The BIS78 Resistive Plate Chambers upgrade of the ATLAS Muon Spectrometer for the LHC Run-3

Abstract

Resistive Plate Chambers (RPCs) are used in the ATLAS experiment for the muon trigger and two coordinate measurements in the central region. In preparation for the coming years of LHC running at higher luminosity, besides the New Small Wheel project which is expected to complement the ATLAS muon spectrometer in the end-cap regions, a smaller size project, known as BIS78 (from Barrel Inner Small sectors), is being developed with a foreseen installation during the LHC Long Shutdown 2 (2019–2020). The BIS78 project will reinforce the fake muon rejection and the selectivity of the muon trigger in the transition region between the ATLAS barrel and end-caps by adding 32 RPC triplets on the edges of the Inner Barrel even sectors (BIS7 and BIS8) as this region is characterized by high rate due to secondary charged tracks generated by beam halo protons and a lack of detector instrumentation. Each triplet will be a station composed by three independent RPC detectors. Due to the narrow available space, the project foresees to replace the existing Monitored Drift Tubes (MDTs), used for the precise position measurement in this area, with muon stations formed by integrated smaller diameter tubes (sMDT) and a new generation of RPCs, capable of withstanding the higher rates and provide a robust standalone muon confirmation. These new RPCs are based on novel design of the gas volume with a thinner gas gap, thinner resistive electrodes, a lower operating voltage and new high gain front-end electronics with respect to the legacy ATLAS RPCs. Besides the use in Run-3 and onwards, this project is also of particular relevance as a pilot test for the High Luminosity LHC upgrade when an additional full layer of new RPC triplets is expected to complement the full barrel region in the innermost plane. The core of the project is presented, together with a description of the production and the test results. Details on the detector infrastructure and services along with a roadmap towards the final installation and commissioning during the Long Shutdown 2 (2019–2020) are also discussed.