Elementary Particles, in physics, the basic components of matter and energy. The study of elementary particles is known as elementary particle physics, or particle physics. Elementary particles are also referred to as fundamental particles. They belong to a class of particles called subatomic particlesthat is, particles smaller than an atom.

Physicists learn about elementary particles primarily through experiments with devices called particle accelerators. Particle accelerators provide the high energy needed to probe the structure of matter.

Particle physicists classify elementary particles into three main groups: quarks, leptons, and gauge bosons.

Quarks

Quarks are the basic constituents of the subatomic particles called hadrons. Two of the most important kinds of hadrons are protons and neutrons, which make up the nuclei of atoms.

There are a number of different kinds of quarks, and each possesses a type of charge called color. For each kind of quark there also exists an antiparticlethat is, a particle with certain properties (such as electrical charge) opposite those of the ordinary particle.

Quarks and antiquarks always occur combined in pairs or groups of three. The combination of a quark and antiquark forms short-lived subatomic particles called mesons, which include kaons and pions. Combinations of three quarks form subatomic particles called baryons, which include protons and neutrons.

Leptons

Leptons include electrons, several kinds of neutrinos, and their respective antiparticles.

Gauge Bosons

A gauge boson is an elementary particle that carries energy between other elementary particles (including other gauge bosons), exerting a force in the process. Quarks (and antiquarks) create and absorb gauge bosons called gluons. Quarks (and antiquarks) are hound together, forming mesons and baryons, through a continuous exchange of gluons. The photon is a gauge boson that is exchanged between leptons.

Before the discovery of the electron by the British physicist J. J. Thomson in 1897, scientists believed that the fundamental units of matter were atoms. (The name atom comes from the Greek word for indivisible.) By the 1950's, physicists had discovered a large number of different subatomic particles, both through studies of cosmic rays (high-energy radiation from space) and through experiments with particle accelerators.

In 1964 Murray Gell-Mann and George Zweig, both physicists from the United States, independently proposed that protons and most other subatomic particles are composed of much smaller particles, which Gell-Mann called quarks. By the 1980's strong experimental evidence had been obtained for the existence of quarks, and a theory called quantum chromodynamics had been developed that explained their properties and interactions.

In the mid-1980's some physicists proposed that elementary particles are string-like, not dotlike as had been previously believed. Through the 1980's and early 1990's, particle physicists sought to further unify the mathematical theories describing elementary particles and their interactions.