Maximization of the acceptance and momentum resolution of the ATLAS muon spectrometer with novel drift-tube and resistive plate chambers

The ATLAS muon spectrometer provides high momentum resolution up to transverse muon momenta in the TeV range in almost the full pseudo-rapidity interval of −2.7 < η < 2.7. The acceptance of the muon spectrometer is currently limited by uninstrumented regions in the vicinity of η = 0 to provide space for cables and services of the inner detector and the calorimeters and in the feet region due to space limitations. A large fraction of these acceptance gaps will successively be closed by installing novel small monitored drift tube (sMDT) chambers with tubes of half the diameter of the present MDT chambers and novel resistive plate chambers (RPC) with smaller gas gaps and higher spatial resolution. The reduced size of the chambers without loss of measurement points does not only make it possible to equip the originally uninstrumented regions with tracking and trigger chambers, but also to operate the new chambers under the highly increased background radiation levels expected for the operation of the muon spectrometer at the High-Luminosity LHC. The design and the performance of these new chambers as well as the installation of the first two stations of sMDT and RPC chambers in the bottom sector of the ATLAS muon spectrometer are presented.