Subatomic particle

In physics, a subatomic particle is any particle that is smaller than an atom. The Standard Model of particle physics divides these particles into two types: composite particles, like protons and neutrons, which are made of smaller constituents, and elementary particles, like electrons, which are not made of anything else. The fields of particle physics and nuclear physics focus on studying these particles and the ways they interact.

Most particles that carry forces—such as photons and gluons—are classified as bosons. These bosons have quanta of energy but lack rest mass and discrete diameters, existing instead as pure energy wavelengths. In contrast, particles that have rest mass and cannot overlap or combine are called fermions. However, the W and Z bosons are exceptions: they are bosons but have relatively large rest masses, around 80 GeV/c² and 90 GeV/c² respectively.

Experiments have shown that light can behave both as a stream of particles (photons) and as a wave, leading to the concept of wave–particle duality. This means quantum-scale particles sometimes act like particles and sometimes like waves, and are occasionally referred to as "wavicles." Another key idea is the uncertainty principle, which states that certain pairs of properties—like a particle’s exact position and momentum—cannot be measured simultaneously with perfect precision.

Within quantum field theory, particle interactions are understood as the creation and annihilation of quanta of fundamental forces, blending particle physics with field theory. Even among particle physicists, there is no single agreed-upon definition of a particle. Professional definitions include: a particle is a collapsed wave function; a particle is an excitation of a quantum field; a particle is an irreducible representation of the Poincaré group; or a particle is simply an observed thing.

**Classification**

**By composition:** Subatomic particles are either elementary (not made of other particles) or composite (made of multiple elementary particles bound together). The elementary particles of the Standard Model include six flavors of quarks (up, down, charm, strange, top, bottom), six types of leptons (electron, electron neutrino, muon, muon neutrino, tau, tau neutrino), twelve gauge bosons (the photon, three W and Z bosons, and eight gluons), and the Higgs boson. All of these have been experimentally discovered, with the top quark (1995), tau neutrino (2000), and Higgs boson (2012) being the most recent. Various extensions of the Standard Model predict an elementary graviton and other particles, but none have been found as of 2026.

**Hadrons:** The term "hadron" comes from Greek and was introduced in 1962 by Lev Okun. Nearly all composite particles contain multiple quarks (or antiquarks) bound by gluons, though a few exceptions (like positronium and muonium) have no quarks. Particles containing few (up to five) quarks (including antiquarks) are called hadrons. Due to color confinement, quarks never appear alone but always within hadrons. Hadrons are divided by quark count: baryons contain an odd number of quarks (almost always three), with protons and neutrons (the nucleons) being the best known; mesons contain an even number (almost always two—one quark and one antiquark), with pions and kaons as the best known. Except for protons and neutrons, all hadrons are unstable and decay within microseconds or less. A proton consists of two up quarks and one down quark; a neutron consists of two down quarks and one up quark. These commonly bind into atomic nuclei—for example, a helium-4 nucleus has two protons and two neutrons. Most hadrons do not survive long enough to form nucleus-like composites; those that do (other than protons and neutrons) create exotic nuclei.

**By statistics:** Any subatomic particle obeying quantum mechanics in three-dimensional space can be either a boson (integer spin) or a fermion (odd half-integer spin). In the Standard Model, all elementary fermions have spin 1/2 and are divided into quarks (which carry color charge and feel the strong force) and leptons (which do not). Elementary bosons include the gauge bosons (photon, W and Z, gluons) with spin 1, while the Higgs boson has spin 0. The hypothetical graviton would have spin 2 but is not part of the Standard Model. Some extensions, like supersymmetry, predict additional elementary particles with spin 3/2, but none have been discovered as of 2023. Due to spin rules for composite particles, baryons (three quarks) have spin 1/2 or 3/2 and are fermions; mesons (two quarks) have integer spin 0 or 1 and are bosons.

**By mass:** In special relativity, a particle’s rest energy equals its mass times the speed of light squared (E = mc²), so mass can be expressed in terms of energy and vice versa. If a particle has a rest frame, it has positive rest mass and is called massive. All composite particles are massive. Baryons (meaning "heavy") tend to have greater mass than mesons (meaning "intermediate"), which in turn tend to be heavier than leptons (meaning "lightweight"), though the heaviest lepton (the tau) is heavier than the two lightest baryons (nucleons). Any particle with an electric charge is necessarily massive. The terms baryons, mesons, and leptons were originally defined in the 1950s.