  The ABJM theory refers to superconformal Chern-Simons-matter theory with
product gauge group U(L)xU(R) and level +k, -k, respectively. The theory is a
candidate for worldvolume dynamics of M2-branes sitting at C(4)/Z(k)k. By
utilizing monopole operators, we prove that ABJM theory gets enhanced N=8
supersymmetry and SO(8) R-symmetry at Chern-Simons levels k=1,2. We first show
that the ABJM Lagrangian can be written in a manifestly SO(8) invariant form up
to certain extra terms. We then show that upon integrating out Chern-Simons
gauge fields these extra terms vanish precisely at levels k=1,2. Utilizing
monopole operators at these levels, we identify new N=2 supersymmetry. We
demonstrate that they combine with the manifest N=6 supersymmetry to close
on-shell on N=8 supersymmetry. We finally show that the ABJM scalar potential
is SO(8) invariant.
