Physics Department

This paper presents the reconstruction of missing transverse momentum (pTmiss$$p_{\text {T}}^{\text {miss}}$$) 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$$\tau $$-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 pTmiss$$p_{\text {T}}^{\text {miss}}$$ ‘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 pTmiss$$p_{\text {T}}^{\text {miss}}$$ 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 140fb-1$$140~\text {fb}^{-1}$$ of data, collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018. The results demonstrate that pTmiss$$p_{\text {T}}^{\text {miss}}$$ reconstruction, and its associated significance, are well understood and reliably modelled by simulation. Finally, the systematic uncertainties on the soft pTmiss$$p_{\text {T}}^{\text {miss}}$$ 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.