Measuring the gluon spin in protons through $\eta'$ central production
  The investigation of a proposed glueball filter in proton-proton collisions led us to show that its action is in fact easily understood in terms of kinematics. While the procedure proposed stays of interest in reducing background when lookingfor centrally produced resonances of specified spin and parity, we stress that it can be put to advantage in giving access to the polarization of gluons in the inital protons 
J.-M. Fr\`ere