Measurement of the energy dependence of the photon-proton total cross section with the ZEUS detector at HERA

The energy dependence of the photon-proton total cross section was determined from positron-proton scattering data collected with the ZEUS detector at HERA at three values of the center-of-mass energy, W, of the \gamma p system in the range 194<W<296 GeV. This is the first determination of the W dependence of the photon-proton total cross section from a single experiment at high W. Parameterizing \sigma tot(\gamma p) \alpha W^(2\epsilon), \epsilon = 0.111 \pm 0.009 (stat.) \pm 0.036 (syst.) was obtained.