  Among numerous hypotheses, recently proposed to explain superconductivity in
iron-based superconductors [1-9], many consider Fermi surface (FS) nesting [2,
4, 8, 10] and dimensionality [4, 9] as important contributors. Precise
determination of the electronic spectrum and its modification by
superconductivity, crucial for further theoretical advance, were hindered by a
rich structure of the FS [11-17]. Here, using the angle-resolved photoemission
spectroscopy (ARPES) with resolution of all three components of electron
momentum and electronic states symmetry, we disentangle the electronic
structure of hole-doped BaFe2As2, and show that nesting and dimensionality of
FS sheets have no immediate relation to the superconducting pairing.
Alternatively a clear correlation between the orbital character of the
electronic states and their propensity to superconductivity is observed: the
magnitude of the superconducting gap maximizes at 10.5 meV exclusively for iron
3dxz;yz orbitals, while for others drops to 3.5 meV. Presented results reveal
similarities of electronic response to superconducting and magneto-structural
transitions [18, 19], implying that relation between these two phases is more
intimate than just competition for FS, and demonstrate importance of orbital
physics in iron superconductors.
