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Consider the vacuum expectation value of a (for simplicity scalar) field $\phi$, we know that its vacuum expectation value can be expressed as

$$\langle\phi\rangle=\frac{1}{\mathcal{Z}}∫\mathcal{D}\phi e^{\frac{i}{\hbar}S[\phi]}\phi.$$

If we introduce a source $J(x)$ then we can also define

$$\phi_{J}\equiv\langle\phi\rangle_{J}\equiv\frac{1}{\mathcal{Z}}∫ \mathcal{D}\phi e^{\frac{i}{\hbar}(S[\phi]+J\cdot\phi)}\phi.$$

Recall also that the quantum effective action is $$\Gamma[\phi_{J}]=W[J]-\phi_{J}\cdot J$$ with $W[J]$ being the Wilsonian effective action of our theory. My question is about the classical solution of $\Gamma$, in particular since

$$\frac{\partial\Gamma[\phi_{J}]}{\partial\phi_{J}(x)}=-J(x)\implies \frac{\partial\Gamma[\phi_{J}]}{\partial\phi_{J}(x)}|_{J=0}=0.$$

Now, my question is rather simple: Does it mean that the minima of the quantum effective action is just $\langle\phi\rangle$ without any source? Because if the answer is positive, then why we do say that the minima of the potential is not really $\langle\phi\rangle$ but can change due to quantum corrections?

For example, for the Higgs we have that the minima $v=\langle\phi\rangle$ is obtained by simply minimizing the tree-level potential but could be corrected using quantum correction (for example using the Coleman-Weinberg potential?), but this seems in contrast with the fact that $\langle\phi\rangle$ should minimize the fully quantum corrected $\Gamma[\phi_{J}]$. So, is possible that the minima of $$V_{\mathrm{Higgs}}(\phi)=\frac{1}{2}\mu^{2}\phi^{2}+\frac{1}{4}\lambda\phi^{4}$$ is not really $\langle\phi\rangle$ but rather something close to it (so in this sense we should write $\langle\phi\rangle=v+\mathrm{corrections}$)?

Filippo
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1 Answers1

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  1. Yes, the Legendre transformed variable $\phi_{\rm cl}=\langle\phi\rangle_J$ is the quantum expectation value. This may differ by quantum corrections from the solution $\phi_0[J]$ to the Euler-Lagrange equation. For details, see e.g. my Phys.SE answer here.

  2. Yes, the quantum effective action $\phi_{\rm cl}\mapsto\Gamma[\phi_{\rm cl}]$ is a convex function and it has at a minimum for $J=0$, i.e. when $\phi_{\rm cl}=\langle\phi\rangle_{J=0}$, cf. e.g. Ref. 1.

  3. Moreover, if $\langle\phi\rangle_{J=0}$ spontaneously breaks a symmetry, then $\phi_{\rm cl}\mapsto\Gamma[\phi_{\rm cl}]$ has a zero-mode, cf. Goldstone's theorem.

References:

  1. M.E. Peskin & D.V. Schroeder, An Intro to QFT, 1995; section 11.3 p. 367-369.
Qmechanic
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