Description
Long-lived particles (LLPs) decaying outside the inner tracker volume leave little or no information in the silicon and drift chamber subsystems, making the muon system the only detector capable of reconstructing such tracks at FCC-ee. We present a comprehensive study of the IDEA muon system, designed as a standalone tracker, with a focus on its capabilities for highly displaced signatures.
A scan of different detector geometries — varying the number of µRWELL layers (3–6), inter-layer iron yoke thickness (25, 30, 50 cm), and asymmetric “paired” arrangements — is performed in full simulation using the Key4hep framework. Performance is evaluated using a dedicated reconstruction algorithm, StandaloneMuonTracking, which addresses the unique challenges of sparse hits, multiple Coulomb scattering through dense iron, and the absence of a vertex constraint. Spatial-resolution and magnetic-field scans complement the geometry comparison.
The optimal layouts deliver acceptable momentum resolution, high standalone efficiency (>99%), and a low fake rate. For a benchmark Heavy Neutral Lepton scenario, the muon system maintains track-finding efficiency well into the multi-meter displacement regime — entirely outside the inner-tracker acceptance — while reconstructing the displaced impact parameter with controlled bias. These results guide the IDEA design choices and quantify, for the first time, the FCC-ee discovery reach for muonic LLP final states obtained from the muon system alone.
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