From Simple English Wikipedia, the free encyclopedia
Six of the particles in theStandard Model are quarks (shown in purple). Each of the first three columns forms ageneration of matter.
Aquark[1] is anelementary particle which makes uphadrons, the most stable of which areprotons andneutrons. Atoms are made of protons, neutrons andelectrons. It was once thought that all three of those were fundamental particles, which cannot be broken up into anything smaller, but after the invention of theparticle accelerator, it was discovered that electrons are fundamental particles, but neutrons and protons are not. Neutrons and protons are made up of quarks, which are held together bygluons.[2]
There are six types of quarks. The types are calledflavours. The flavours areup (u),down (d),strange (s),charm (c),top (t), andbottom (b). Up, charm and top quarks have acharge of +2⁄3, while down, strange and bottom quarks have a charge of -1⁄3. Each quark has a matchingantiquark. Antiquarks have a charge opposite to that of their quarks; meaning that up, charm and top antiquarks have a charge of -2⁄3 and down, strange and bottom antiquarks have a charge of +1⁄3.
Only up and down quarks are found inside of atoms of normal matter. Two up quarks and one down make a proton (2⁄3 +2⁄3 -1⁄3 = +1 charge) while two down quarks and one up make a neutron (2⁄3 -1⁄3 -1⁄3 = 0 charge). The other four flavours are not seen naturally on Earth, but they can be made in particle accelerators. Some of them may also exist inside of stars.
When two or more quarks are held together by thestrong nuclear force, the particle formed is called ahadron. Quarks that make thequantum number of hadrons are named 'valence quarks'. The two families of hadrons arebaryons (made of three valence quarks) andmesons (which are made from a quark and an antiquark). Some examples of baryons are protons and neutrons, and examples of mesons arepions andkaons.
When quarks are stretched farther and farther, the force that holds them together becomes bigger. When it comes to the point when quarks are separated, they form two sets of quarks, because the energy that is put into trying to separate them is enough to form two new quarks. So scientists think it is not possible to have one quark by itself.
Quarks also have color charge and react via theweak force. For baryons, each quark is green, red or blue. One can be one color at one time. For mesons, the quark is red, green or blue and the antiquark is antired (cyan), antiblue (yellow) or antigreen (pink). Quarks can change color by emitting aW boson. For example, if a green color-charged quark emits a red-antigreen W boson to a red quark, the green quark will become red and the red quark will become green. Therefore, W andZ bosons (which mediate the weak force forneutrinos) help particles react via the weak interaction.
This also causes thebeta and beta-plus decay of atoms where a neutron decays into a proton, electron and electron antineutrino, and makes a proton turn into a neutron,positron andelectron neutrino.
A picture of a neutron. The 'u' stands for an up quark, and the 'd' stands for a down quark. A neutron is made of three quarks, and is abaryon (a type ofhadron). The colors represent thecolour strong force in quantum theory.
The idea (or model) for quarks was proposed by physicistsMurray Gell-Mann andGeorge Zweig in 1964. Other scientists began searching for evidence of quarks, and succeeded in 1968 through the use ofdeep inelastic scattering; a process used to probe the inside of hadrons.
