Exclusive ρ⁰ meson photoproduction with a leading neutron at HERA

A first measurement is presented of exclusive photoproduction of ρ⁰ mesons associated with leading neutrons at HERA. The data were taken with the H1 detector in the years 2006 and 2007 at a centre-of-mass energy of √(s)=319 GeV and correspond to an integrated luminosity of 1.16 pb^-1. The ρ⁰ mesons with transverse momenta p_T<1 GeV are reconstructed from their decays to charged pions, while leading neutrons carrying a large fraction of the incoming proton momentum, x_L>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 20γp<100 GeV and the polar angle of the leading neutron θ_n<0.75 mrad. The cross section of the reaction γp→ρ⁰nπ⁺ is measured as a function of several variables. The data are interpreted in terms of a double peripheral process, involving pion exchange at the proton vertex followed by elastic photoproduction of a ρ⁰ meson on the virtual pion. In the framework of one-pion-exchange dominance the elastic cross section of photon-pion scattering, σ^el(γπ⁺→ρ⁰π⁺), is extracted. The value of this cross section indicates significant absorptive corrections for the exclusive reaction γp→ρ⁰nπ⁺.