New Detectors to Explore the Lifetime Frontier

TL;DR

The work argues that ultra-long-lived neutral particles are a natural signature in many BSM theories and remain largely inaccessible to main detectors due to their enormous lifetimes. It proposes the MATHUSLA surface detector for the HL-LHC to observe LLP decays with minimal background, leveraging timing and vetoes to reject cosmic rays and atmospheric neutrinos, and estimates sensitivity to exotic Higgs decays into LLPs. It extends the concept to a future 100 TeV collider with a compact underground forward detector, which could probe sub-percent exotic Higgs branching fractions and reach lifetimes near the BBN bound. Collectively, the study demonstrates that dedicated LLP detectors can comprehensively explore the finite parameter space for ULLPs, complementing MET-based searches and potentially revealing new hidden-sector dynamics.

Abstract

Long-lived particles (LLPs) are a common feature in many beyond the Standard Model theories, including supersymmetry, and are generically produced in exotic Higgs decays. Unfortunately, no existing or proposed search strategy will be able to observe the decay of non-hadronic electrically neutral LLPs with masses above $\sim$ GeV and lifetimes near the limit set by Big Bang Nucleosynthesis (BBN), $c τ\lesssim 10^7 - 10^8$~m. We propose the MATHUSLA surface detector concept (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 (ULLPs), whether produced in exotic Higgs decays or more general production modes. We also advocate for a dedicated LLP detector at a future 100 TeV collider, where a modestly sized underground design can discover ULLPs with lifetimes at the BBN limit produced in sub-percent level exotic Higgs decays.

Figures (4)

• Figure 1: Possible geometric configurations for the MATHUSLA surface detector at the HL-LHC. Gray shading indicates areas assumed to be sensitive to LLP decays. The surface detector is a 200m square building, centered along the beam line.
• Figure 2: Schematic of a possible design for MATHUSLA, with a robust multi-layer tracker at the top and a segmented scintillator veto surrounding the entire detector. Also shown are two possible displaced vertex signals from LLP decays (top) and the five most important backgrounds (bottom). Black arrows indicate charged particles and their direction of travel.
• Figure 3: HL-HLC sensitivity to LLP production in exotic Higgs decays. Solid lines: Required $\mathrm{Br}(h \to XX)$ required to see 4 events in MATHUSLA. Dotted lines: projected ATLAS $\mathrm{Br}(h \to X X)$ exclusions Coccaro:2016lnz. Purple shading: projected CMS $\mathrm{Br}(h \to \mathrm{invis})$ exclusion CMS:2013xfa, which applies roughly beyond the blue shaded region.
• Figure 4: Projected 100 TeV reach for $\mathrm{Br}(h \to X X)$, the exotic Higgs branching fraction to LLPs. Purple shading represents the $\mathrm{Br}(h \to \mathrm{invis})$ bound projected for a TLEP-like future lepton collider Gomez-Ceballos:2013zzn, which applies roughly beyond the blue shaded region.