Micromegas chambers performance studies for the ATLAS upgrade

The MICRO MEsh GAseous Structure, micromegas (MM), are highly innovative micro-pattern gas detectors, designed to achieve high spatial resolution (σ$_{x}$ 100 μm) and high efficiency (>95%) in highly irradiated environments ($\phi$ ∼ 15 kHz/cm$^{2}$). For this reason, these chambers were chosen as precision detectors for the upgrade of the ATLAS muon spectrometer (New Small Wheel project, NSW), in view of the expected increase of the luminosity at LHC for Run 3 and HL-LHC (High Lumi LHC) programme. Two identical NSW will be built to replace the innermost stations in the two forward regions of the ATLAS muon spectrometer. Each NSW will consists of two different detector technologies: MM and sTGC (small strip Thin Gap Chamber) to provide information both to the ATLAS trigger and to the track reconstruction. The NSW consists of 8 large sectors (LM) and 8 small sectors (SM). To ensure redundancy, each sector will consist of 2MM chambers and two sTGCs with 4 layers each, for a total of 16 points per track. Four types of MM chambers called LM 1-2 and SM 1-2 are currently under construction, whose production is distributed among different countries: Italy (SM1), Germany (SM2), France (LM1), Greece and Russia (LM2). All the chambers will be trapezoidal in shape, with areas of 2–3m$^{2}$. Each MM chamber, composed of 4 reading layers, will provide information about the precision coordinate in ATLAS ($\eta$) and, through the two planes with stereo reading (±1.5 degrees), also about the azimuthal coordinate ($\phi$). In this paper the validation procedures of the chambers will be shown as well as the results obtained on the first MM SM1 modules at the cosmic rays stand of the National Laboratories of Frascati.