family. Hadrons and leptons, for all their differences, form a stable partnership in all forms of matter.

What are these particles made of? One way to find out is to split them open. A hadron has messy stuff inside, like an egg. Drop an egg on the counter, and yolk and white come spilling out of the cracked shell. Throw it against a wall and the yolk breaks open with egg innards flying everywhere. Throw one egg at another and get twice the impact, and twice the mess—all mixed together. You might be better off just tapping the shell open with a spoon to see what’s inside an egg.

Smashing hadrons

But this sort of gentle tapping does not work for hadrons. They are too small, and their shells are too tough, so we are forced to resort to the smash-it-open method. What we find inside looks different, too—it’s not as simple as an egg. At low energies, when we’re just lobbing it at a target, the proton appears to be made of three quarks, called valence quarks; specifically, two up quarks and one down quark. But throw it harder and other particles can pop out as well.

Here at Fermilab, we collide protons at higher energies than anywhere else in the world. This is a messy business, just like egg smashing, necessitating the huge CDF and DZero detectors. Protons traveling near the speed of light in the Tevatron are so energetic that, upon impact, they split apart into many pieces that themselves spray far out in all directions. At about four stories tall, each detector provides dense material to contain these particle products, so we can count them, measure them and see where they went flying.

What do we see? At Tevatron energies, we find a whole particle sea in there, with quark and anti-quark pairs—the quark sea—in addition to the three valence quarks. The valence and sea quarks can also recombine to form other particles such as pions, a new type of hadron discovered in 1947. With two valence quarks instead of three, the pion is a different class of hadron from the proton and neutron. The lighter pion is known as a meson, from the Greek mesos, or middle. Hadrons with three quarks, such as protons, are known as baryons (Greek: varys, or heavy). There are a total of six quarks, paired in twos according to properties they have in common. All six can combine in many different ways to form groups of two (mesons) and groups of three (baryons). There are many more particles in the hadron families than those I’ve mentioned, including perhaps, some we have not even discovered yet.

Loner leptons

The lepton family is small by comparison, both in numbers and size, comprising only six elementary particles. Three leptons are electrically charged: the electron, the heavier muon, and the still heavier tau. Three are electrically neutral and unusually small: the electron neutrino, the muon neutrino and the tau neutrino. These neutrinos have properties in common with their respective charged lepton siblings and are therefore paired with them. Like the quarks, leptons appear to be structureless, fundamental particles, the building blocks of matter. Unlike the clingy quarks, which appear only in groups as mesons or baryons, the leptons are real individualists, preferring to be on their own.

Since leptons are structureless (as far as we know), they are a good tool for probing hadrons. They allow us to crack the shell and see what’s inside. Colliding a lepton and a hadron is a lot like firing a tiny bullet at an egg. A small enough bullet may not even crack the whole egg open but rather pass right through leaving a little hole and taking a little bit of the inside. Fire lots of tiny bullets at the egg and you can map out the entire inside. In this way, leptons have proven to be a very precise way to find out what’s inside hadrons.

So far we’ve seen that hadrons are bigger, messier and more complicated than leptons. They are also more talkative. In the Standard Model, particles interact by exchanging special particles that carry forces. The electromagnetic force is transferred by exchange of the photon. The strong force exchanger is the gluon; the weak force exchangers are the W and Z particles. Neutrinos, the uncharged leptons, are particularly shy. They will talk with other particles only via the weak force. The charged leptons are a little more outgoing and will interact via the weak force or the electromagnetic force. Hadrons are gregarious by comparison, willing to interact via any of the three Standard Model forces—weak, electromagnetic or strong.

Hadrons and leptons look different, behave differently and, in general, have very different personalities. But their ability to work together has stood the test of time in forming the stable constituents of every atom in the universe as we know it. So far.