Observation of Events with an Energetic Forward Neutron in Deep   Inelastic Scattering at HERA
  In deep inelastic neutral current scattering of positrons and protons at the center of mass energy of 300 GeV, we observe, with the ZEUS detector, events with a high energy neutron produced at very small scattering angles with respect to the proton direction. The events constitute a fixed fraction of the deep inelastic, neutral current event sample independent of Bjorken $x$ and $Q^2$ in the range $ 3 \cdot 10^{-4} < \xbj < 6\cdot 10^{-3}$ and $10 < Q^2 <100$~GeV$^2$. 
ZEUS Collaboration, M. Derrick et al