Performance of ATLAS RPC detectors and L1 Muon Barrel Trigger with a new CO2-based gas mixture

Resistive Plate Chambers are used in the ATLAS experiment for triggering muons in the barrel region. These detectors use a Freon-based gas mixture containing C2 H2 F4 and SF6 , high global warming potential greenhouse gases. To reduce the greenhouse gas emissions and cost, it is crucial to search for new environmentally friendly gas mixtures. In August 2023, at the end of the proton-proton data-taking campaign, ATLAS collaboration decided to replace the standard gas mixture (94.7% C2 H2 F4 , 5.0% i-C4 H10 , 0.3% SF6 ) with a new CO2 -based gas mixture: 64% C2 H2 F4 , 30% CO2 , 5.0% i-C4 H10 , 1% SF6 . The performance of the RPC detectors with the new gas mixture is presented with a particular emphasis on detector efficiency, cluster size and timing performance, as well as the efficiency of the L1 Muon Barrel trigger system.