Helium atom (schematic)
Showing two protons (red), two neutrons (green) and two electrons (yellow).

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. Most of the particles that have been discovered and studied are not encountered under normal earth conditions; they have to be produced during scattering processes in particle accelerators. The study of subatomic particles is the most active branch of particle physics.

Subatomic particles found in atoms

The only subatomic particles found in ordinary atoms are the electron, proton and neutron. The electron (symbol e^-) makes up the bulk of an atom's volume and is responsible for the chemistry of atoms; an electron has 1/1836 the mass of a hydrogen atom and a negative charge. Protons (symbol p⁺) are found in atomic nuclei; 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 also found in nuclei and are responsible for differing isotopes of atoms; neutrons are electrically neutral and have slightly greater mass than protons. The proton and neutron are both composed of two kinds of elementary subatomic particles: the up and down quarks (symbols u and d).

Other subatomic particles

Hadrons are particles composed of quarks. They are divided into two classes: baryons and mesons. Baryons are composed of three quarks and have a large rest mass for subatomic particles. The proton and neutron are the only stable baryons (and the neutron is only stable inside atomic nuclei). Every baryon has an antiparticle composed of three antiquarks.

Mesons are composed of a normal quark and an antiquark, which gives them baryon number zero. There are no stable mesons; the most stable have half lives on the order of nanoseconds. They have a rest mass starting with 140 MeV for the lightest mesons, the pion. The antiparticles of mesons are other mesons.

Leptons are not composed of quarks, but are elementary particles (no smaller constituent is currently known). The only known leptons are electrons, muons, tauons and neutrinos. Until recently neutrinos were thought to have zero rest mass; their masses are much smaller than the masses of any other subatomic particles. In addition to the electron, all three types of neutrino are stable. Each lepton also has an antiparticle.

See also

• Boson
• Fermion
• Elementary particle
• Particle physics
• List of particles

External links

References

• particleadventure.org: The Standard Model
• particleadventure.org: Particle chart
• University of California: Particle Data Group
• Annotated Physics Encyclopædia: Quantum Field Theory
• Jose Galvez: Chapter 1 Electrodynamics (pdf)

Recent News

• Number 644 #1, June 30, 2003, Physics News Update: A Five-Quark State Has Been Discovered
• UniSci: Anti-Proton Mass And Charge Measured For First Time Citat: "...In this case, the values agree with those of the proton (allowing for the opposite charge) to within 60 parts per billion...."
• 22 August, 2001, BBCNews: Physicists make 'strange' matter
• 21-Jan-2002 UniSci: Quantum Gravitational States Observed For First Time Citat: "...The researchers report seeing a minimum (quantum) energy of 1.4 picoelectron volts (1.4 x 10^-12 eV)..."
• LHCB: Everything you ever wanted to know about CP violation and never dared to ask
• BBC News 5-3-1999: 'Sensational' anti-matter discovery "...The phenomenon they think they spotted is technically called direct Charge-Parity (CP) violation. It means that particles behave differently if you swap matter for anti-matter and also swap left and right. ...The observation of direct CP violation is an exciting one for physicists as it disagrees with all the currently held theories about the nature of matter. "
• Number 660 #2, November 4, 2003, Physics News Update: Acceleration Disrupts Quantum Teleportation Citat: "...While this effect is small for typical accelerations in Earthly labs the result shows an interesting relationship between the effects of space-time motion and the quantum world..."