  We report on a theoretical study of inelastic quantum transport in $(3m,0)$
carbon nanotubes. By using a many-body description of the electron-phonon
interaction in Fock space, a novel mechanism involving optical phonon emission
(absorption) is shown to induce an unprecedented energy gap opening at half the
phonon energy, $\hbar\omega_{0}/2$, above (below) the charge neutrality point.
This mechanism, which is prevented by Pauli blocking at low bias voltages, is
activated at bias voltages in the order of $\hbar\omega_{0}$.
