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<xL<0.925 of the incoming proton energy, and the four-momentum transfer squared at the proton-neutron vertex is small, |t|<0.425 GeV2. The xL distribution of the neutrons is measured in bins of t. The (1-xL) distributions in the t bins studied satisfy a power law dN/dxL∝(1-xL)a(t), with the powers a(t) following a linear function of t: a(t)=0.88±0. 09(stat.)-0.39+0.34(syst.)-(2.81±0.42(stat.)-0.62+1.13(syst.) GeV-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.