  The diffraction slope parameter is investigated for elastic proton-proton and
proton-antiproton scattering based on the all available experimental data at
intermediate square of momentum transfer in the main. Energy dependence of the
elastic diffraction slope is approximated by various analytic functions in a
model-independent fashion. The expanded standard logarithmic approximations
allow to describe experimental slopes in all available energy range at
qualitative level reasonably. Various fitting functions differ from each other
both in low energy and very high energy domains. Predictions for diffraction
slope parameter are obtained for elastic proton-proton scattering at NICA, RHIC
and LHC energies, for proton-antiproton elastic reaction in FAIR energy domain
for various approximation functions at intermediate square of momentum
transfer. Difference of nuclear slopes for proton-antiproton and proton-proton
scattering is investigated in wide momentum transfer range also.
