Higgs field (classical)
Spontaneous symmetry breaking, a vacuum Higgs field, a Higgs boson are quantum phenomena. A vacuum Higgs field is responsible for spontaneous symmetry breaking the gauge symmetries of fundamental interactions and provides the Higgs mechanism of generating mass of elementary particles. However, no adequate mathematical model of this Higgs vacuum has been suggested in the framework of quantum gauge theory, though somebody treats it as sui generis a condensate by analogy with that of Cooper pairs in condensed matter physics.
At the same time, classical gauge theory admits comprehensive geometric formulation where gauge fields are represented by connections on principal bundles. In this framework, spontaneous symmetry breaking is characterized as a reduction of the structure group of a principal bundle to its closed subgroup . By the well-known theorem, such a reduction takes place if and only if there exists a global section of the quotient bundle . This section is treated as a classical Higgs field.
A key point is that there exists a composite bundle where is a principal bundle with the structure group . Then matter fields, possessing an exact symmetry group , in the presence of classical Higgs fields are described by sections of some composite bundle , where is some associated bundle to . Herewith, a Lagrangian of these matter fields is gauge invariant only if it factorizes through the vertical covariant differential of some connection on a principal bundle , but not .
An example of a classical Higgs field is a classical gravitational field identified with a pseudo-Riemannian metric on a world manifold . In the framework of gauge gravitation theory, it is described as a global section of the quotient bundle where is a principal bundle of the tangent frames to with the structure group .
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