  The electronic structure and superconducting gap structure are prerequisites
to establish microscopic theories in understanding the superconductivity
mechanism of iron-based superconductors. However, even for the most extensively
studied optimally-doped (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$, there remain
outstanding controversies on its electronic structure and superconducting gap
structure. Here we resolve these issues by carrying out high-resolution
angle-resolved photoemission spectroscopy (ARPES) measurements on the
optimally-doped (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$ superconductor using both
Helium lamp and laser light sources. Our results indicate the "flat band"
feature observed around the Brillouin zone center in the superconducting state
originates from the combined effect of the superconductivity-induced band
back-bending and the folding of a band from the zone corner to the center. We
found direct evidence of the band folding between the zone corner and the
center in both the normal and superconducting state. Our resolution of the
origin of the flat band makes it possible to assign the three hole-like bands
around the zone center and determine their superconducting gap correctly.
Around the zone corner, we observe a tiny electron-like band and an M-shaped
band simultaneously in both the normal and superconducting states. The obtained
gap size for the bands around the zone corner ($\sim$5.5 meV) is significantly
smaller than all the previous ARPES measurements. Our results establish a new
superconducting gap structure around the zone corner and resolve a number of
prominent controversies concerning the electronic structure and superconducting
gap structure in the optimally-doped (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$. They
provide new insights in examining and establishing theories in understanding
superconductivity mechanism in iron-based superconductors.
