Measurement of the total cross section and rho-parameter from elastic scattering in pp collisions at sqrt(s) = 13 TeV with the ATLAS detector

In a special run of the LHC with β* = 2.5 km, proton–proton elastic-scattering events were recorded at sqrt(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 ρ-parameter 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.