  Using a combined analytical and numerical approach, we study the collective
spin and orbital excitations in a spin-orbital chain under a crystal field.
Irrespective of the crystal field strength, these excitations can be
universally described by fractionalized fermions. The fractionalization
phenomenon persists and contrasts strikingly with the case of a spin chain,
where fractionalized spinons cannot be individually observed but confined to
form magnons in a strong magnetic field. In the spin-orbital chain, each of the
fractional quasiparticles carries both spin and orbital quantum numbers, and
the two variables are always entangled in the collective excitations. Our
result further shows that the recently reported separation phenomenon occurs
when crystal fields fully polarize the orbital degrees of freedom. In this
case, however, the spinon and orbiton dynamics are decoupled solely because of
a redefinition of the spin and orbital quantum numbers.
