Exclusive $\ddot{I}{}^0$ Meson Photoproduction with a Leading Neutron at HERA

Abstract

A first measurement is presented of exclusive photoproduction of $\ddot{I}{}^0$ 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 $\ddot{I}{}^0$ 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 $20<W_{{\hat{I}}^{3}p}<100$ GeV and the polar angle of the leading neutron $\hat{I}{¸}_n<0.75$ mrad. The cross section of the reaction ${\hat{I}}^{3}p\to \ddot{I}{}^{0}n\ddot{I}{€}^{+}$ 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 $\ddot{I}{}^0$ meson on the virtual pion. In the framework of one-pion-exchange dominance the elastic cross section of photon-pion scattering, $\ddot{I}{ƒ}^{\mathrm{el}}({\hat{I}}^3\ddot{I}{€}^{+}\to \ddot{I}{}^0\ddot{I}{€}^{+})$, is extracted. The value of this cross section indicates significant absorptive corrections for the exclusive reaction ${\hat{I}}^{3}p\to \ddot{I}{}^{0}n\ddot{I}{€}^{+}$.

9 pages, 5 figures. Writeup for the Proceedings of the Conference EDS Blois 2017, Prague, Czech Republic, June 26-30, 2017