Measurement of the Diffractive Deep-Inelastic Scattering Cross Section with a Leading Proton at HERA

The cross section for the diffractive deep-inelastic scattering process ep -> e X p is measured, with the leading final state proton detected in the H1 Forward Proton Spectrometer. The data sample covers the range x_pom<0.1 in fractional proton longitudinal momentum loss, 0.1 < |t| < 0.7 GeV^2 in squared four-momentum transfer at the proton vertex and 4 < Q^2 < 700 GeV^2 in photon virtuality. The cross section is measured four-fold differentially in t, x_pom, Q^2 and beta=x/x_pom, where x is the Bjorken scaling variable. The t and x_pom dependences are interpreted in terms of an effective pomeron trajectory and a sub-leading exchange. The data are compared to perturbative QCD predictions at next-to-leading order based on diffractive parton distribution functions previously extracted from complementary measurements of inclusive diffractive deep-inelastic scattering. The ratio of the diffractive to the inclusive ep cross section is studied as a function of Q^2, beta and x_pom.