Leading Baryons and σtot(γp) at HERA

Events with a baryon carrying a large fraction of the proton beam energy have been observed in ep scattering at HERA [1, 2]. The dynamical mechanisms for their production are not completely understood. They may be the result of hadronization of the proton remnant, leaving a baryon in the final state. Exchange of virtual particles is also expected to contribute. In this picture, the target proton fluctuates into a virtual meson-baryon state. The virtual meson scatters with the projectile lepton, leaving the fast forward baryon in the final state. Leading neutron (LN) production occurs through the exchange of isovector particles, notably the π meson. For leading proton (LP) production isoscalar exchange also contributes, including diffraction mediated by Pomeron exchange. In the exchange picture, the cross section for some process in ep scattering with e.g. LN production factorizes: σep→enX = fπ/p(xL, t) · σeπ→eX . Here fπ/p is the flux of virtual pions in the proton, xL = En/Ep is the fraction of the proton beam energy carried by the neutron, and t is the virtuality of the exchanged pion. σeπ→eX is the cross section for electroproduction on the pion.