Many extensions of the Standard Model (SM) include particles that are neutral, weakly coupled, and long-lived that can decay to final states containing several hadronic jets. Long-lived particles (LLPs) can be detected as displaced decays from the interaction point, or missing energy if they escape. ATLAS and CMS have performed searches at the LHC and significant exclusion limits have been set in recent years.
However, the current searches performed at colliders have limitations. An LLP does not interact with the detector and it is only visible once it decays. Unfortunately, no existing or proposed search strategy will be able to observe the decay of non-hadronic electrically neutral LLPs with masses above $\sim~\textrm{GeV}$ and lifetimes near the limit set by Big Bang Nucleosynthesis ($c\tau \sim 10^7-10^8$ m). Therefore, ultra-long-lived particles (ULLPs) produced at the LHC will escape the main detector with extremely high probability.
We describe the concept of the MATHUSLA surface detector (MAssive Timing Hodoscope for Ultra Stable neutraL pArticles), which can be implemented with existing technology and in time for the high luminosity LHC upgrade to find such ultra-long-lived particles, whether produced in exotic Higgs decays or more general production modes. The MATHUSLA detector will consist of resistive plate chambers (RPC) and scintillators with a total sensitive area of $200\times200$ m square. It will be installed on the surface, close to the ATLAS or CMS detectors.
A small-scale test detector ($\sim 6 \, \textrm{m}^2$) has been installed on the surface above ATLAS in autumn 2017. It consists of three layers of RPCs used for timing/tracking and two layers of scintillators for timing measurements. It has been placed above the ATLAS interaction point to estimate cosmic backgrounds and proton-proton backgrounds coming from ATLAS during nominal LHC operations.