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

Abstract

In a special run of the LHC with beta ^{star } = 2.5 km, proton–proton elastic-scattering events were recorded at sqrt{s} = 13 TeV with an integrated luminosity of 340~upmu {text {b}}^{-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 cdot 10^{-4} GeV^{2} to -t = 0.46 GeV^{2} using 6.9 million elastic-scattering candidates. This paper presents measurements of the total cross section sigma _{text {tot}}, parameters of the nuclear slope, and the rho -parameter defined as the ratio of the real part to the imaginary part of the elastic-scattering amplitude in the limit t rightarrow 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 sigma _{text {tot}} and rho are σtot(pp→X)=104.7±1.1mb,ρ=0.098±0.011.\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\begin{aligned} \\sigma _{\ ext {tot}}(pp\\rightarrow X) = 104.7 \\pm 1.1 \\; \ ext{ mb },\\quad \\rho = 0.098 \\pm 0.011 . \\end{aligned}$$\\end{document}The uncertainty in sigma _{text {tot}} is dominated by the luminosity measurement, and in rho by imperfect knowledge of the detector alignment and by modelling of the nuclear amplitude.