Impact of PileUp mitigation with the Inner Tracker in the reconstruction of the Missing Transverse Energy in the ATLAS detector at HL-LHC

A precise measurement of the missing transverse energy, E_{T}^{miss}, is fundamental to experimentally measure the transverse momentum carried by non interacting particles produced in the proton-proton collisions. A good E_{T}^{miss} resolution is essential in any analysis at LHC and HL-LHC signated by events with large E_{T}^{miss} both for searches of new physics, such as SuperSymmetry and extra dimensions, and precise measurement of the Standard Model, such as the top-quark mass and the properties of the Higgs boson in the \tau\tau channel. In ATLAS, the E_{T}^{miss} is computed as the sum of the four-momentum of high-pt physics objects, i.e. leptons, photons and jets, plus the vector sum of the tracks from the Hard Scatter vertex not associated to them. The high PileUp density environment expected under the HL-LHC conditions, with a <\mu>~190-210, makes crucial an efficient rejection of PileUp jets and an accurate selection of tracks from the Hard Scatter vertex to achieve a good reconstruction of the E_{T}^{miss}. PileUp mitigation techniques developed in ATLAS during RunI and RunII exploit tracks and their vertex pointing information. Their major limitation is that they are not applicable outside the inner tracker detector acceptance. The Inner Tracker, ITk, for the Phase-II upgrade of ATLAS will extend the current |\eta|<2.5 tracker acceptance to |\eta|<4.0, allowing to apply vertex tagging techniques also to forward tracks and jets. The performances of the E_{T}^{miss} reconstruction derived from simulated events under the expected PileUp condition at HL-LHC for ATLAS plus ITk will be presented.