Exclusive diffractive production of real photons and vector mesons in ep collisions has been studied at HERA in a wide kinematic range. Here we present and discuss a Regge-type model of real photon production (Deeply Virtual Compton Scattering), as w
ell as production of vector mesons (VMP) treated on the same footing by using an extension of a factorized Regge-pole model proposed earlier. The model has been fitted to the HERA data. Despite the very small number of the free parameters, the model gives a satisfactory description of the experimental data, both for the total cross section as a function of the photon virtuality Q2 or the energy W in the center of mass of the gammastar-p system, and the differential cross sections as a function of the squared four-momentum transfer t with fixed Q2 and W.
The following effects in the nearly forward (soft) region of the LHC are proposed to be investigated: 1) At small |t| the fine structure of the cone (Pomeron) shouldbe scrutinized: a) a break of the cone near $tapprox - 0.1 ~ GeV$^2, due to the two-p
ion threshold, and required by t-channel unitarity, is expected, and b) possible small-period oscillations between $t=0$ and the dip region. 2) In measuring the elastic $pp$ scattering and total $pp$ cross section at the LHC, the experimentalists are urged to treat the total cross section $sigma_t,$ the ratio $rho$, the forward slope $B$ and the luminosity ${cal L}$ as free arameters, and to publish model-independent results on ${dN/{dt}}.$ 3) Of extreme interest are the details of the expected diffraction minimum in the differential cross section. Its position, expected in the interval $0.4<-t<1$ GeV$^2$ at the level of about $10^{-2} {rm mb} cdot$ GeV$^{-2}div 10^{-1} {rm mb}cdot$ GeV$^{-2}$, cannot be predicted unambiguously, and its depth, i.e. the ratio of $dsigma/dt$ at the minimum to that at the subsequent maximum (about $-t=5 $GeV$^2$, which is about 5 is of great importance. 4) The expected slow-down with increasing $|t|$ of the shrinkage of the second cone (beyond the dip-bump), together with the transition from an exponential to a power decrease in $-t$, will be indicative of the transition from soft to hard physics. Explicit models are proposed to help in quantifying this transition. 5) In a number of papers a limiting behavior, or saturation of the black disc limit (BDL) was predicted. This controversial phenomenon shows that the BDL may not be the ultimate limit.
