Pile-up Suppression in Missing Transverse Momentum Reconstruction in the ATLAS Experiment in Proton-Proton Collisions at sqrt{s} = 8 TeV

The ATLAS experiment recorded proton collision data at $\sqrt{s} = 8$ TeV at the LHC in 2012 corresponding to an integrated luminosity of about 20 fb$^{-1}$. The high instantaneous luminosity achieved (up to $8 \cdot 10^{33}$ cm$^{-2}$ s$^{-1}$), combined with bunch crossings every 50 ns, led to unprecedented backgrounds (pile-up) from additional proton-proton collisions occurring at the same bunch crossing as the triggered collision of interest, and from remnants of electronic signals from previous bunch crossings in the detectors. Typical run conditions were characterised by an average of 21 pile-up interactions, increasing to 35 by the end of the data taking. This pile-up led to a significant deterioration of the missing transverse momentum reconstruction performance. Both the hard part of the recorded event, which is comprised of identified leptons, photons, and reconstructed jets, and the soft part containing detector signals from the underlying event, are affected by these pile-up contributions. While the objects contributing to the hard part are fully calibrated and corrected for pile-up, the signals from the ATLAS calorimeters and tracking detectors contributing to the soft part are typically not. Several correction methods for this soft part have been developed in ATLAS to mitigate the effect of pile-up on the missing transverse momentum reconstruction performance. In this note, these methods as well as relevant features of their respective inputs are described. In addition, performance improvements for various final states are evaluated and systematic uncertainties arising from the application of these corrections are presented.