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In quantum field theory, a nonlinear ''σ'' model describes a scalar field which takes on values in a nonlinear manifold called the target manifold ''T''. The non-linear ''σ''-model was introduced by , who named it after a field corresponding to a spinless meson called ''σ'' in their model. ==Description== The target manifold ''T'' is equipped with a Riemannian metric ''g''. is a differentiable map from Minkowski space ''M'' (or some other space) to ''T''. The Lagrangian density in contemporary chiral form is given by : where we have used a + − − − metric signature and the partial derivative is given by a section of the jet bundle of ''T''×''M'' and is the potential. In the coordinate notation, with the coordinates , ''a'' = 1, ..., ''n'' where ''n'' is the dimension of ''T'', : In more than two dimensions, nonlinear ''σ'' models contain a dimensionful coupling constant and are thus not perturbatively renormalizable. Nevertheless, they exhibit a non-trivial ultraviolet fixed point of the renormalization group both in the lattice formulation and in the double expansion originally proposed by Kenneth G. Wilson. In both approaches, the non-trivial renormalization-group fixed point found for the ''O(n)''-symmetric model is seen to simply describe, in dimensions greater than two, the critical point separating the ordered from the disordered phase. In addition, the improved lattice or quantum field theory predictions can then be compared to laboratory experiments on critical phenomena, since the ''O(n)'' model describes physical Heisenberg ferromagnets and related systems. The above results point therefore to a failure of naive perturbation theory in describing correctly the physical behavior of the ''O(n)''-symmetric model above two dimensions, and to the need for more sophisticated non-perturbative methods such as the lattice formulation. This means they can only arise as effective field theories. New physics is needed at around the distance scale where the two point connected correlation function is of the same order as the curvature of the target manifold. This is called the UV completion of the theory. There is a special class of nonlinear σ models with the internal symmetry group ''G'' *. If ''G'' is a Lie group and ''H'' is a Lie subgroup, then the quotient space ''G''/''H'' is a manifold (subject to certain technical restrictions like H being a closed subset) and is also a homogeneous space of ''G'' or in other words, a nonlinear realization of ''G''. In many cases, ''G''/''H'' can be equipped with a Riemannian metric which is ''G''-invariant. This is always the case, for example, if ''G'' is compact. A nonlinear σ model with G/H as the target manifold with a ''G''-invariant Riemannian metric and a zero potential is called a quotient space (or coset space) nonlinear model. When computing path integrals, the functional measure needs to be "weighted" by the square root of the determinant of ''g'', : 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「nonlinear sigma model」の詳細全文を読む スポンサード リンク
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