AN Underground Belayed In-Shaft experiment to search for long-lived particles using LHC service shafts at CERN

The Standard Model of Elementary Particle Physics (SM) is the deepest, most complete and accurate scientific theory and has passed decades of experimental tests. Yet, it cannot answer many fundamental questions. These include: - the nature of Dark Matter, which contributes 80% of the total mass in the Universe, - the puzzle of neutrino particles from e.g. nuclear reactions that are predicted to be massless by the SM but are experimentally known to have a very small albeit non-zero mass, and - the riddle of the origin of the matter-antimatter asymmetry that would explain why the Universe -- as we know it -- is composed of matter rather than antimatter, - etc. Many proposed theoretical scenarios that address these fundamental questions predict new, electrically neutral particles with long lifetimes on the scale that can be probed by typical collider experiments like ATLAS or CMS at the Large Hadron Collider (LHC) of CERN. Yet, there is a striking gap in sensitivity for such long-lived particles that are electrically neutral and have a mass above 1 Gigaelectronvolt (GeV), i.e., are more massive than a hydrogen atom. This sensitivity gap spans several orders of magnitude in the lifetimes, which translates into decay lengths from 100 m and up to the Big-Bang nucleosynthesis bound of 100,000,000 m, quite reasonably assuming the particles move at the speed of light when produced at the LHC. This proposal aims to close the present gap in sensitivity to long-lived particles in a timely manner and at up to one order of magnitude lower costs than proposals with competing sensitivity, resulting in a transformative impact on the field. This can be achieved by instrumenting the existing service shafts of the ATLAS experiment at the Large Hadron Collider (LHC) of CERN. The foreseen detector structure, AN Underground Belayed In-Shaft search experiment (ANUBIS), will consist of a number of tracking stations belayed into the shaft and affixed to its walls, instrumenting approximately 15,000 m^3 with dedicated tracking detectors with excellent timing capability. For scenarios with electrically neutral long-lived particles with masses above 1 GeV, the lifetime reach is increased by a factor of up to 1,000 compared to currently operating and approved future experiments. To master this challenge cost-effectively, the tracking stations of ANUBIS will employ the next generation of detectors using the resistive plate chamber (RPC) technology that I will develop in the proposed FLF project using the low-cost, large-scale specifications of ANUBIS. In addition to the ambitious research programme above, another important objective is to expand the full potential of the existing ATLAS detector to search for long-lived particles on a short time scale. The detector technology that I will develop in the proposed FLF project using the low-cost, large-scale specifications of ANUBIS is an ideal candidate for the scanning of buildings, bridges, and other large-scale infrastructure using muon tomography based on naturally occurring cosmic rays. Cosmic ray tomography provides crucial advantages over currently available approaches that suffer from a limited depth reach and resolution (radar), are expensive and carry significant health and safety risks (X-ray), or even prohibitive (destructive methods). Preliminary research reveals a wide range of applications, given the large number of ageing buildings and infrastructure constructed in the 1950-70s with questionable structural integrity. The environmental human footprint can be dramatically reduced by using existing structures for longer in a safe manner, which, combined with substantial cost savings, will enormously benefit our society. One of the main goals of the FLF proposal is to realise a full-scale demonstrator prototype for cosmic ray tomography including field tests, with an intermediate-term goal of commercialisation.