Letter of Intent for the Phase-I Upgrade of the ATLAS Experiment

After the first successful years of running at the LHC, the ATLAS Collaboration is preparing to fully exploit the unprecedented physics opportunities offered by exploration of a completely new energy domain. This program builds on the excellent LHC accelerator complex performance demonstrated to date. A plan to consolidate and improve the physics capabilities of the current detector over the next decade, targeting the 2018 LHC shutdown as installation milestone, is presented in this Letter of Intent. The document primarily addresses the proposed enhancements to the ATLAS trigger system to cope with luminosities beyond the LHC nominal design value, while retaining the same physics performance. The Phase-I upgrades will allow ATLAS to maintain low pT trigger thresholds for isolated leptons by increasing the granularity of the calorimeters involved in the Level-1 trigger and by introducing new muon trigger and tracking detectors in the forward direction. Precision measurements of the couplings of the Higgs boson, if found in the low mass region, as well as searches for supersymmetric particles in a large region of the SUSY parameter space, rely on the capability of efficiently selecting low pT isolated leptons. Fast accurate tracking information provided near the start of the Level-2 trigger processing will lead to much more effective identification of events with isolated t and b-hadrons, improving the selection of Higgs boson decays and sensitivity to many other physics channels. Finally, a new set of very far forward detectors will enable ATLAS to explore the new diffractive physics domain made accessible by the LHC energies and luminosities, providing an unprecedented sensitivity to large momentum transfer processes.