Performance studies of resistive-strip bulk micromegas detectors in view of the ATLAS New Small Wheel upgrade

The ATLAS collaboration at the Large Hadron Collider at CERN has endorsed the resistive-strip micromegas technology for the high luminosity upgrade of the first muon station in the high-rapidity region, the so called “New Small Wheel” project. It requires detectors with a spatial resolution of $\sim$100$\mu$m , fully efficient up to a particle rate of $\sim$20kHz/cm$^2$ . In order to demonstrate that the resistive-strip micromegas technology fulfils these requirements, small resistive bulk micromegas have been studied with radioactive sources and with high energy beams. The micromegas chambers were operated with an Ar+7%CO$_2$ gas mixture and read out using the APV25 chip. Results on the detection efficiency and the position resolution are presented for track impact angles from 0° to 40°. A position reconstruction method has been developed for inclined tracks, called the “micro-TPC method”. A description of the method along with performance studies is presented. In addition, the impact of the unavoidable presence of pillars and the relative alignment of readout and resistive strips on the micromegas performance has been quantified. In view of the fact that the micromegas detectors will also contribute to the trigger in ATLAS their time response has been studied