Can't the Higgs vacuum be a superposition?

Question

Supose you have a complex scalar field $\phi$ statisfying the typical Higgs lagrangian \begin{equation} L = \partial_{\mu}\phi\partial^{\mu}\phi + \mu^2\phi^*\phi-\frac{\lambda^2}{2}(\phi^*\phi)^2 \end{equation}

It has an infinitely degenerated vacuum state parametrized by an angle $\theta$ like $\phi_0=e^{i\theta}v$ where $\phi_0$ is a posible vacuum state for each posible theta.

Now, any election of a vacuum would indeed break the symmetry of the system and, classically (I'm thinking in magnets now) this seems kind of inevitable. But since this is a quantum theory.. why can't we just make a vacuum that is a superposition of every posible vacuum? Something like:

\begin{equation} |\phi_0'\rangle=\frac{1}{2\pi}\int_0^{2\pi}|{\phi_0(\theta)}\rangle d\theta \end{equation}

would this vacuum state break symmetry? is it even a posible state? is it more stable that just a regular symmetry-breaking one?

Any comments on the idea of a superposition of symmetry-breaking vacuum states will be welcome!

P.S.: I'm not saying that $\phi_0'$ is the correct superposition, I just wrote it down to give an example of what I was thinking.

Answer 1

Goldstone's theorem is unforgiving. It somehow feels your idiosyncratic notation for the vacuum is expressly designed to moot it. Work in the polar parameterization, $$ \phi(x)\equiv r(x) e^{i\Theta(x)}, $$ so $\langle r(x)\rangle =v$ for any and all vacua at the bottom of the Goldstone sombrero.

I imagine, hoping I am right, that your vacua are defined/labelled by $$ \langle \theta| \Theta(x)|\theta\rangle=\theta. $$ Recall a small rotation by angle ε will always shift the Goldstone field and thus its v.e.v. by ε--no choice there.

Any vacuum $|\Omega\rangle$, in any case, will have $$ \langle \Omega| \phi(x)/v|\Omega\rangle=e^{i\theta'}, $$ for some θ'.

For your $|\theta\rangle$, θ'=θ. For your exemplar Ansatz $|\phi_0'\rangle$, (unnormalizable, multi-superselectioned),the integral of θ would net you θ'=0, etc.

The key point: it really does not matter what your θ' is. You simply redefine your goldston to $\Theta'\equiv \Theta -\theta'$ so $$ \langle \Theta' \rangle =0, $$ but $\langle \delta_\epsilon \Theta' \rangle =\epsilon\neq 0$, which is all that matters: a shift from an arbitrary origin. SSB appears inescapable, in the absence of other fields.

Answer 2

You're completely right, it's completely possible, and in fact expected, to get superpositions of $| \theta \rangle$ vacua. If one adopts the "many worlds" view, once spontaneous symmetry breaking takes place (maybe after the universe cools somehow) then the universes built atop these different vacua (by acting with creation operators on these vacua) will represent different branches of the QFT wave functional. These branches represent parallel realities that do not "talk to each other."

The magical vocabulary word you might want to investigate further is "super selection." It would take an infinite amount of energy to transition from one $| \theta \rangle$ vacuum to another, so this is impossible in the real world. Therefore, once you've measured yourself to be living in one such vacuum, you'll never be able to physically construct a state which is the super position of multiple different vacua.

(In the actual Higgs mechanism, of course, the broken symmetry is a gauge symmetry, and the gauge symmetry is used to "eat" the $\theta$ direction, and does not represent a physical degree of freedom. But obviously here you are talking about the non gauge version of the Higgs mechanism.)