Goldstone boson

The Goldstonetheorem of theoretical physics states that massless scalar particles occur in the spontaneous breaking of a symmetry, the Goldstone - Nambu - Goldstone or bosons. Spontaneous symmetry breaking is available if a system of a symmetrical condition in the lowest energy state, the ground state is about, is no longer symmetrical.

The Goldstone bosons were discovered by Nambu in studies of superconductivity. Goldstone worked the theory further and extended it to the field of quantum field theory.

Examples

Solid State Physics

As an example of solid state physics one can consider the ferromagnetism: In ferromagnetic materials are the laws that describe invariant under rotations in space. Above the Curie temperature the magnetization is equal to zero - that is also invariant under spatial rotations. Below the Curie temperature, the magnetization is of constant, non-zero value and indicates in a predetermined direction, the preferred direction. The invariance under spatial rotations (ie the symmetry) is broken. The Goldstone bosons in this case magnons: quasiparticles that represent a magnetic spin wave.

Particle Physics

In particle physics, Goldstone bosons are massless elementary particles ( bosons ) with spin 0, ie scalar particles. Their number corresponds to the number of generators that do not leave the vacuum, the ground state of elementary particle physics, invariant.

In supersymmetric theories there are also gold Tinos ( Goldstonefermionen ). In the case of global symmetry, this is an ordinary particles with local symmetry, it gives the gravitino mass (similar to the Higgs field the W and Z bosons are mass ). The bosonic superpartners of the gold Tinos hot Sgoldstinos.

Chiral symmetry breaking in QCD

An example of Goldstone bosons in quantum chromodynamics ( QCD ), the pions: the mass of the two light u and d quarks are compared to the mass scale of the strong interaction close to 0, so that the strong interaction has an approximate global symmetry ( a so-called chiral symmetry, the left - and right-handed fields transform independently). That is invariant under transformation of

Where and voneinder independent matrices. The QCD vacuum breaks this symmetry spontaneously, in the particle spectrum is observed only one that rotates left - and right-handed components simultaneously (ie the matrices and in the above transformation must be identical. This residual symmetry is known as isospin symmetry). The pions play the role of Goldstone bosons. Since u and d quarks, however, are not exactly massless (only then can the left - and right-handed quark fields independently transform ), the symmetry is not only spontaneously but also explicitly broken, so that the Goldstone bosons are not exactly massless, which is why also referred to as pseudo - Goldstone bosons. However, the mass of the pion is very small compared to the mass of the proton or neutron.