Leading nucleons from peripheral processes in lepton deep inelastic scattering and the nucleon structure

The experimental information on nucleon production in lepton deep inelastic scattering (DIS) is rather scarce. Recently there is a growing interest in understanding the mechanism of the production of baryons in DIS, stimulate d by recent results on leading protons and neutrons from electron-proton sc attering at HERA. I review on different peripheral mechanisms of nucleon (p roton or neutron) production in lepton DIS and discuss their role in unders tanding the spectra of nucleons for both fixed target and collider experime nts. In DIS ep --> e'Xh, the QCD hardness scale gradually diminishes from the ha rd scale, Q(2), in the virtual photon (current) fragmentation region to the soft, hadronic, scale in the proton (target) fragmentation region. This su ggests a similarity of the inclusive spectra of leading protons and neutron s, h = p, n, in high energy hadron-proton collisions and in lepton DIS at s mall Bjorken-x. The semi-inclusive cross section for production of slow protons in charged- current deep inelastic (anti)neutrino scattering on protons and neutrons is discussed. The standard hadronization models based upon the color neutrali zation mechanism appear to underestimate the rate of slow proton production on hydrogen. The presence of the virtual mesons (pions) ill the nucleon le ads to an additional mechanism for proton production, referred to as specta tor process. The results of the calculations are compared with the CERN bub ble chamber (BEBG) data. I discuss the structure of deep-inelastic events induced by the pionexchang e at HERA kinematics. Most of the event characteristics do not provide a po ssibility to distinguish the ordinary DIS fragmentation events from those i nduced by the pion-exchange mechanism. The only exception is the energy dis tribution of outgoing neutrons. The pion cloud model predicts energy distri bution of neutrons which differs significantly from the standard hadronizat ion models which can in principle be verified experimentally. A detailed analysis of semi-exclusive diffractive processes in ep DIS at HE RA, with the diffractive final states in the forward direction is presented . The contributions of the subleading f(0), w, a(2), rho reggeons and the p ion exchanges to the diffractive structure function with the forward proton or neutron are estimated. It is found that the (a(2), rho) reggeons are en tirely responsible for the forward neutron production at x(IP) < 10(-3). Th e pi N production in the forward region is estimated using the Deck mechani sm. The significance of this reaction for the processes measured at HERA is discussed. In the DIS production of leading protons DIS off neutral pions must be supp lemented by a contribution from isoscalar reggeon (f(0)) exchange extrapola ted down to moderate values of x(L). I comment on the Ic and Q(2) dependenc e 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 sm all x. rate of slow proton production on hydrogen. The presence of the virtual mes ons (pions) ill the nucleon leads to an additional mechanism for proton pro duction, referred to as spectator process. The results of the calculations are compared with the CERN bubble chamber (BEBG) data. I discuss the struct ure of deep-inelastic events induced by the pion-exchange at HERA kinematic s. Most of the event characteristics do not provide a possibility to distin guish the ordinary DIS fragmentation events from those induced by the pion- exchange mechanism. The only exception is the energy distribution of outgoi ng neutrons. The pion cloud model predicts energy distribution of neutrons which differs significantly from the standard hadronization models which ca n in principle be verified experimentally. A detailed analysis of semi-excl usive diffractive processes in ep DIS at HERA, with the diffractive final s tates in the forward direction is presented. The contributions of the suble ading f(0); w, a(2), rho reggeons and the pion exchanges to the diffractive structure function with the forward proton or neutron are estimated. It is found that the (alpha(2), rho) reggeons are entirely responsible for the f orward neutron production at x(IP), < 10(-3). The pi N production in the fo rward region is estimated using the Deck mechanism. The significance of thi s reaction for the processes measured at NERA is discussed. In the DIS prod uction of leading protons DIS off neutral pions must be supplemented by a c ontribution from isoscalar reggeon (f(0)) exchange extrapolated down to mod erate values of x(L). I comment on the Ic and Q(2) dependence of leading pr oton production as a probe of a universal pattern of the x, Q(2) evolution of the nucleon and meson (reggeon) structure functions at small:.