Searches for long-lived particles with ANUBIS: first commissioning results from proANUBIS

Many extensions of the Standard Model with Dark Matter candidates predict new long-lived particles (LLPs). The LHC provides an unprecedented possibility to search for such LLP produced at the electroweak scale and above. The ANUBIS concept foresees instrumenting the ceiling and service shafts above the ATLAS experiment with tracking stations in order to search for LLPs with decay lengths of O(10m) and above. A prototype detector, proANUBIS, has been constructed within the ATLAS experimental cavern to demonstrate the ANUBIS concept and verify the expected performance. The proANUBIS design and initial performance during 2024 LHC collisions are presented, and the planned analyses using prototype data are discussed.