Sidebar: The Basics of Particle Physics

QUARKS These particles make up protons, neutrons and a veritable zoo of lesser-known particles. They have never been observed in isolation.

Electric charge: +2/3 Mass: 2 MeV Constituent of ordinary matter; two up quarks, plus a down, make up a proton.

Electric charge: -1/3 Mass: 5 MeV Constituent of ordinary matter; two down quarks, plus an up, compose a neutron.

Electric charge: +2/3 Mass: 1.25 GeV Unstable heavier cousin of the up; constituent of the J/theta particle, which helped physicists develop the Standard Model.

Electric charge: -1/3 Mass: 95 MeV Unstable heaver cousin of the down; constituent of the much studied kaon particle.

Electric charge: +2/3 Mass: 171 GeV Heaviest known particle, comparable in mass to an atom of osmium. Very short-lived.

Electric charge: -1/3 Mass: 4.2 GeV Unstable and still heavier copy of the down; constituent of the much studied B-meson particle.

LEPTONS These particles are immune to the strong force and are observed as isolated individuals. Each neutrino shown here is actually a mixture of neutrino species, each of which has a definite mass of no more than a few eV.

Electric charge: 0 Immune to both electromagnetism and the strong force, it barely interacts at all but is essential to radioactivity.

Electric charge: -1 Mass: 0.511 MeV The lightest charged particle, familiar as the carrier of electric currents and the particles orbiting atomic nuclei.

Electric charge: 0 Appears in weak reactions involving the muon.

Electric charge: -1 Mass: 106 MeV A heavier version of the electron, with a lifetime of 2.2 microseconds; discovered as a component of cosmic-ray showers.

Electric charge: 0 Appears in weak reactions involving the tau lepton.

Electric charge: -1 Mass: 1.78 GeV Another unstable and still heavier version of the electron, with a lifetime of 0.3 picosecond.

BOSONS At the quantum level, each force of nature is transmitted by a dedicated particle or set of particles.

Electric charge: 0 Mass: 0 Carrier of electromagnetism, the quantum of light acts on electrically charged particles. It acts over unlimited distances.

Electric charge: 0 Mass: 91 MeV Mediator of weak reactions that do not change the identity of particles. Its range is only about 10^-18 meter.

Electric charge: +1 or -1 Mass: 80.4 GeV Mediators of weak reactions that change particle flavor and charge. Their range is only about 10^-18 meter.

Electric charge: 0 Mass: 0 Eight species of gluons carry the strong interaction, acting on quarks and on other gluons. They do not feel electromagnetic or weak interactions.

Electric charge: 0 Mass: Expected below 1 TeV, most likely between 113 and 192 GeV. Believed to endow W and Z bosons, quarks and leptons with mass.

HOW THE FORCES ACT An interaction among several colliding particles can change their energy, momentum or type. An interaction can even cause a single particle in isolation to decay spontaneously.

ELECTROMAGNETIC INTERACTION The electromagnetic interaction acts on charged particles, leaving the particles unchanged. It causes like-charged particles to repel.

STRONG INTERACTION The strong force acts on quarks and gluons. It binds them together to form protons, neutrons and more. Indirectly, it also binds protons and neutrons into atomic nuceli.

WEAK INTERACTION The weak interaction acts on quarks and leptons. Its best-known effect is to transmute a down quark into an up quark, which in turn causes a neutron to become a proton plus an electron and a neutrino.

HIGGS INTERACTION The Higgs field (gray background) is thought to fill space like a fluid, impeding the W and Z bosons and thereby limiting the range of weak interactions. The Higgs also interacts with quarks and leptons, endowing them with mass.