Searches for Long-Lived Particles with MATHUSLA

Abstract: Long-Lived Particles (LLPs) beyond the Standard Model emerge in numerous theoretical frameworks that address key open questions in physics, including the hierarchy problem, dark matter, neutrino masses, and the baryon asymmetry of the universe. The Large Hadron Collider (LHC) may be producing a significant number of neutral LLPs with masses above a GeV, yet these elusive particles could be escaping detection by the main experiments. To bridge this gap, we have proposed the MATHUSLA detector (MAssive Timing Hodoscope for Ultra-Stable neutraL pArticles), a dedicated surface-based experiment positioned above CMS, designed to operate during the High-Luminosity LHC era. MATHUSLA consists of multiple layers of plastic scintillators with wavelength-shifting fibers coupled to silicon photomultipliers, surrounding a large, air-filled decay volume. In this talk, we will present a 40-meter MATHUSLA design optimized for lower cost while maintaining world-leading sensitivity to LLPs. Additionally, we will discuss background studies of rare Standard Model processes and provide updates of measurements of the "demonstrator modules" at the University of Victoria and the University of Toronto.