  Using high-resolution angle-resolved photoemission, we observe a strong
spin-dependent renormalization and lifetime broadening of the quasiparticle
excitations in the electronic band structure of Ni(111) in an energy window of
$\sim$0.3 eV below the Fermi level. We derive a quantitative result for the
spin-dependent lifetime broadening by comparing the scattering rates of
majority and minority $d$ states, and further show that spin-dependent electron
correlations are instead negligible for $sp$ states. From our analysis we
experimentally determine the effective on-site Coulomb interaction $U$ caused
by Stoner-like interband transitions between majority and minority $d$ states.
The present results unambiguously demonstrate the remarkable impact of
spin-dependent electron correlation effects originating from single-particle
excitations in a prototypical 3$d$ transition metal, paving the way for further
refinement of current many-body theoretical approaches.
