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The | {{Merge to|Chiral symmetry breaking|discuss=Talk:Chiral symmetry breaking#Merger proposal|date=February 2013}} | ||
{{Main| Chiral symmetry breaking}} | |||
'''Pseudo-Goldstone bosons''' arise in a [[quantum field theory]] with both spontaneous and [[explicit symmetry breaking]]. The controlling approximate [[symmetries]], if they were exact, would be [[spontaneous symmetry breaking|spontaneously broken]] (hidden), and would thus engender massless Nambu-[[Goldstone boson]]s. The additional explicit symmetry breaking gives these bosons a small mass. The properties of these pseudo-Goldstone bosons can normally be found by an expansion around the (exactly) symmetric theory in terms of the explicit symmetry-breaking parameters. | |||
[[Quantum chromodynamics]] (QCD), the theory of strong particle interactions, provides the best known example in nature, through its [[chiral symmetry breaking]]; also see the article on the [[QCD vacuum]] for details. Experimentally, it is observed that the masses of the octet of [[pseudoscalar (physics)|pseudoscalar]] [[meson]]s (such as the [[pion]]) are much lighter than the next heavier [[quantum state|state]]s, e.g., the octet of [[vector meson]]s (such as the [[rho meson]]). | |||
In QCD, this is interpreted as a consequence of [[spontaneous symmetry breaking]] of chiral symmetry in a sector of QCD with 3 flavours of light quarks.<ref>Ta-Pei Cheng and Ling-Fong Li, ''Gauge Theory of Elementary Particle Physics'', (Oxford 1984) ISBN 978-0198519614; {{cite doi|10.1063/1.882879|noedit}}</ref> Such a theory, for idealized massless quarks, has global <math>SU(3) \times SU(3)</math> chiral [[flavour (particle physics)|flavour]] symmetry. Under SSB, this is '''spontaneously broken''' to the diagonal ''SU''(3), generating eight Nambu-[[Goldstone boson]]s, which are the pseudoscalar mesons transforming as an [[Adjoint representation of a Lie group|octet representation]] of flavour [[Special unitary group|''SU''(3)]]. | |||
In actual full QCD, the small [[quark]] masses further break the chiral symmetry '''explicitly''' as well.<ref>({{cite doi|10.1103/PhysRev.175.2195|noedit}})</ref> The masses of the actual pseudoscalar meson octet are found by an expansion in the quark masses,<ref> J Donoghue, E Golowich and B Holstein, ''Dynamics of the Standard Model'', ( Cambridge University Press, 1994) ISBN 9780521476522 .</ref> which goes by the name of [[chiral perturbation theory]]. The internal consistency of this argument is further checked by [[lattice QCD]] computations, which allow one to vary the quark mass and check that the variation of the pseudoscalar masses with the quark masses is as dictated by [[chiral perturbation theory]], effectively as the square-root of the quark masses. | |||
==See also== | |||
*[[Spontaneous symmetry breaking]] | |||
*[[Goldstone boson]]s | |||
*[[QCD vacuum]] | |||
*[[Little Higgs]] | |||
==References== | |||
{{reflist}} | |||
{{DEFAULTSORT:Pseudo-Goldstone Boson}} | |||
[[Category:Theoretical physics]] | |||
[[Category:Quantum field theory]] | |||
[[Category:Quantum chromodynamics]] | |||
[[Category:Mathematical physics]] | |||
[[it:Pseudo bosone di Goldstone]] | |||
{{phys-stub}} | |||
Revision as of 12:04, 20 April 2013
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Pseudo-Goldstone bosons arise in a quantum field theory with both spontaneous and explicit symmetry breaking. The controlling approximate symmetries, if they were exact, would be spontaneously broken (hidden), and would thus engender massless Nambu-Goldstone bosons. The additional explicit symmetry breaking gives these bosons a small mass. The properties of these pseudo-Goldstone bosons can normally be found by an expansion around the (exactly) symmetric theory in terms of the explicit symmetry-breaking parameters.
Quantum chromodynamics (QCD), the theory of strong particle interactions, provides the best known example in nature, through its chiral symmetry breaking; also see the article on the QCD vacuum for details. Experimentally, it is observed that the masses of the octet of pseudoscalar mesons (such as the pion) are much lighter than the next heavier states, e.g., the octet of vector mesons (such as the rho meson).
In QCD, this is interpreted as a consequence of spontaneous symmetry breaking of chiral symmetry in a sector of QCD with 3 flavours of light quarks.[1] Such a theory, for idealized massless quarks, has global chiral flavour symmetry. Under SSB, this is spontaneously broken to the diagonal SU(3), generating eight Nambu-Goldstone bosons, which are the pseudoscalar mesons transforming as an octet representation of flavour SU(3).
In actual full QCD, the small quark masses further break the chiral symmetry explicitly as well.[2] The masses of the actual pseudoscalar meson octet are found by an expansion in the quark masses,[3] which goes by the name of chiral perturbation theory. The internal consistency of this argument is further checked by lattice QCD computations, which allow one to vary the quark mass and check that the variation of the pseudoscalar masses with the quark masses is as dictated by chiral perturbation theory, effectively as the square-root of the quark masses.
See also
References
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it:Pseudo bosone di Goldstone
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- ↑ Ta-Pei Cheng and Ling-Fong Li, Gauge Theory of Elementary Particle Physics, (Oxford 1984) ISBN 978-0198519614; Template:Cite doi
- ↑ (Template:Cite doi)
- ↑ J Donoghue, E Golowich and B Holstein, Dynamics of the Standard Model, ( Cambridge University Press, 1994) ISBN 9780521476522 .