Central Production of $\eta$ via Double Pomeron Exchange and Double   Reggeon Exchange in the Sakai-Sugimoto Model
  We use holographic QCD to study Pomeron- and Reggeon-mediated central production in Regge regime hadronic scattering. We focus specifically on $\eta$ production in proton-proton collisions. While previous work studied the Pomeron-mediated process, the mesonic Regge trajectories ("Reggeons") we now incorporate contribute significantly at experimentally probed energies. We use the five-point open string amplitude (in flat space) to construct approximate propagators for the Reggeon states, and the five-point closed string amplitude for the Pomeron. Using these "holographic" Reggeons and Pomerons, and low-energy couplings derived in the Sakai-Sugimoto model, we compute the differential and total cross sections at $\sqrt{s}$ =29.1 GeV. Our calculation of the total cross section is $\sigma =236 \ \mathrm{nb}$, while the experimentally measured value is $\sigma = 3859 \ \mathrm{nb}$. The discrepancy between our result and the measured value may indicate that our model systematically underestimates the values of the relevant coupling constants. 
Neil Anderson, Sophia Domokos, Nelia Mann
hep-ph hep-th