  The two-photon momentum distribution of annihilating electron-positron pairs
in ferromagnetic nickel (Ni) was determined by measuring the spin-polarized
two-dimensional angular correlation of annihilation radiation (ACAR). The
spectra were compared with theoretical results obtained within LDA+DMFT, a
combination of the local density approximation (LDA) and the many-body
dynamical mean-field theory (DMFT). The self-energy describing the electronic
correlations in Ni is found to make important anisotropic contributions to the
momentum distribution which are not present in LDA. Based on a detailed
comparison of the theoretical and experimental results the strength of the
local electronic interaction U in ferromagnetic Ni is determined as 2.0 +- 0.1
eV.
