PHENOMENOLOGICAL APPROACH TO UNPOLARIZED AND POLARIZED PARTON DISTRIBUTIONS AND EXPERIMENTAL TESTS

We recall our recent description of quark parton densities of the proton at $Q^2=4 GeV^2$ in terms of Fermi-Dirac distributions parametrized with very few free parameters. We have also proposed some simple assumptions to relate unpolarized and polarized quark parton densities which lead to a fair description of the spin-dependent structure functions $xg^p_1(x,Q^2)$ and $xg^n_1(x,Q^2)$ at low $Q^2$. We will show the predictions we obtain after a straightforward DGLAP $Q^2$ evolution and comparison in a much broader $x$ and $Q^2$ range, with several recent and accurate deep-inelastic scattering data. In particular, we will see that we get an excellent agreement with the sharp rise of $F^{ep}_2(x,Q^2)$ for small $x$, recently observed at HERA. Finally, we give several predictions for lepton pair and gauge boson production in $pp$ and $pn$ collisions at high energies which will be tested in the future at RHIC.