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where <math> \eta </math> are external probing fields coupled to the system and <math>g</math> is the ground state, i.e., an eigenstate of the GTO picked up from the set of eigenstates with the lowest real part of their eigenvalues, a requirement needed to ensure stability of the response. The ground state represents the system which has been allowed to evolve for a long time without perturbations. The generating functional describes how the ground state responses to external perturbations.
When TS is spontaneously broken, the ground state is degenerate and the system can be effortlessly excited. In higher-dimensional theories, this degeneracy evolves into a gapless branch of excitations above the ground state called [[Goldstino|goldstinos]]. Due to gaplessness of goldstinos, the resulting effective field theory must be scale-invariant, or, a conformal field theory
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