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Spontaneous Symmetry Breaking and the Goldstone Theorem

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Part of the book series: Lecture Notes in Physics ((LNP,volume 830))

Abstract

So far we have concentrated on the chiral symmetry of the QCD Hamiltonian and the explicit symmetry breaking by the quark masses. We have discussed the importance of chiral symmetry for the properties of Green functions with particular emphasis on the relations among different Green functions as expressed through the chiral Ward identities. Now it is time to address a second aspect which, for the low-energy structure of QCD, is equally important, namely, the concept of spontaneous symmetry breaking. A (continuous) symmetry is said to be spontaneously broken or hidden if the ground state of the system is no longer invariant under the full symmetry group of the Hamiltonian. In this chapter we will first illustrate this by means of a discrete symmetry and then turn to the case of a spontaneously broken continuous global symmetry.

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Notes

  1. 1.

    The case of a quantum field theory with an infinite volume \(V\) has to be distinguished from, say, a nonrelativistic particle in a one-dimensional potential of a shape similar to the function of Fig. 2.2. For example, in the case of a symmetric double-well potential, the solutions with positive parity always have lower energy eigenvalues than those with negative parity (see, e.g., Ref. [11]).

  2. 2.

    The field \(\Upphi'\) instead of \(\Upphi\) is assumed to vanish at infinity.

  3. 3.

    For continuous symmetry groups one may have a non-countably infinite number of ground states.

  4. 4.

    The linear sigma model [6, 7, 17] is constructed in terms of the O(4) multiplet \((\sigma,\pi_1,\pi_2,\pi_3).\) Since the group O(4) is locally isomorphic to SU(2) \(\times\) SU(2), the linear sigma model is a popular framework for illustrating the spontaneous symmetry breaking in two-flavor QCD.

  5. 5.

    In the beginning, the discussion of spontaneous symmetry breaking in field theories [9, 1315] was driven by an analogy with the theory of superconductivity [1, 2, 4, 5].

  6. 6.

    The Lagrangian is invariant under the full group O(3) which can be decomposed into its two components: the proper rotations connected to the identity, SO(3), and the rotation-reflections. For our purposes it is sufficient to discuss SO(3).

  7. 7.

    We say, somewhat loosely, that \(T_1\;\hbox{and}\;T_2\) do not annihilate the ground state or, equivalently, finite group elements generated by \(T_1\;\hbox{and}\;T_2\) do not leave the ground state invariant. This should become clearer later on.

  8. 8.

    The restriction to compact groups allows for a complete decomposition into finite-dimensional irreducible unitary representations.

  9. 9.

    Using the replacements \(Q_k\to \hat l_k\;\hbox{and}\;\Upphi_l\to\hat x_l,\) note the analogy with \(i[\hat l_k,\hat x_l]=-\varepsilon_{klm}\hat x_m.\)

  10. 10.

    The abbreviation \(\sum\!\!\!\!\!\!\!\!\int_n|n\rangle\langle n|\) includes an integral over the total momentum \(\vec{p}\) as well as all other quantum numbers necessary to fully specify the states.

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Scherer, S., Schindler, M.R. (2011). Spontaneous Symmetry Breaking and the Goldstone Theorem. In: A Primer for Chiral Perturbation Theory. Lecture Notes in Physics, vol 830. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-19254-8_2

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  • DOI: https://doi.org/10.1007/978-3-642-19254-8_2

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