Electronic Access: www-h1.desy.de/publications/H1preliminary.short list.html Exclusive photoproduction of ρ0 with forward neutron at HERA

The first measurement of exclusive photoproduction of ρ0 mesons associated with lead-ing neutrons at HERA is presented. The data are taken with the H1 detector in the years 2006-2007 and correspond to an integrated luminosity of 1.16 pb−1. ρ0 mesons are reconstructed from their decays to pions in the central tracking cham-ber, while leading neutrons carrying a large fraction of the incoming proton momentum, xL> 0.35, are detected in the Forward Neutron Calorimeter. The phase space of the measurement is defined by the photon virtuality Q2 < 2 GeV2, the total energy of the photon-proton system 20 < Wγp < 100 GeV and the polar angle of the leading neutron, θn < 0.75 mrad. The cross section of the reaction γp → ρ0nY, where Y is a small mass system escaping undetected in the proton direction, is measured as a function of the neutron energy, of the effective mass of the pi+pi − system as well as of the transverse momentum squared, p2t, and of the pseudorapidity η of ρ meson. The data are interpreted in terms of the double peripheral process, involving pion ex-change at the proton vertex followed by elastic photoproduction of a ρ meson on the virtual pion. In the framework of OPE approach elastic cross section of photon-pion scattering, σel(γpi+ → ρ0pi+) at average energy 〈Wγpi 〉 = 22 GeV is extracted.