Study of the performance of the Micromegas chambers for the ATLAS Muon Spectrometer upgrade

Micromegas (MICRO MEsh GAseous Structure) chambers are Micro-Pattern Gaseous Detectors designed to provide a high spatial resolution in highly irradiated environments. In 2007 an ambitious long-term R&D activity was started in the context of the ATLAS experiment, at CERN: the Muon ATLAS Micromegas Activity (MAMMA). After years of tests on prototypes and technology breakthroughs, Micromegas chambers were chosen as tracking detectors for an upgrade of the ATLAS Muon Spectrometer. These novel detectors will be installed in 2018 and 2019 during the second long shutdown of the Large Hadron Collider, and will serve as precision detectors in the innermost part of the ATLAS Muon Spectrometer. Eight layers of Micromegas modules of unprecedented size, up to 3 $\rm{{m^2}}$, will cover a surface of 150 $\rm{{m^2}}$ for a total active area of about 1200 $\rm{{m^2}}$. This upgrade will be crucial to ensure high quality performance for the ATLAS Muon Spectrometer in view of the third run of the Large Hadron Collider and of the High-Luminosity LHC, as the luminosity of the collider will significantly exceed the one the machine was originally designed for. To meet the demanding performance requirements of the ATLAS Muon Spectrometer, Micromegas chambers are required to achieve a single plane resolution of 100 ${\mu{}}\rm{{m}}$ with an efficiency better than 95% for tracks up to an inclination of 32${}^\circ$ and in a magnetic field up to 0.3 T. A thorough test program on Micromegas prototypes has been performed during the past years and is still ongoing, with the goal of driving the design of the ATLAS Micromegas chambers and of fully characterising and certifying their performance. These tests produced excellent results, proving that the prototypes fully meet the performance requirements. The methodology and the results of this activity will be reviewed in this paper.