  The mechanism of atomic collisions in excited bands plays an important role
in the study of the orbital physics in optical lattices and simulation of
condensed matter physics. Atoms distributing in one excited bands of an optical
lattice would collide and decay to other bands through different scattering
channels. In excited bands of one dimensional lattice, due to lack of geometry,
there is no significant difference between cross section of scattering
channels. Here, we investigate the collisional scattering channels for atoms in
the excited bands of a triangular optical lattice and demonstrate a dominant
scattering channel in the experiment. A shortcut method is utilized to load
Bose-Einstein condensates of $^{87} {\rm Rb}$ atoms into the first D band with
zero quasi-momentum. After some time for evolution, the number of atoms
scattering to S band due to two-body collisions is around four times more than
that to the second most band. We reveal that the scattering channel to $ss$
band is dominant by theoretical calculation, which agrees with experimental
measurements. The appearance of dominant scattering channels in triangular
optical lattice is owing to geometric dimension coupling. This work is helpful
for the study of many-body systems and directional enhancement in optical
lattices.
