In physics, a subatomic particle is a particle smaller than an atom. These include atomic constituents such as electrons, protons, and neutrons (protons and neutrons are actually composite particles, made up of quarks), as well as particles produced by radiative and scattering processes, such as photons, neutrinos, and muons. Many of the particles that have been discovered and studied are actually not encountered naturally; they have to be produced during scattering processes in particle accelerators. The study of subatomic particles is the most active branch of particle physics.
The electron (symbol e-) is present in all atoms; it has a mass of 1/1836 the mass of a hydrogen atom, and a negative charge. Protons (symbol p+) are also present in all atoms; a proton is about the same mass as a hydrogen atom and carries positive charge equal in magnitude but opposite in sign to electron. Neutrons (symbol n) are electrically neutral and have about the same mass as a proton.
Hadrons are particles composed of quarks. Examples include baryons and mesons. Baryons are composed of three quarks, usually of the "up" or "down" variety. They all have a large rest mass for subatomic particles. Examples of baryons are protons and neutrons.
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Mesons are composed of a normal quark and an antiquark, which causes the baryon number to go to zero. None of these are very stable, but have half lives on the order of nanoseconds. They have a rest mass starting with 140 MeV for the lightest mesons, the pion.
Leptons contain no quarks, but are small irreducible particles (no smaller constituent is currently known). Some have no rest mass, while others have very large rest mass. Types of leptons include electrons, muons, tauons and neutrinos.