Lepton (particle physics)

Leptons are elementary particles that are among the smallest points of matter in the universe. According to the most current scientific understanding of particle physics—a theory known as the Standard Model—leptons, along with quarks, are the two fundamental building blocks of all matter. Leptons and quarks are divided into six different types, each of which comes in pairs known as generations. The six known leptons are the electron, muon, tau, electron neutrino, muon neutrino, and the tau neutrino. Electrons, muons, and tau leptons all have mass and carry an electrical charge. Their corresponding neutrinos have almost no mass and are electrically neutral. All stable matter in the universe is made up of particles that include the first generation of leptons, the electron, while its pair, the electron neutrino, is generally not a constituent of bound stable matter. The other generations of leptons are unstable and quickly decay to form a more stable combination.

Background

Matter—the substance that makes up the universe—is defined by physicists as anything that has mass and takes up space. Ordinary baryonic matter is made up of atoms, the smallest particle an element can be divided into without losing its chemical properties. Atoms themselves are made up of three smaller subatomic particles called protons, neutrons, and electrons. Protons have a positive electrical charge, electrons have a negative charge, and neutrons are electrically neutral. The protons and neutrons are bound together in the center, or nucleus, of an atom, while the electrons orbit the nucleus in a sort of cloud.

In turn, these subatomic particles, specifically protons and neutrons, are made up of even smaller particles called elementary particles. Elementary particles are divided into two main types: fermions and bosons. Fermions are the particles that make up matter. They have an angular momentum, or spin, that is measured in half-integers, such as 1/2 or 3/2. Bosons are force-carrying particles that transfer energy between matter particles. They have spin rates of full integers, such as 0, 1, 2, and so on.

According to the Standard Model, the universe is controlled by four fundamental forces: the strong nuclear force, the weak nuclear force, the electromagnetic force, and gravity. As its name suggests, the strong nuclear force is the strongest of the four. A residual effect of this force is what holds protons and neutrons together in atomic nuclei.. The strong nuclear force is transferred by force-carrying bosons known as gluons. The weak nuclear force causes changes at the subatomic level, resulting in the formation of new elements or nuclear decay—the process that causes radiation. The weak force is transferred through W and Z bosons.

The electromagnetic force is the interaction between electricity and magnetism and is responsible for magnetic attraction and repulsion, electric power, and even light. This force is transferred by a boson known as a photon. Gravity, the weakest of the four forces, is the attractive force exerted by objects that have mass and energy, which warps the fabric of spacetime. Physicists have yet to discover a corresponding force-carrying boson for gravity; however, they refer to the theoretical particle as a graviton.

Overview

Quarks and leptons are examples of fermions, the particles that make up matter. Like leptons, quarks are also divided into six types and combined into generations. Physicists refer to the types of quarks as “flavors” and divide them into the up quark, down quark, charm quark, strange quark, top quark, and bottom quark. The up and down quarks are combined into the first generation, the charm and strange quarks make up the second generation, and the top and bottom quarks make up the third generation. Quarks are subject to the strong nuclear force and can never exist alone. For example, protons are made up of two up quarks and one down quark, while neutrons are made up of two down quarks and one up quark.

Leptons are not subject to the strong nuclear force, and can exist alone in nature. They do interact with each other, but only through the electromagnetic, weak and gravitational forces. The most common lepton is the electron, a negatively charged particle that typically orbits the nucleus of an atom. Electrons are stable and can also exist freely in nature. The electron was first discovered by English physicist J.J. Thomson in 1897, which earned him a Nobel Prize.

Muons are short-lived, more-massive versions of electrons that are created by cosmic rays striking the upper levels of Earth’s atmosphere. Muons are more than two hundred times more massive than electrons and are so unstable they only have a lifespan of about two millionths of a second. They were discovered by American physicists Carl Anderson and Seth Neddermeyer in 1936. The tau lepton is even more massive and more unstable than the muon, with a mass almost 3,500 times that of an electron and a lifespan about ten million times shorter than a muon. The tau was discovered in the 1970s by American Martin Perl, who used a particle accelerator at Stanford University to observe collisions between high-energy charged particles.

Each of these three leptons is accompanied by a corresponding neutrino, an elementary particle with a neutral charge and almost no mass. Like the electron, the electron neutrino is stable and, together with the electron, make up the first generation of leptons, though only the electron actually serves as a building block for bound stable matter. Electron neutrinos are incredibly abundant in the universe. They were produced in great numbers in the early universe and are still generated by nuclear fusion in stars and by stellar explosions known as supernovas. Neutrinos are not affected by electromagnetic or strong forces, and, as a result, they almost never interact with physical matter. Trillions of electron neutrinos bombard each square centimeter of Earth each second, with the vast majority passing through matter without ever interacting with a single particle. Muon neutrinos and tau neutrinos are associated with their respective generation of leptons—muons and muon neutrinos in the second generation and tau leptons and tau neutrinos in the third. Unlike their charged counterparts, these neutrinos are effectively stable within the Standard Model, though they do dynamically oscillate between different neutrino flavors as they travel.

Bibliography

Bourdin, Thomas. “What Is the Difference between Quarks & Leptons?” Sciencing, 30 Aug. 2022, sciencing.com/difference-between-quarks-leptons-8076741.html. Accessed 29 May 2026.

Barberio, Elisabetta. “Explainer: What Are Leptons?” Phys.org, 7 Jan 2014, phys.org/news/2014-01-leptons.html. Accessed 29 May 2026.

Donnelly, T. William, et al. Foundations of Nuclear and Particle Physics. Cambridge UP, 2017.

Kyberd, Paul. “Explainer: What Are Fundamental Particles?” The Conversation, 20 Mar. 2015, theconversation.com/explainer-what-are-fundamental-particles-38339. Accessed 29 May 2026.

Lincoln, Don. “Subatomic Puzzle: Are Quarks and Leptons Hiding Another Level of Matter?” Big Think, 5 Aug. 2024, bigthink.com/hard-science/standard-model-quarks-leptons/. Accessed 29 May 2026.

Mann, Adam. “What Are Elementary Particles?” LiveScience, 20 Sept. 2022, www.livescience.com/65427-fundamental-elementary-particles.html. Accessed 29 May 2026.

Nave, C. R. “Leptons.” HyperPhysics, 2016, hyperphysics.phy-astr.gsu.edu/hbase/Particles/lepton.html. Accessed 29 May 2026.

“The Standard Model.” CERN, 2021, home.cern/science/physics/standard-model. Accessed 29 May 2026.

“Tau Lepton.” Symmetry, 2020, www.symmetrymagazine.org/article/octobernovember-2004/tau-lepton. Accessed 29 May 2026.

“Tau Lepton Mass Is Measured at Belle II with the Highest Precision to Date – Deutsches Elektronen-Synchrotron DESY.” Desy.de, 31 May 2023, www.desy.de/news/news_search/index_eng.html?openDirectAnchor=2781&two_columns=1. Accessed 29 May 2026.