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,
.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
