Photon '97: Theory Summary

I’m going to restrict myself to diffractive phenomena in γp interactions (where the photon can be real or virtual). For the present purposes “diffraction” means that there is a rapidity gap in the final state (I’ll have more to say on this later). Let’s start by recalling some results on total rates . In particular, I want to discuss the W -dependence of total rates (W is the γp invariant mass). A convenient way to parameterize the data on the W -dependence is to extract an effective pomeron intercept, αP (0). It is to be understood that this value would be the true pomeron intercept if the physics were solely due to exchange of a single Regge pole. Recall that total hadronic cross-sections and exclusive photoproduction of light vector mesons (e.g. γp → V p where V is a vector meson and the photon is close to its mass-shell, i.e. Q ≈ 0) can be described with a single value of αP (0) ≈ 1.08 . However, there are processes which do not follow this trend. The dissociation process, γ∗p→ Xp (X denotes the dissociation products which are distant in rapidity from the outgoing proton), is characterized by a rather large effective intercept, i.e. αP (0) = 1.18 ± 0.02 ± 0.04 . There is also a tendency for the intercept to grow as Q increases . In addition, the growth for γp → J/ψ p is larger still, αP (0) ≈ 1.3 (even for photoproduction, i.e. Q ≈ 0). The situation for light meson production off virtual photons is less clear. There is a suggestion that the W dependence is much steeper than it would be for on-shell photons, and that it steepens as Q rises. However the conclusion relies upon extrapolating from low energy data, where the recent E665 measurement 8 (of a high crosssection value at W ≈ 11 GeV) confuses the issue. Data from Hermes should help sort this out . It seems that a large photon virtuality or a large quark mass is correlated with a more rapidly rising cross-section (recall also that the deep inelastic total cross-section rises rapidly with increasing W , i.e. at small