Leading proton spectrum from DIS at HERA

The QCD hardness scale for secondary particles ($h$) production in semi-inclusive deep inelastic scattering (DIS), $e p \to e' X h$, gradually decreases from $Q^{2}$, the photon virtuality which determines the hard scale in the virtual photon (current) fragmentation region to a soft, hadronic, scale in the proton fragmentation region. This suggests similarity of the inclusive spectra of leading protons and neutrons, $h=p,n$, in high energy hadron-proton and virtual photon-proton collisions. We explore this similarity extending to the DIS regime the nonperturbative peripheral mechanisms of inelastic scattering traditionally used in hadronic interactions to explain fast nucleons production. While the production of leading neutrons is known to be exhausted by DIS off charged pions, the production of leading protons by DIS off neutral pions must be supplemented by a substantial contribution from isoscalar reggeon ($f_{0}$) exchange extrapolated down to moderate values of $x_{L}$. We comment on the $x$ and $Q^{2}$ dependence of leading proton production as a probe of a universal pattern of the $x,Q^{2}$ evolution of the nucleon and meson (reggeon) structure functions at small $x$.