|
In particle physics, the parton model was proposed by Richard Feynman in 1969 as a way to analyze high-energy hadron collisions.〔 〕 It was later recognized that partons describe the same objects now more commonly referred to as quarks and gluons. Therefore a more detailed presentation of the properties and physical theories pertaining indirectly to partons can be found under quarks. ==Model== In this model, a hadron (for example, a proton) is composed of a number of point-like constituents, termed "partons". Additionally, the hadron is in a reference frame where it has infinite momentum — a valid approximation at high energies. Thus, parton motion is slowed by time dilation, and the hadron charge distribution is Lorentz-contracted, so incoming particles will be scattered "instantaneously and incoherently". The parton model was immediately applied to electron-proton deep inelastic scattering by Bjorken and Paschos.〔 〕 Later, with the experimental observation of Bjorken scaling, the validation of the quark model, and the confirmation of asymptotic freedom in quantum chromodynamics, partons were matched to quarks and gluons. The parton model remains a justifiable approximation at high energies, and others have extended the theory over the years. In the parton model, partons are defined with respect to a physical scale (as probed by the inverse of the momentum transfer). For instance, a quark parton at one length scale can turn out to be a superposition of a quark parton state with a quark parton and a gluon parton state together with other states with more partons at a smaller length scale. Similarly, a gluon parton at one scale can resolve into a superposition of a gluon parton state, a gluon parton and quark-antiquark partons state and other multiparton states. Because of this, the number of partons in a hadron actually goes up with momentum transfer. At low energies (i.e. large length scales), a baryon contains three valence partons (quarks) and a meson contains two valence partons (a quark and an antiquark parton). At higher energies, however, observations show ''sea partons'' (nonvalence partons) in addition to valence partons. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Parton (particle physics)」の詳細全文を読む スポンサード リンク
|