Absorptive corrections to the one pion exchange and measurablity of the small-x pion structure function at HERA

We compaWe compare the absorptive corrections to the one pion exchange in $pp\to Xn$ and $ep\to e'Xn$ reactions. It is shown that the absorption is stronger in the case of $pp$ collision. The difference in strength of the absorption for the $pp\to Xn$ and $ep\to e'Xn$ reactions breaks the factorization peculiar to the pure one pion exchange. We evaluate the emerging model dependence of extraction of the small-$x$ pion structure function from an analysis of the HERA data on the neutron production at physical values of $t$.