UBC Faculty Research and Publications

Measurement of the total cross section and ρ-parameter from elastic scattering in pp collisions at √s = 13 TeV with the ATLAS detector ATLAS Collaboration


In a special run of the LHC with β = 2.5 km, proton–proton elastic-scattering events were recorded at √s = 13 TeV with an integrated luminosity of 340 µb−1 using the ALFA subdetector of ATLAS in 2016. The elastic cross section was measured differentially in the Mandelstam t variable in the range from −t = 2.5 · 10−4 GeV2 to −t = 0.46 GeV2 using 6.9 million elastic-scattering candidates. This paper presents measurements of the total cross section σtot, parameters of the nuclear slope, and the ρparameter defined as the ratio of the real part to the imaginary part of the elastic-scattering amplitude in the limit t → 0. These parameters are determined from a fit to the differential elastic cross section using the optical theorem and different parameterizations of the t-dependence. The results for σtot and ρ are σtot(pp → X) = 104.7 ± 1.1 mb, ρ = 0.098 ± 0.011. The uncertainty in σtot is dominated by the luminosity measurement, and in ρ by imperfect knowledge of the detector alignment and by modelling of the nuclear amplitude.

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