  We discuss and compare different approaches to include gluon transverse
momenta for heavy quark-antiquark pair production. The correlations in
azimuthal angle and in heavy quark, heavy antiquark transverse momenta are
studied in detail. The results are illustrated with the help of different
unintegrated gluon distribution functons (UGDF) from the literature. We compare
results obtained with on-shell and off-shell matrix elements and kinematics and
quantify where these effects are negligible and where they are essential. We
concentrate on the region of asymmetric transverse momenta of charm quark and
charm antiquark. Most of UGDFs lead in this corner of the phase space to almost
full decorellation in azimuthal angle. We propose correlation observables to be
best suited in order to test the existing models of UGDFs.
