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In physics, an elementary particle or fundamental particle is a particle that is not made of other particles. An elementary particle can be a fermion or a boson. Fermions have mass and are the building blocks of matter, while bosons are massless and behave as force carriers for fermion interactions. The Standard Model is the most accepted way to explain how particles behave, and the forces that affect them. According to this model, the elementary particles are further grouped into quarks, leptons, and gauge bosons, with the Higgs boson having a special status.
There are three basic properties that describe an elementary parcticle: "mass", "charge", and "spin". Each property is assigned a number value, which may be zero. For example a photon has zero mass and a neutrino has zero charge.
- Mass: A particle has mass if it takes energy to increase its speed, or to accelerate it. The table to the right gives the mass of each elementary particle. Special relativity tells us that energy equals mass times a constant, the square of the speed of light. If distance and time are measured so that light travels one unit of distance in one unit of time, then mass equals energy. This is why the masses in the table to the right are given in units of energy over the speed of light squared, MeV/c2 (that is pronounced megaelectronvolts over "c" squared). All particles with mass produce gravity. All particles are affected by gravity, even particles with no mass like the photon (see general relativity).
- Charge: An electron has charge -1. A proton has charge +1. A neutron has an average charge 0. Normal quarks have charge of ⅔ or -⅓. If one particle has a negative charge, and another particle has a positive charge, the two particles are attracted to each other. If the two particles both have negative charge, or both have positive charge, the two particles are pushed apart. At short distances, this force is much stronger than the force of gravity which pulls all particles together. Charge has always been conserved in all measured experiments.
- Spin: The angular momentum or constant turning of a particle has a particular value, called its spin number, which is a natural number (positive whole number) times ½. Spin is always conserved in all reactions that do not involve the weak force. Subatomic particles with "spin" are not spinning in the usual sense, but instead "spin" in quantum physics is a more abstract concept invented by scientists to describe what is really going on with the particle.
Mass and charge are properties we can see in everyday life, because gravity and electricity affect things that humans can see and touch. But spin is affects only the very, very small world of subatomic particles. And so we do not see their effect in our everyday life.
Fermions (named after the scientist Enrico Fermi) have a spin number of -½ or ½, and are either quarks or leptons. There are 12 different types of fermions (not including antimatter. Each type is called a "flavor." The flavors are:
- Quarks: up, down, strange, charm, bottom, top. Quarks come in three pairs, called "generations." One member of each pair has a charge of ⅔. The other member has charge -⅓.
- Leptons: electron, muon, tau, electron neutrino, mu neutrino, tau neutrino. The neutrinos have charge 0, hence the neutr- prefix. The other leptons have charge -1. Each neutrino is named after its corresponding original lepton: the electron, muon, and tauon.
Six of the 12 fermions are thought to last forever: up and down quarks, the electron, and the three kinds of neutrinos (which constantly switch flavor). The other fermions decay. That is, they break down into other particles a fraction of a second after they are created. Fermi-Dirac statistics is a theory that describes how collections of fermions behave. Essentially, you can't have more than one fermion in the same place at the same time.
Bosons (named after the Indian physicist Satyendra Nath Bose (1894-1974)) have spin numbers that are integers (e.g. -1, 0, 1). Although most bosons are made of more than one particle, there are two kinds of elementary bosons:
- Gauge bosons: gluons, W+ and W- bosons, Z0 bosons, and photons. These bosons carry 3 of the 4 fundamental forces, and have a spin number of 1;
- Higgs boson: Physicists believe that massive particles have mass (that is, they are not pure bundles of energy like photons) because of the Higgs interaction.
The photon and the gluons have no charge, and are the only elementary particles that have a mass of 0 for certain. The photon is the only boson that does not decay. Bose-Einstein statistics is a theory that describes how collections of bosons behave. Unlike fermions, it is possible to have more than one boson in the same space at the same time.
The Standard Model includes all of the elementary particles described above. All these particles have been observed in the laboratory.
The Standard Model does not talk about gravity. If gravity works like the three other fundamental forces, then gravity is carried by the hypothetical boson called the graviton. (The graviton has yet to be found, and is not included in the table above.)
The first fermion to be discovered, and the one we know the most about, is the electron. The first boson to be discovered, and also the one we know the most about, is the photon. The theory that most accurately explains how the electron, photon, electromagnetism, and electromagnetic radiation all work together is called quantum electrodynamics.
- Sylvie Braibant; Giorgio Giacomelli; Maurizio Spurio (2012). Particles and Fundamental Interactions: An Introduction to Particle Physics (2nd ed.). Springer. pp. 1–3. . http://books.google.com/books?id=e8YUUG2pGeIC&pg=PA1.