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Types of quantum field thories
For $Conf$ a configuration space and
an action functional that is invariant under a group $G$ of symmetries acting on $Conf$, in that
a solution $\phi_0$ to the Euler-Lagrange equations of motion is said to exhibit spontaneously broken symmetry if it is not a fixed-point of that group action: if there is $g \in G$ such that $g(\phi_0) \neq \phi_0$.
The “breaking” refers to the fact that the group no longer acts. It is called “spontaneous” because one imagines that by a physical process the theory “finds” one of its solutions. This comes from the class of examples where a statistical system is first considered at high temperature and then cooled down. At some point it will “spontaneously” freeze in one allowed configuration. A standard example is a ferromagnet?: at high temperature its magnetization? vanishes, while at very low termperature it spontaneously finds a direction of magnetization, thus “breaking” rotational symmetry.
One calls the subgroup $G_{\phi_0} \subset G$ that fixes the given configuration $\phi_0$ the unbroken symmetry group .
In the context of the quantum field theory arising by quantization of this action functional one considers the given classical solution $\phi_0$ as a background about which to consider perturbations of the remaining effective quantum field theory.
The fields in this effective QFT are then small excitations $\delta \phi$ around the given $\phi_0$. Since the original symmetry group still acts on the full fields $\phi_0 + \delta \phi$, the remaining symmetry group of the effective field theory is $G_{\phi_0}$, whose elements $g$ send
Since in the effective theory around $\phi_0$ the vacuum state where all the $\delta \phi$ have no excitations (or rather: are in their ground state) corresponds to $\phi_0$ itself one says in this context that a quantum theory exhibits spontaneouly broken symmetry if its vacuum state is not invariant under the pertinent symmetries .
A standard example which is both very simple but at the same time of central importance in one of the main applications in the standard model of particle physics – the electroweak symmetry breaking via the Higgs mechanism – is this:
Let $Conf = C^\infty(X = [0,1]^d, \mathbb{R}^N)$ be the configuration space of $N$ real scalar fields and take the action functional to be
for some $h, g \in \mathbb{R}$. This is manifestly invariant under the canonical action of the orthogonal group $G = O(N)$ on $Conf$.
This action functional has a class of critical points given by constant maps $\phi : X \to \mathbb{R}^n : \phi(x) = \Phi$. These extremize the action functional precisely if the $\Phi$ extremize the potential energy
If both $g$ and $h$ are positive, then there is only one such critical point, given by $\Phi = 0$. Therefore in this case the unique constant solution does not break the symmetry, in that $g( \Phi = 0) = (\Phi = 0)$ for all $g \in O(N)$.
However, if $h$ is negative and $g$ positve, then the solutions are all those $\Phi$ with
The set of all these is closed under the action of $G = O(N)$ – this group takes one of these solutions into another – but none of these solutions is fixed by this action.
One says in this case that any such solution $\phi : x \mapsto \Phi$ is a solution that spontaneously breaks the symmetry of the theory.
The theory of gravity on a given topological manifold $X$ has as configurations pseudo-Riemannian metrics on $X$ and its action functional – the Einstein-Hilbert action or one of its variants – is invariant under the action of the diffeomorphism group on $X$.
The corresponding Euler-Lagrange equations are Einstein's equations . A given solution $(X,g)$ breaks the symmetry given by a diffeomorphism $f : X \to X$ unless $f$ is an isometry. This means that the unbroken symmetries connected to the identiy correspond precisely to the Killing vector fields on $(X,g)$.
Spontaneous symmetry breaking in gravity plays a central role for instance in the context of the Kaluza-Klein mechanism. For instance for $dim X = 5$ the effective field theory of gravity around a solution of the form $(X = X_4 \times S^1, g_4 \otimes g_1)$ is 4-dimensional gravity coupled to electromagnetism (and a dilaton field): the components of the field of gravity along the circle transmute into the electromagnetic field. The ansatz breaks all the symmetries that would mix the remaining 4-dimensional gravitational excitations with these new electromagnetic excitations.
This is discussed in a bit of detail for instance in (Strominger, lecture 1).
The above discussion has a direct analog in theories of higher supergravity. By the same logic, one finds that the effective quantum field theory around classical solutions that are Kaluza-Klein reductions of the form $(X^4 \times Y^d, (g_4 \otimes g_d))$ exhibts as global symmetries all those diffeomorphisms that are not spontaneously broken by this solution.
In this case, though, there are also supersymmetry analogs of the plain diffeomorphism action. Such a local supersymmetry remains unbroken in the given solution if it comes from a Killing spinor field.
Therefore KK-reductions to 4-dimensional Minkowski space in supergravity that admit precisely four Killing spinors of the form $(\psi_4 = const \otimes \psi_d = covariantly const)$ give rise to effective field theories with exactly one remaining global supersymmetry.
For more see supersymmetry and Calabi-Yau manifolds.
This is discussed in a bit of detail for instance in (Strominger, lecture 2).
Goldstone boson?
topological defects in the vacuum
In
sponaneously broken global gauge group symmetry is discssued in vol I, section 19, and spontaneously broken local gauge group symmetry in vol I, section 21.4.
Textbook discussion of broken symmetry in gravity and supergravity in the context of the Kaluza-Klein mechanism is in
Andrew Strominger (notes by John Morgan), Kaluza-Klein compactifications, Supersymmetry and Calabi-Yau spaces , volume II, starting on page 1091 in
Pierre Deligne, Pavel Etingof, Dan Freed, L. Jeffrey, David Kazhdan, John Morgan, D.R. Morrison and Edward Witten, eds. , Quantum Fields and Strings, A course for mathematicians, 2 vols. Amer. Math. Soc. Providence 1999. (web version)
Discussion of spontaneous supersymmetry breaking is in
Yael Shadmi, Supersymmetry breaking (arXiv:hep-th/0601076)
Joseph Polchinski, volume II, appendix B of String theory
The article
points out that for symmetric systems with a symmetric ground state, already a tiny perturbation mixing the ground state with the first excited stated causes spontaneous symmetry breaking in the suitable limit, and suggests that this already resolves the measurement problem in quantum mechanics.