|
A sphaleron ((ギリシア語:σφαλερός) "slippery") is a static (time-independent) solution to the electroweak field equations of the Standard Model of particle physics, and it is involved in processes that violate baryon and lepton numbers. Such processes cannot be represented by Feynman diagrams, and are therefore called non-perturbative. Geometrically, a sphaleron is simply a saddle point of the electroweak potential energy (in the infinite-dimensional field space), much like the saddle point of the surface z(x,y)=x2−y2 in three-dimensional analytic geometry. In the standard model, processes violating baryon number convert three baryons to three antileptons, and related processes. This violates conservation of baryon number and lepton number, but the difference B−L is conserved. In fact, a sphaleron may convert baryons to anti-leptons and anti-baryons to leptons, and hence a quark may be converted to 2 anti-quarks and an anti-lepton, and an anti-quark may be converted to 2 quarks and a lepton. A sphaleron is similar to the midpoint (=0) of the instanton, so it is non-perturbative. This means that under normal conditions sphalerons are unobservably rare. However, they would have been more common at the higher temperatures of the early universe. == Sphalerons and baryogenesis == Since a sphaleron may convert baryons to antileptons and antibaryons to leptons, if the density of sphalerons was at some stage high enough, they would wipe out any baryon asymmetry (i.e. any net excess of baryons or anti-baryons). This has two important implications in any theory of baryogenesis within the Standard Model:〔 〕〔 〕 : * Any baryon net excess arising before the electroweak symmetry breaking was wiped out because sphalerons were abundant due to the high temperatures existing in the universe. * While a baryon net excess can be created during the electroweak symmetry breaking, it can be preserved only if this phase transition was first-order. This is because in a second-order phase transition, sphalerons would wipe out any baryon asymmetry as it is created, while in a first-order phase transition, sphalerons would wipe out baryon asymmetry only in the unbroken phase. In some theories of baryogenesis, an imbalance of the number of leptons and antileptons is formed first by leptogenesis and sphaleron transitions then convert this to an imbalance in the numbers of baryons and antibaryons. 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「sphaleron」の詳細全文を読む スポンサード リンク
|