  We theoretically study an emergent SU(2) symmetry which is suggested by
recent magneto-transport measurements, carried out near two electrons filling
of a carbon nanotube quantum dot. It emerges in the case where the spin and
orbital Zeeman splittings cancel each other out for two of the one-particle dot
levels among four. Using the Wilson numerical renormalization group, we show
that a crossover from the SU(4) to SU(2) Fermi-liquid behavior occurs at two
impurity-electrons filling as magnetic field increases. We also find that the
quasiparticles are significantly renormalized as the remaining two one-particle
levels move away from the Fermi level and are frozen at high magnetic fields.
In order to clarify how the ground state evolves during such a crossover, we
also reexamine the SU(N) Kondo singlet state for M impurity-electrons filling
in the limit of strong exchange interactions. We show that the nondegenerate
Fermi-liquid fixed point of Nozi\`{e}es and Blandin can be described as a
bosonic Perron-Frobenius vector for M hard-core bosons, each of which consists
of one impurity-electron and one conduction hole. This interpretation can also
be applied to the Fermi-liquid fixed-point without the SU(N) symmetry.
