DisplacedTracking

A Gaudi/Key4hep algorithm for reconstructing displaced tracks in the IDEA muon system at FCC-ee. Designed for long-lived particle (LLP) searches where signal tracks originate from a displaced vertex and leave hits only in the muon system, bypassing the inner tracker entirely.

Physics motivation

Targets signatures such as HNL → μμ where the heavy neutral lepton decays at a displaced vertex (O(200–500 cm) from the IP). The decay products appear as muon-system-only tracks with large impact parameters (d0, z0) relative to the interaction point.

Algorithm overview

1. Triplet seeding — forms seeds from hits in the three innermost muon layers using a circle fit and cross-product charge determination
2. Hit extension — extends seeds to a 4th layer hit using a distance+angle compatibility check
3. 4-hit circle fit — weighted least-squares circle fit to all 4 hits for improved pT and charge
4. GenFit Kalman fit — full track fit with material effects using the GenFit library (DD4hep material interface)
5. Inner propagation — optional extrapolation of the fitted track back toward the solenoid

Track states stored per track:

• AtFirstHit — 3-hit analytical seed
• AtLastHit — 4-hit WLS circle fit
• AtOther — GenFit Kalman fit result
• AtVertex — inner propagation result (when enabled)

Dependencies

• Key4hep nightly stack (DD4hep, EDM4hep, Gaudi, k4FWCore, podio, ROOT, Eigen3)
• GenFit2 — built and installed separately

Build

source /cvmfs/sw-nightlies.hsf.org/key4hep/setup.sh

mkdir build && cd build
cmake .. -DCMAKE_INSTALL_PREFIX=../install
make install -j10

Note: The GenFit install path is currently hardcoded in CMakeLists.txt. Update GENFIT_INSTALL_DIR to match your local GenFit installation before building.

Running

source /cvmfs/sw-nightlies.hsf.org/key4hep/setup.sh
source build/displacedtrackingenv.sh

k4run options/runDisplacedTracking.py

Edit options/runDisplacedTracking.py to set the input file, geometry XML, and algorithm parameters before running.

Key configuration parameters

Parameter	Default	Description
DetectorName	"Muon-System"	Detector readout name
MaxChi2	10.0	Maximum chi2 for hit acceptance
MaxDist	160.0 cm	Maximum hit-to-hit distance in extension loop
MinCosAngle2d	0.7	Minimum cos(angle) between chord vectors for seeding
MaxSeedPT	200.0 GeV	Upper pT cut on triplet seeds
ParticleType	"muon"	Particle hypothesis for GenFit material effects
UseGenFit	true	Enable/disable GenFit Kalman fit
MaxFitIterations	100	GenFit Kalman filter max iterations
DoInnerPropagation	false	Enable extrapolation toward solenoid
InnerPropTargetRadius	100.0 cm	Target radius for inner propagation

Output collections

Collection	Type	Contents
DisplacedTracks	TrackCollection	Reconstructed tracks with all states
DisplacedTrackHits	TrackerHitPlaneCollection	Hits associated to tracks
DisplacedTrackHitSimLinks	TrackerHitSimTrackerHitLinkCollection	Reco–sim hit links

Author

Mahmoud Al-Thakeel