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I have seen all kinds of questions and answers about how to identify a higher-form symmetries, but they all seem rather abstract. What I would like to do is investigate two very simple examples.

  1. Consider ordinary E&M theory with Lagrangian $$\mathcal L = -\frac{1}{4} F_{\mu\nu}F^{\mu\nu}. $$ The equations of motion are $\partial_\nu F^{\mu\nu}=0$ and the Bianchi identity tells us that $\partial_\nu\star F^{\mu\nu}=0$. Thus, we have the two conserved two-form currents $F^{\mu\nu}=0$ and $\star F^{\mu\nu}=0$.

  2. Consider massive E&M theory with Lagrangian $$\mathcal L = -\frac{1}{4} F_{\mu\nu}F^{\mu\nu}-\frac{1}{2} m^2 A_\mu A^\mu. $$ Now the equations of motion are $\partial_\nu F^{\mu\nu} = m^2 A^\mu$ and the Bianchi identity is as before $\partial_\nu\star F^{\mu\nu}=0$.

QUESTION: Evidently, in the massive case $F^{\mu\nu}$ is no longer a conserved two-form current. But it appears that $\star F^{\mu\nu}$ is a conserved current. Does this mean that massive E&M has a one-form symmetry?

I have never heard anyone claim that massive gauge fields have higher-form symmetries, leading me to suspect they do not. But I cannot figure out why this should be so.

Qmechanic
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user105620
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1 Answers1

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There is a one-form symmetry in the Higgs phase of electromagnetism. And as the Higgs action can be rewritten into a Proca action, I would think the answer to your question is affirmative.

In Maxwell electromagnetism, there are indeed two 1-form symmetries, which we can call electric and magnetic. Furthermore, these are both spontaneously broken, in the sense that their Wilson loops obey a perimeter law. The photon is the associated Nambu-Goldstone boson.

Coupling the Maxwell field to any source, the electric 1-form symmetry is explicitly broken. The magnetic symmetry persists.

Moreover, in the Higgs phase of the Abelian-Higgs model where the matter field condenses, the Wilson loop now obeys an area law and the magnetic 1-form symmetry is unbroken.

In the Higgs phase, one can define a new gauge field $B_\mu = A_\mu - \frac{1}{e} \partial_\mu \phi$ with $\phi$ the phase of the Higgs field and $e$ the coupling constant; the action for $B_\mu$ is the Proca action, while its field strength is identical to the field strength of $A_\mu$. Therefore, it has the same unbroken magnetic 1-form symmetry.

References

  1. https://physics.stackexchange.com/a/522942/288815
  2. 1412.5148
  3. 1802.07747
  4. 1802.09512
  5. 1102.0468 (for the redefinition of the gauge field)
Aron Beekman
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