  Superfluid helium, describable by a two-component order parameter, exhibits
only the Bogolubov mode with energy $\to 0$ at long wavelengths, while a
Lorentz-invariant theory with a two-component order parameter exhibits a finite
energy mode at long wavelengths (the Higgs Boson), besides the above mass-less
mode. The mass-less mode moves to high energies if it couples to
electromagnetic fields (the Anderson-Higgs mechanism). Superconductors, on the
other hand have been theoretically and experimentally shown to exhibit both
modes. This occurs because the excitations in superconductors have an
(approximate) particle-hole symmetry and therefore show a similarity to
Lorentz-invariant theories.
