We calculate the cross section of diffractive charge-parity C = +1 neutral meson production in virtual photon proton collision at high energies. Due to the opposite C-parities of photon and meson M + (M + = η C , π 0 , a 2 ) this process probes the t-channel C = −1 odderon exchange which is described here as noninteracting three-gluon exchange. Estimates for the cross section of inelastic diffractive process γ * p → η C X p are presented. The total cross section of diffractive η C meson photoproduction is found to be 47 pb. The cross sections for the diffractive production of light mesons (π 0 , a 2 ) in γ * p collisions are of the same order if the photon virtuality Q 2 is m 2 C .1 Supported by German Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie, grant No. 05 7LP91 P0, and in the framework of the German-Polish agreement on scientific and technological cooperation, grant No. N -115 -95. Odd charge-parity exchangeThe increase of luminosity at HERA offers the possibility of experimental investigations of diffractive processes of photo-and electroproduction of neutral charge-parity even mesons M + = π 0 , a 2 , η C at high energies (see, for example, [1]). Studies of these reactions are of great interest since the opposite chargeparities C of photon (C = −1) and M + meson cause definite C-parity C = −1 of the t-channel exchange. Consequently the pomeron exchange being most important at high energies does not contribute there. Therefore these processes allow for clean investigations of other Regge trajectories which are characterized by negative C = −1 parity.Reggeons having C = −1 are the ω trajectory and the odderon. According to fits to data on total hadron-hadron cross sections the intercept of ω trajectory is close to 0.5 [2] and its contribution decreases with energy. The odderon is the C odd partner of pomeron with an intercept ≥ 1 which has been introduced in phenomenology long time ago [3]. It is assumed that, like the pomeron, the odderon is related to gluonic degrees of freedom. The odderon exchange results in a difference between, for example, the total pp and pp cross sections which does not decrease with increasing energy. Up to now there is no indication of that difference in present high energy pp and pp data. Also odderon effects have not been observed in other soft hadronic collisions.The reasons for the absence of the odderon exchange in soft hadronic reactions remains to be understood since in perturbative QCD the pomeron and the odderon appear on the equal footing. In Leading Logarithmic Approximation (LLA) of perturbative QCD the pomeron corresponds to the exchange of two interacting reggeized gluons (hard pomeron) whereas the odderon is described by exchange of three interacting reggeized gluons (hard odderon), see [4]. In Born approximation, i.e. considering the pure three-gluon exchange in the t-channel, the intercept of the odderon is equal to 1. The effect of gluon interactions is expected to increase of odderon intercept [5]. Therefore the hard odderon ...

We consider pion electroproduction close to threshold for Q 2 in the region 1 − 10 GeV 2 on a nucleon target. The momentum transfer dependence of the S-wave multipoles at threshold, E0+ and L0+, is calculated in the chiral limit using light-cone sum rules. Predictions for the cross sections in the threshold region are given taking into account P-wave contributions that, as we argue, are model independent to a large extent. The results are compared with the SLAC E136 data on the structure function F2(W, Q 2 ) in the threshold region.

The gluon content of the proton, in the high-energy regime, is embodied by the unintegrated gluon distribution (UGD), which describes the gluon emission probability, with a given longitudinal momentum fraction and transverse momentum. The UGD, formulated within the κ-factorization approach, has universal validity and several models for it have been proposed so far. We will show that the polarized ρ-meson leptoproduction at HERA is a not trivial testfield for discriminating among existing models of UGD.

We calculate the electromagnetic contribution to the scattering amplitude of pion diffractive dissociation into di-jets which is described by one photon exchange. The result shows that the factorization procedure known for the description of exclusive reactions holds also for this quasi-exclusive process. We find that the longitudinal momentum distribution of di-jets does not depend on the form of the pion distribution amplitude. We discuss the magnitude of the cross section.Comment: 7 pages, 3 .eps figures, Late