Plans for the Phase II Upgrade to the ATLAS Detector

CERN has planned a series of upgrades for the Large Hadron Collider (LHC). The last in this current series of planned upgrades is designated the High Luminosity LHC (HL-LHC) and as the name suggests will bring the Luminosity up to $5\times 10^{34}$~cm$^{-2}$s$^{-1}$. The ATLAS detector will be extensively changed to meet the challenges of this upgrade (termed the ``Phase~II'' upgrade). There are many systems that require modification in this regime, but this paper focuses on the subsystems requiring the most radical changes. The ATLAS inner tracker is being completely rebuilt for Phase~II. The TRT is removed in favour of an all-silicon tracker. The changes to the pixel system, barrel and end-cap strip detectors are explained. In addition, the muon detector will be modified and the muon and electron triggers will be modified to include tracking regions of interest and to improve muon resolution. In this way trigger rates can be brought under control while maintaining constant trigger thresholds.