  We study a lattice model of interacting Dirac fermions in $(2+1)$ dimension
space-time with an SU(4) symmetry. While increasing interaction strength, this
model undergoes a {\it continuous} quantum phase transition from the weakly
interacting Dirac semimetal to a fully gapped and nondegenerate phase without
condensing any Dirac fermion bilinear mass operator. This unusual mechanism for
mass generation is consistent with recent studies of interacting topological
insulators/superconductors, and also consistent with recent progresses in
lattice QCD community.
