Lepton

In particle physics, a lepton is a tiny piece that helps make up the universe. It is called an elementary particle.

Leptons spin in a special way called half-integer spin. This means they spin at a rate of 1/2. They do not join together through a strong force called the strong interaction.

There are two main types of leptons. The first type includes charged leptons, such as the electron, muon, and tauon. These can link with other particles to make bigger things like atoms. The second type is made of neutral leptons called neutrinos. Neutrinos hardly ever bump into anything, so they are very hard to see. The most familiar lepton is the electron, which helps with electricity and the structure of atoms.

Scientists have found six kinds of leptons, in three groups. The lightest and most common is the electron, which stays the same. Heavier leptons, like muons and taus, quickly change into electrons and neutrinos. This change is called particle decay. Electrons are everywhere in the universe, while muons and taus can only be made in very powerful collisions, such as those from cosmic rays or in big machines called particle accelerators.

Etymology

The word "lepton" comes from the Greek word leptós, meaning "fine, small, thin." A scientist named Léon Rosenfeld first used this word in 1948 for tiny particles like electrons and neutrinos. Later, the muon was also called a lepton. These particles are much lighter than protons. Electrons and muons have small masses, and neutrinos have almost no mass at all.

History

See also: Electron § Discovery, Muon § History, and Tau (particle) § History

The first lepton found was the electron, discovered by J.J. Thomson and his team in 1897. Later, in 1930, Wolfgang Pauli suggested that an invisible particle, called the electron neutrino, helped keep important rules balanced during a process called beta decay.

In 1936, Carl D. Anderson found the muon. At first, it was thought to be a different kind of particle, but it was later grouped with electrons and neutrinos as leptons. In 1962, scientists showed that another type of neutrino exists, linked to the muon.

The tau was discovered between 1974 and 1977 by Martin Lewis Perl and his team. Scientists later found its matching neutrino in the year 2000. Today, scientists are looking for a possible fourth group of these particles, but none has been found yet.

Properties

Leptons are tiny particles that spin in a special way — they have a spin of 1/2. This makes them act like tiny magnets. Because of this spin, each lepton can only point in two directions: up or down.

Leptons come in two types. Charged leptons include the electron, muon, and tau, and they can stick to other particles to build bigger things, like the atoms that make up everything around us. Neutrinos are different — they hardly ever bump into anything else, so scientists don’t see them very often.

Charged leptons can feel a special kind of push and pull called the electromagnetic force, which helps them stick to other things. Neutrinos don’t feel this force, which is why they zip right through most matter without a hitch.

The weak interactions of the first generation leptons.

Universality

See also: LHCb experiment § Lepton flavour universality

Leptons are tiny particles with a spin of ¹⁄₂. They interact with other particles in a special way. This means that no matter which lepton you look at, the way it interacts is the same.

Scientists test this idea by studying how muons and tau particles break apart. They do this in experiments like the Stanford Linear Collider and the Large Electron–Positron Collider.

They also look at how these particles decay, or break apart. They find that the patterns are very similar, though not exactly the same because of small differences in their weights. These studies help scientists understand the rules that govern these tiny particles.

Table of leptons