Measurement of the total cross section and ρ -parameter from elastic scattering in pp collisions at √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 √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 σ_tot , 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 σ_tot and ρ are σ_tot(pp → X) = 104.7 ± 1.1 mb, ρ = 0.098 ± 0.011. The uncertainty in σ_tot is dominated by the luminosity measurement, and in ρ by imperfect knowledge of the detector alignment and by modelling of the nuclear amplitude.