In BCS theory, it's said that electrons that form the pairs only lie within $\omega_D$ (Debye frequency) of the Fermi energy. Why is that?
Edit -
In Quantum field theory for the gifted amateur, Chapter 44 - Superconductors, In page-no 404, it says,
Since the electrons that form the pairs only lie within $\omega_D$ of the Fermi energy, we can restrict our sum to $|\epsilon_\mathbf{p}-\mu|<\omega_D$.
in units $\hbar=1$.