MICRO MEsh GAseous Structure (MM) chambers are Micro-Pattern Gaseous Detectors designed to provide a high spatial resolution and reasonable good time resolution in highly irradiated environments. This detector was chosen by the ATLAS collaboration as tracking detectors for the upgrade of the first station of the forward Muon Spectrometer. To meet the demanding performance requirements of the ATLAS Muon Spectrometer, MM chambers are required to achieve a single plane resolution better than $100~\mu m$ with an efficiency better than $95\%$ for tracks up to an inclination of $32~\deg$ in a magnetic field up to $0.3~T$.
A thorough test program on MM full size prototypes is just bring to an end and the assembly of 128 MM modules to be produced and certified for the New Small Wheel (NSW) of ATLAS experiment is started. The tests performed on the first module-0 produced excellent results, proving that the prototype fully meet the performance requirements.
The replacement in LS3 of the Inner Detector by the Inner Tracker (ITk) extends the pseudo-rapidity coverage for tracking up to $|\eta | < 4.0$. As a consequence, it becomes interesting to identify muons in this pseudo-rapidity region. This can be accomplished installing micro-pattern gaseous or silicon pixel detectors between the endcap calorimeters and the shielding disks in front of the NSW in the region $2.7 < \eta < 4$. These detectors could be used to identify (tag) ITk tracks as muons, relying entirely on the ITk for the momentum measurement. There are currently different proposals for the instrumentation of this region.
The ATLAS NSW MM assembly methodologies, the related quality controls and the candidate technologies for the $\eta-$tagger are presented in this document.