The Missing Odderon
  In contrast to theoretical expectations, experimental results at sqrt(s)=200 GeV for the reaction gamma p --> pi0 X show no evidence for odderon exchange. The upper limit on the cross section is an order of magnitude smaller than the theoretical estimate. It is argued that chiral symmetry leads to a large suppression, taking the thoeretical estimates well below the data. Two additional arguments are presented which may decrease the theoretical estimate further. The calculations are more sensitive to the assumptions made in evaluating the hadronic scattering amplitude than in the processes considered previously and lattice gauge calculations indicate that the odderon intercept may be appreciably lower than usually assumed. These two latter effects are particularly relevant for the reactions gamma p --> f2(1270)X and gamma p --> a2(1320)X for which the data upper limits are also below the theoretical predictions, but not so dramatically as for gamma p --> pi0 X. 
A Donnachie, H G Dosch and O Nachtmann