Study of the pion trajectory in the photoproduction of leading neutrons   at HERA
  Energetic neutrons produced in $ep$ collisions at HERA have been studied with the ZEUS detector in the photoproduction regime at a mean photon-proton center-of-mass energy of 220 GeV. The neutrons carry a large fraction $0.64 < x_L <0.925$ of the incoming proton energy, and the four-momentum-transfer squared at the proton-neutron vertex is small, $|t|<0.425$ GeV$^2$. The $x_L$ distribution of the neutrons is measured in bins of $t$. The $(1-x_L)$ distributions in the $t$ bins studied satisfy a power law $dN/dx_L \propto (1-x_L)^{a(t)}$, with the powers $a(t)$ following a linear function of $t$: \mbox{$a(t)=0.88\pm 0.09 ({\rm stat.})^{+0.34}_{-0.39}({\rm syst.})-(2.81\pm 0.42({\rm stat.})^{+1.13}_{-0.62}({\rm syst.}) {\rm Ge V}^{-2})t$.} This result is consistent with the expectations of pion-exchange models, in which the incoming proton fluctuates to a neutron-pion state, and the electron interacts with the pion. 
ZEUS Collaboration, S. Chekanov, et al