Exclusive $ρ^0$ Meson Photoproduction with a Leading Neutron at HERA

A first measurement is presented of exclusive photoproduction of $ρ^0$ 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 $ρ^0$ 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 $20 < W_{γp} < 100$ GeV and the polar angle of the leading neutron $θ_n < 0.75$ mrad. The cross section of the reaction $γp \to ρ^0 n π^+$ 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 $ρ^0$ meson on the virtual pion. In the framework of one-pion-exchange dominance the elastic cross section of photon-pion scattering, $σ^{\rm el}(γπ^+ \to ρ^0π^+)$, is extracted. The value of this cross section indicates significant absorptive corrections for the exclusive reaction $γp\toρ^0 n π^+$.

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