2018 Impact factor 4.843
Particles and Fields
Eur. Phys. J. C 5, 303-315
DOI 10.1007/s100529800845

A unitarity bound and the components
of photon-proton interactions

E. Gotsman1,2 - E.M. Levin1,2,3 - U. Maor1

1 School of Physics and Astronomy, Tel Aviv University, Ramat Aviv, 69978, Israel
(e-mail: gotsman@post.tau.ac.il, leving@post.tau.ac.il, maor@post.tau.ac.il)
2 DESY Theory, Notkestr. 85, D-22603, Hamburg, Germany
3 Theory Department, Petersburg Nuclear Physics Institute, 188350, Gatchina, St. Petersburg, Russia

Received: 7 August 1997 / Revised version: 11 December 1997 / Published online: 10 March 1998

Abstract
We show how and why the short distance ("hard") interaction, which is calculated in perturbative QCD, provides a mass cutoff in Gribov's formula for photon-proton collisions. This enables us to find a new and more restrictive unitarity bound for this process, $\sigma(\gamma^{*}p)\leq C(ln\frac{1}{x})^{\frac{5}{2}}$.We develop a simple model that consists of "soft" and "hard" contributions, which yields a qualitative description of the published experimental data over a wide range of photon virtualities (Q2) and energies (W). This model provides a quantitative way of evaluating the relative rate of the short and long distance contributions, in the different kinematic regions. The main results of the analysis are (i) that even at Q2 =0 and high energies the short distance contribution is not small, and it provides a possible explanation for the experimental rise of the photoproduction cross section; and (ii) at large values of Q2, the long distance processes still contribute to the total cross section.


Copyright Springer-Verlag