  The diffraction slope parameter is investigated for elastic proton-proton and
proton-antiproton scattering based on the all available experimental data at
low and intermediate momentum transfer values. Energy dependence of the elastic
diffraction slopes is approximated by various analytic functions. The expanded
"standard" logarithmic approximations with minimum number of free parameters
allow to describe experimental slopes in all available energy range reasonably.
The estimations of asymptotic shrinkage parameter $\alpha'_{\cal{P}}$ were
obtained for various $|t|$ domains based on the all available experimental
data. Various approximations differ from each other both in the 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.
