Long-lived particles (LLPs) evade many standard searches because their decays occur millimeters to kilometers from the collision point. Experiments improve sensitivity by resolving displaced vertices, rejecting pileup, and triggering on unusual topologies. The most impactful upgrades combine precision timing, extended and higher-resolution tracking, and flexible trigger/DAQ systems that allow offline-quality reconstruction at trigger level. The CMS Collaboration at CERN and the ATLAS Collaboration at CERN have documented how these components change reachable lifetimes and masses, and the LHCb Collaboration at CERN emphasizes gains from a rebuilt vertex detector for forward acceptance.
Precision timing
Adding a dedicated timing layer with per-track resolution of order tens of picoseconds dramatically reduces the combinatorial background from pileup and separates near-simultaneous collisions. The CMS Collaboration at CERN describes the MIP Timing Detector delivering roughly 30 picoseconds per track to assign tracks to the correct interaction and to time displaced decays. This reduces fake displaced vertices and enables identification of slow-moving charged LLPs by their delayed arrival. Timing is most powerful when combined with improved spatial reconstruction: it converts ambiguous spatial information into four-dimensional vertices.
Tracking, acceptance, and triggers
Upgrades to the inner tracker that increase granularity, reduce material, and extend geometric acceptance improve vertex resolution and the efficiency for reconstructing low-momentum or highly displaced tracks. The ATLAS Collaboration at CERN and the LHCb Collaboration at CERN have shown that extended coverage in pseudorapidity and lower thresholds for displaced-track reconstruction recover signals that the pre-upgrade detectors missed. Equally important are trigger and data-acquisition upgrades that allow displaced-vertex or trackless-jet signatures to be retained. Online algorithms that use precision timing and updated tracking substantially increase the fraction of LLP decays saved for analysis.
Together, these upgrades shift the dominant limitations from detector-induced inefficiency to fundamental production rates and backgrounds. The consequence is tangible: expanded discovery potential for dark-sector mediators, heavy neutral leptons, and other LLP scenarios that produce displaced leptons, jets, or missing-energy signatures. Beyond physics reach, these projects reflect international coordination, long-term workforce training, and material-resource considerations; choices about detector complexity and energy consumption influence regional lab priorities and collaboration structures. Verifiable technical studies and collaboration technical design reports from the CMS Collaboration at CERN, the ATLAS Collaboration at CERN, and the LHCb Collaboration at CERN document the quantitative improvements expected from these upgrades.