The performance of missing transverse momentum reconstruction and its significance with the ATLAS detector using 140 fb^-1 of s=13 TeV pp collisions

Abstract This paper presents the reconstruction of missing transverse momentum (p ₓ^ {miss} p T miss) in proton–proton collisions, at a center-of-mass energy of 13 TeV. This is a challenging task involving many detector inputs, combining fully calibrated electrons, muons, photons, hadronically decaying τ -leptons, hadronic jets, and soft activity from remaining tracks. Possible double counting of momentum is avoided by applying a signal ambiguity resolution procedure which rejects detector inputs that have already been used. Several p ₓ^ {miss} p T miss ‘working points’ are defined with varying stringency of selections, the tightest improving the resolution at high pile-up by up to 39% compared to the loosest. The p ₓ^ {miss} p T miss performance is evaluated using data and Monte Carlo simulation, with an emphasis on understanding the impact of pile-up, primarily using events consistent with leptonic Z decays. The studies use 140~ fb^-1 140 fb - 1 of data, collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018. The results demonstrate that p ₓ^ {miss} p T miss reconstruction, and its associated significance, are well understood and reliably modelled by simulation. Finally, the systematic uncertainties on the soft p ₓ^ {miss} p T miss component are calculated. After various improvements the scale and resolution uncertainties are reduced by up to 76\% 76 % and 51\% 51 %, respectively, compared to the previous calculation at a lower luminosity.