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

The ATLAS collaboration at LHC has chosen the Micromegas (Micro Mesh Gaseous Structure) technology along with the small-strip Thin Gap Chambers (sTGC) for the high luminosity upgrade of the inner muon station in the high-rapidity region, the so called New Small Wheel (NSW). It employs eight layers of Micromegas detectors and eight layers of sTGC. The NSW project requires fully efficient Micromegas chambers with spatial resolution down to 100 μm in the precision coordinate for momentum reconstruction, and at mm level in the azimuthal (second) coordinate, over a total active area of 1200 m², with a rate capability up to about 15 kHz/cm² and operation in a moderate magnetic field up to B = 0.4 T. The required tracking capability is provided by the intrinsic space resolution combined with a mechanical precision at the level of 30 μm along the precision coordinate. Together with the precise tracking capability the Micromegas chambers should provide a trigger signal. Several tests have been performed on small (10x10 cm²) and large (1 x 1 m²) size single gap chambers prototypes using high energy hadron beams at CERN, low and intermediate energy (0.5-5 GeV) electron beams at Frascati and DESY, neutron beams at Demokritos (Athens) and Garching (Munich) and cosmic rays. More recently two quadruplets with dimensions 1.2 x 0.5 m² and the same configuration and structure foreseen for the NSW upgrade have been built at CERN and tested with high energy pions/muons beam. Results obtained in the most recent tests, in different configurations and operating conditions, in dependence with the magnetic field, will be presented, along with a comparison between different read-out electronics, either based on the APV25 chips, or based on a new digital front-end ASIC developed in its second version (VMM2) as a new prototype of the final chip that will be employed in the NSW upgrade. (authors)