Probing dissociation of space - like photons in deep - inelastic lepton - nucleon scattering

It is shown that the hadronic dissociation of space-like (Q > 0) photons can be directly probed by performing measurements in the fragmentation region of transversely polarized and unpolarized proton beams at the electron-proton collider HERA. Measurements of momentum-distributions in the photon-fragmentation region in normal and in LRG (large rapidity gap) events are also suggested — especially when the scattered proton or neutron in the proton-beam direction is tagged. It is pointed out that such distributions can yield useful information on the mechanisms of hadronic fragmentation in general, and answer the following questions in particular: Is the well-known hypothesis of limiting fragmentation (HFL) valid in color-exchange, in flavor-exchange, or only in vacuum-quantum-number-exchange processes ?