Nonperturbative effects in the proton sea

We revisit the evaluation of the pionic mechanism of the $\bar{u}-\bar{d}$-asymmetry in the proton structure function. Our analysis is based on the unitarity relation between contributions of different mechanisms to the inclusive particle production and the total photoabsorption cross-section (i.e. the proton structure function). We reanalyze the role of isovector reggeons in inclusive production of nucleons and Delta isobars in hadronic reactions. A rather large contribution of reggeon-exchange induced production of Delta isobars is found. This leaves much less room for the pion-exchange induced mechanism of $\Delta$ production and provides a constraint on the $\pi N \Delta$ form factor. The production of leading pions in proton-proton collisions puts additional constraints on the $\pi NN$ vertex form factors. All these constraints are used then to estimate the pion content of the nucleon and allow to calculate parameter-free the x-dependence of $\bar d - \bar u$. We discuss the violation of the Gottfried Sum Rule and $\bar d$-$\bar u$ asymmetry and compare to the one obtained from the E866 experiment at Fermilab.