Enhancing the ATLAS Level-1 endcap muon trigger with New Small Wheel integration in Run 3

The Level-1 muon endcap trigger in the ATLAS experiment utilises signals from the Thin Gap Chambers (TGCs). A long-standing challenge for this system has been the high background rate from particles not originating at the proton–proton interaction point. These background particles mimic muon signatures and lead to an increased Level-1 trigger rate. To address this issue, the New Small Wheel (NSW) detectors, installed before the start of LHC Run 3, were integrated into the Level-1 endcap muon trigger system. The NSW detectors provide real-time tracking with fine spatial resolution, confirming that the candidates identified by the TGCs are consistent with originating from interaction point. By requiring consistency between signals from the TGCs and track segments reconstructed in the NSW, the trigger system can efficiently reject fake muon candidates from background particles. In 2024, the full integration of the NSW detectors into the Level-1 trigger decision was successfully completed. This advancement reduced the Level-1 muon trigger rate by approximately 14 kHz, corresponding to about 15% of the total Level-1 trigger rate, while introducing only a small additional loss in efficiency. As a result, the load on the ATLAS data acquisition (DAQ) system was alleviated, and the overall data-taking efficiency of the experiment was substantially improved. This upgrade represents an essential step toward maintaining efficient operations in the challenging environment of the High-Luminosity LHC.