Exclusiv $\rho^0$ Meson Photoproduction with a Leading Neutronat HERA

A first measurement is presented of exclusive photoproduction of $\rho^0$ 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 $\sqrt{s}$=319 GeV and correspond to an integrated luminosity of 1.16 pb$^{−1}$. The $\rho^0$ mesons with transverse momenta $p_T $ 0.35, are detected in the Forward Neutron Calorimeter. The phase space of the measurement is defined by the photon virtuality Q$^2$ < 2 GeV$^2$, the total energy of the photon-proton system 20 < W$_{γp}$ < 100 GeV and the polar angle of the leading neutron $θn <$ 0.75 mrad. The cross section of the reaction $γp → ρ^0 nπ^+$ 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, $σ^{el}(γπ^+ → ρ^0 π^+)$, is extracted. The value of this cross section indicates significant absorptive corrections for the exclusive reaction $γp → ρ^0 nπ^+$