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

Abstract

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, stimulated by recent results on leading protons and neutrons from electron-proton scattering at HERA. I review on different peripheral mechanisms of nucleon (proton or neutron) production in lepton DIS and discuss their role in understanding the spectra of nucleons for both fixed target and collider experiments. In DIS ep → e′Xh, the QCD hardness scale gradually diminishes from the hard scale, Q 2, in the virtual photon (current) fragmentation region to the soft, hadronic, scale in the proton (target) fragmentation region. This suggests a similarity of the inclusive spectra of leading protons and neutrons, h = p, n, in high energy hadron-proton collisions and in lepton DIS at small Bjorken-χ. 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 neutralization mechanism appear to underestimate the rate of slow proton production on hydrogen. The presence of the virtual mesons (pions) in the nucleon leads to an additional mechanism for proton production, referred to as spectator process. The results of the calculations are compared with the CERN bubble chamber (BEBC) data. I discuss the structure of deep-inelastic events induced by the pione-xchange at HERA kinematics. Most of the event characteristics do not provide a possibility to distinguish the ordinary DIS fragmentation events from those induced by the pion-exchange mechanism. The only exception is the energy distribution of outgoing neutrons. The pion cloud model predicts energy distribution of neutrons which differs significantly from the standard hadronization models which can in principle be verified experimentally. A detailed analysis of semi-exclusive diffractive processes in ep DIS at HERA, with the diffractive final states in the forward direction is presented. The contributions of the subleading f 0; ω,a 2, ρ reggeons and the pion exchanges to the diffractive structure function with the forward proton or neutron are estimated. It is found that the ( a 2, ρ) reggeons are entirely responsible for the forward neutron production at χ IP < 10 −3. The π N production in the forward region is estimated using the Deck mechanism. 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 supplemented by a contribution from isoscalar reggeon ( f0) exchange extrapolated down to moderate values of χ L . I comment on the χ and Q 2 dependence of leading proton production as a probe of a universal pattern of the χ, Q 2 evolution of the nucleon and meson (reggeon) structure functions at small χ.