Soft Interaction Processes at HERA: Leading Baryon Production, Multi-Parton Interactions

Experimental results from HERA on the production of leading protons and neutrons in ep collisions are reviewed. The measurements are well described by the models which include virtual meson exchange. Assuming the validity of pion exchange model, the leading neutron data are used to constrain the pion structure function. A refinement of the simple factorisation picture is provided by baryon absorption, occurring through rescattering. Exchange models accounting for absorption describe the Q2 evolution of the data. Furthermore, the influence of underlying event and multi-parton interactions on the charged particle multiplicity in the photoproduction of jets is investigated.