User:Army1987/Quark
From Wikipedia, the free encyclopedia
In physics, a quark (/kwɔrk/ or /kwɑrk/) is a type of subatomic particle.[1] In technical terms, quarks are elementary fermions which engage in the strong interaction due to their color charge.[2] Because of a phenomenon known as color confinement, quarks are never found on their own in nature; they are always bound together in composite particles named hadrons;[3] the most common hadrons are protons and neutrons, that compose atomic nuclei.
There are six different types of quarks, known as flavors: up (symbol:
u
), down (
d
), charm (
c
), strange (
s
), top (
t
), and bottom (
b
).[4]
The flavors with least masses, the up quark and the down quark, are generally stable and are very common in the universe, as they are found in protons and neutrons and are two of the primary building blocks of matter. The more massive charm, strange, top and bottom quarks are unstable and rapidly decay; these can only be produced under high energy conditions, such as in particle accelerators and in cosmic rays. For every quark flavor there is a corresponding antiparticle, called antiquark, that differs from the quark only in that some of its properties have the opposite sign.
The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964.[5] There was little evidence for the theory until 1968, when electron–proton scattering experiments indicated the existence of substructure within the proton resembling three "sphere-like" regions.[6][7] By 1995, when the top quark was observed at Fermilab, all the six flavors had been observed.
Since quarks are not found in isolation, their properties can usually only be deduced from experiments on hadrons.[3] An exception to this is the top quark, which decays so rapidly that it does not hadronize at all, and instead is observed through the identification of the particles it has decayed into.[8] Furthermore, in some of the Big Bang theories, it has been postulated that in the very beginning, the extremely hot early universe may have contained single quarks, including "free" top quarks, in a quark-gluon plasma.