How to measure the pion structure function at HERA
  We suggest a method of determination of the pion structure function down to x \simeq 10^{-4} based on semi-exclusive deep inelastic scattering off protons. The idea is to exploit the nonperturbative \pi N$ and \pi\Delta Fock components of the nucleon, which contribute significantly to deep inelastic scattering and dominate the fragmentation of protons into fast neutrons and deltas. The intrinsic factorization properties of the semi-exclusive cross section give a good test for the validity of this approach. 
H. Holtmann, G. Levman, N.N. Nikolaev, A. Szczurek and J. Speth