arXiv:0807.0191v1 [hep-ex] 1 Jul 2008
Energy dependence of σtot(γp) at HERA
Aharon Levy1,2,∗
on behalf of the ZEUS collaboration
1- Max-Planck-Institute
Föhringer Ring 6, 80805 Munich, Germany
2- School of Physics and Astronomy
Tel Aviv University, 69978 Tel Aviv, Israel
The energy dependence of the total photon-proton cross-section is determined from data
collected with the ZEUS detector at HERA with two different proton beam energies.
1
Introduction
Donnachie and Landshoff (DL) [1] showed that the energy dependence of all hadron-hadron
total cross sections can be described by a simple Regge motivated form,
σtot = A · (W
2)αIP (0)−1 + B · (W 2)αIR(0)−1 ,
(1)
where A and B are process dependent constants, W is the hadron-hadron center-of-mass
energy, and αIP (0) (αIR(0)) is the Pomeron (Reggeon) trajectory intercept.
The σtot(γp) dependence on W is particularly interesting because of the nature of the
photon, which is known to exhibit properties of both a point-like particle (direct photon) and
a hadron-like state (resolved photon). At the ep collider HERA, σtot(γp) can be extracted
from ep scattering at very low momentum transferred squared at the electron vertex, Q2 ≃
0 GeV2.
The first measurements of the total γp cross section at HERA [2, 3] showed that the
total photoproduction cross section has a W dependence similar to that of hadron-hadron
reactions. Further measurements of σtot(γp) at HERA [4, 5, 6] have reduced its statistical
errors but the systematic uncertainties remained too large for a precise determination of the
W dependence of the cross section. The original fits of DL gave ǫ ≡ αIP (0)−1 = 0.0808 and
no uncertainties were determined. Cudell et al. [7], using a data set [9] that contained 2747
data points for total cross sections and 303 data points for real part of hadronic amplitudes,
determined ǫ = 0.093 ± 0.003. However, only very few points were present for the highest
center-of-mass energies. The data were from different experiments and had a large spread.
Furthermore, the value of the Po