H1

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 $p_T$ < 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 $Q^2$ < 2 GeV$^2$, the total energy of the photon-proton system 20 < $W_\gamma$, $p$ < 100 GeV and the polar angle of the leading neutron $\theta_n$ < 0.75 mrad. The cross section of the reaction $\gamma p \to \rho^0 n \pi^+$ 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 rho0 meson on the virtual pion. In the framework of one-pion-exchange dominance the elastic cross section of photon-pion scattering, $\sigma_{el}(\gamma \pi^+ \to \rho^0 π^+)$, is extracted. The value of this cross section indicates significant absorptive corrections for the exclusive reaction $\gamma p \to \rho^0 n \pi^+$.