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We report results from a study of events with a leading antiproton of beam

W e report a measurement o f the diffraction dissociation differential cross section d 2 u s~/ d M 2 d t for ?Sp + pX at f i = 546 and 1800 G e V , M 2 / s < 0.2 and 0 < -t < 0.4 G~v~. Our results are compared t o theoretical predictions and t o extrapolations from experimental results at lower energies. PACS number(s): 13.85.Hd, 12.40.Nn

A measurement of the total pp cross section at the LHC at √ s = 7 TeV is presented. In a special run with high-β beam optics, an integrated luminosity of 80 µb −1 was accumulated in order to measure the differential elastic cross section as a function of the Mandelstam momentum transfer variable t. The measurement is performed with the ALFA sub-detector of ATLAS. Using a fit to the differential elastic cross section in the |t| range from 0.01 GeV 2 to 0.1 GeV 2 to extrapolate to |t| → 0, the total cross section, σ tot (pp → X), is measured via the optical theorem to be:where the first error is statistical, the second accounts for all experimental systematic uncertainties and the last is related to uncertainties in the extrapolation to |t| → 0. In addition, the slope of the elastic cross section at small |t| is determined to be B = 19.73 ± 0.14 (stat.) ± 0.26 (syst.) GeV −2 .

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