  We study relaxation of an excited electron in the conduction band of
intrinsic graphene at zero temperature due to production of interband
electron-hole pairs. The electronic band curvature, being anisotropic because
of trigonal warping, is shown to suppress relaxation for a range of directions
of the initial electron momentum. For other directions, relaxation is allowed
only if the curvature exceeds a finite critical value; otherwise, a
non-decaying quasiparticle state is found to exist.
