Construction and test of the SM1 MicroMegas chambers for the upgrade of the forward muon spectrometer of the ATLAS experiment

MICRO MEsh GASeous structure chambers, MicroMegas (MM (Giomataris et al., 1996 [1] )), is an innovative design concept for Micro-Pattern Gaseous Detectors, designed in order to provide a high spatial resolution and to cope with highly irradiated environments. These chambers have been chosen as new precision tracking detectors for the upgrade of the forward muon spectrometer of the ATLAS experiment during Long Shutdown 2 (LS2, 2019–2020). MM trapezoidal detector modules, with areas of 2–3 m$^2$ , will be part of the two New Small Wheels (NSW (ATLAS collaboration, 2013 [2] )) composing the innermost station of the ATLAS muon tracking system. ATLAS MM detectors are composed out of 4 read-out (RO) layers, two for the reconstruction of the precision coordinate and two with stereo reading ( ±1.5 °) for the reconstruction of the second coordinate. MM chambers are designed to provide a resolution better that 100μm on the precision coordinate with an efficiency of at least 95% per single plane for a rate capability up to 15 kHz/cm$^2$ , in presence of magnetic fields up to 0.3 T. The Istituto Nazionale di Fisica Nucleare (INFN) is deeply involved in the construction of these chambers and their validation at the Cosmic Ray Stand (CRS) in the Frascati INFN site (LNF). Preliminary results on the tests performed on the INFN SM1 prototypes are presented.