The platypus particle

A leptoquark would be a strange amalgam of familiar leptons and quarks, the way that a platypus has features of both mammals and birds. Image from Charles Baker, Animals, Their Nature and Uses (1877)

All of the atoms in our bodies are made of electrons, protons and neutrons, and the protons and neutrons can be further decomposed into quarks. At the bottom level, then, we are made of only two types of particles: electrons and quarks. But what do these labels mean? Why do we even say that electrons and quarks are different from each other?

Since they don’t come with nametags, we have to define particles by how they interact. It is a bit like cataloging wildlife on a new continent — at first, everything is strange, but eventually we see how the species can be grouped into patterns. Some animals quack and waddle, so we call them all ducks, while others are furry and build dams, and we choose to call them beavers. When physicists first explored the subatomic world, they noticed that there are two basic types of nuclear interactions, one much stronger than the other. To this day, they are called the weak force and the strong force because they never got better names.

Particles of matter were similarly grouped into two classes, leptons and hadrons, which come from Greek words for small and big. Curiously, leptons seem to be completely unaffected by the strong force while hadrons are utterly dominated by it. Although leptons, such as the familiar electron, can turn into other leptons – muons, taus and neutrinos – the total number of leptons in the universe appears to be constant (counting a matter lepton as plus one and an antimatter lepton as minus one). The same is independently true of quarks, the fundamental building block of hadrons. There may be a deep reason for this similarity, but it isn’t yet known.

The resemblance between leptons and quarks is even more striking when we arrange them by the ways they interact with the weak force (see bottom figure). Many physicists suspect that the similarity between leptons and hadrons is not an accident, and that they might be connected somehow. If so, then there could be a new particle that is a little of both — a “leptoquark.” Such a thing would be as shocking as the discovery of the platypus, a mammal that lays eggs like a duck yet is furry like a beaver.

As the missing link between leptons and quarks, leptoquarks might explain how more matter emerged from the big bang than antimatter. They might also determine our fate, since they would satisfy the accounting that currently keeps electrons and quarks in atoms from annihilating with each other. If there were a bridge between leptons and quarks, then even ordinary matter could spontaneously decay into pure energy. It would only take a billion-trillion-trillion years.

—Jim Pivarski

Leptons and quarks naturally fit into three generations of doublets based on the way they interact with the weak force. Physicists do not know why both types of particles conform to the same pattern.