Subatomic particle

Experiments show that light can behave like a stream of particles (called photons) as well as exhibiting wave-like properties. This led to the concept of wave–particle duality to reflect that quantum systems exhibit both particle-like and wave-like behaviors.

Another concept, the uncertainty principle, states that certain pairs of physical properties, such as simultaneous position and momentum, cannot be measured simultaneously with arbitrary precision. Interactions of particles in the framework of quantum field theory are understood as the creation and annihilation of quanta associated with fundamental interactions.

Even among particle physicists, the exact philosophical definition of a particle varies. Common perspectives include:

• A particle as a collapsed wave function
• A particle as an excitation of a quantum field
• A particle as an irreducible representation of the Poincaré group
• A particle as a localized detection event in an apparatus

Classification

By composition

Subatomic particles are either "elementary", i.e. not made of multiple other particles, or "composite" and made of more than one elementary particle bound together.

The elementary particles of the Standard Model are:

• Six flavors of quarks: up, down, charm, strange, top, and bottom;
• Six types of leptons: electron, electron neutrino, muon, muon neutrino, tau, tau neutrino;
• Twelve gauge bosons (force carriers): the photon of electromagnetism, the three W and Z bosons of the weak force, and the eight gluons of the strong force;
• The Higgs boson.

All of these have now been discovered through experiments, with the latest being the top quark (1995), tau neutrino (2000), and Higgs boson (2012).

Various extensions of the Standard Model predict the existence of an elementary graviton particle and many other elementary particles, but none have been discovered.

Hadrons

The word hadron comes from Greek and was introduced in 1962 by Lev Okun. Nearly all composite particles contain multiple quarks (and/or antiquarks) bound together by gluons (with a few exceptions with no quarks, such as positronium and muonium). Those containing few (≤ 5) quarks (including antiquarks) are called hadrons. Due to a property known as color confinement, quarks are never found singly but always occur in hadrons containing multiple quarks. The hadrons are divided by number of quarks (including antiquarks) into the baryons containing an odd number of quarks (almost always 3), of which the proton and neutron (the two nucleons) are by far the best known; and the mesons containing an even number of quarks (almost always 2, one quark and one antiquark), of which the pions and kaons are the best known.

Except for the proton and neutron, all other hadrons are unstable and decay into other particles in microseconds or less. A proton is made of two up quarks and one down quark, while the neutron is made of two down quarks and one up quark. These commonly bind together into an atomic nucleus, e.g. a helium-4 nucleus is composed of two protons and two neutrons. Most hadrons do not live long enough to bind into nucleus-like composites; those that do (other than the proton and neutron) form exotic nuclei.

By statistics

Any subatomic particle, like any particle in the three-dimensional space that obeys the laws of quantum mechanics, can be either a boson (with integer spin) or a fermion (with odd half-integer spin).

In the Standard Model, all the elementary fermions have spin 1/2, and are divided into the quarks which carry color charge and therefore feel the strong interaction, and the leptons which do not. The elementary bosons comprise the gauge bosons (photon, W and Z, gluons) with spin 1, while the Higgs boson is the only elementary particle with spin zero.

The hypothetical graviton is required theoretically to have spin 2, but is not part of the Standard Model. Some extensions such as supersymmetry predict additional elementary particles with spin 3/2, but none have been discovered as of 2026.

Due to the laws for spin of composite particles, the baryons (3 quarks) have spin either 1/2 or 3/2 and are therefore fermions; the mesons (2 quarks) have integer spin of either 0 or 1 and are therefore bosons.

By mass

In special relativity, the energy of a particle at rest equals its mass times the speed of light squared, E = mc². That is, mass can be expressed in terms of energy and vice versa. If a particle has a frame of reference in which it lies at rest, then it has a positive rest mass and is referred to as 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"), but the heaviest lepton (the tau particle) is heavier than the two lightest flavours of baryons (nucleons). It is also certain that any particle with an electric charge is massive.

When originally defined in the 1950s, the terms baryons, mesons and leptons referred to masses; however, after the quark model became accepted in the 1970s, it was recognised that baryons are composites of three quarks, mesons are composites of one quark and one antiquark, while leptons are elementary and are defined as the elementary fermions with no color charge.

All massless particles (particles whose invariant mass is zero) are elementary. These include the photon and gluon, although the latter cannot be isolated.

By decay

Most subatomic particles are unstable. All leptons (except the electron) as well as baryons undergo particle decay via the strong or weak force. Protons are not known to decay, although whether they are strictly stable remains an open question in physics, as some Grand Unified Theories (GUTs) predict proton decay. Muons and tau leptons decay via the weak force. Neutrinos do not decay, but undergo neutrino oscillations. The electron is theoretically stable because it is the lightest charged particle; conservation of charge prevents it from decaying into lighter particles, as no lighter charged particles exist.