  We report the observation of a novel effect in the bilayer Pb-Bi2212 high-TC
superconductor by means of angle-resolved photoemission with circularly
polarized excitation. Different scattering rates, determined as a function of
energy separately for the bonding and antibonding copper-oxygen bands, strongly
imply that the dominating scattering channel is odd with respect to layer
exchange within a bilayer. This is inconsistent with a phonon-mediated
scattering and favours the participation of the odd collective spin excitations
in the scattering mechanism in near-nodal regions of the k-space, suggesting a
magnetic nature of the pairing mediator.
