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| LHCb Vertex Locator | |
|---|---|
| Name | Vertex Locator |
| Caption | Schematic of the silicon modules and RF foil in the VELO system |
| Facility | Large Hadron Collider |
| Experiment | LHCb |
| Location | CERN |
| Type | Pixel and strip silicon detector |
| Status | Active (upgraded) |
| First beam | 2009 |
| Primary physics | Flavour physics, CP violation, rare decays |
LHCb Vertex Locator
The LHCb Vertex Locator is a high-precision silicon tracking subdetector located around the interaction point of the Large Hadron Collider experiment LHCb at CERN. Designed to reconstruct primary and secondary vertices for studies of CP violation and flavour physics, the system supports measurements of heavy-flavour hadrons produced in proton–proton collisions and contributes to searches for physics beyond the Standard Model. The detector operates close to the LHC beamline and integrates complex mechanics, cooling, readout electronics, and software developed by collaborations including institutions such as University of Oxford, Imperial College London, Ecole Polytechnique Fédérale de Lausanne, and Max Planck Society groups.
The Vertex Locator provides precise vertexing and tracking information within a few millimetres of the interaction point to resolve decay vertices of beauty quark and charm quark hadrons. The device was commissioned during early LHC runs following installation at Point 8 (LHC), and it has undergone a major upgrade aligned with the LHC Run 3 programme. It interfaces with the LHCb Trigger and Data Acquisition systems to deliver low-latency position measurements crucial for online selection of heavy-flavour decays and rare processes studied by the collaboration.
The system comprises semicircular silicon modules mounted on retractable halves that can be moved relative to the beam pipe during injection and operation. Key elements include high-voltage silicon sensors, front-end ASICs, low-mass support structures, precision mechanics, and an RF foil that shields the LHC beam. The upgraded design employs hybrid pixel sensors based on the VeloPix ASIC derived from developments for the Timepix3 and Medipix families, whereas the original detector used microstrip sensors and Beetle ASIC readout. Cooling is provided via CO2 evaporative systems similar to those used in detectors like ATLAS IBL and CMS subsystems, and the mechanical alignment relies on precision stages manufactured by industrial partners and academic workshops associated with University of Manchester and CERN engineering groups.
During operation, the halves are retracted for beam injection and closed to a nominal distance of a few millimetres from the beam during stable conditions, enabling transverse impact-parameter resolutions better than 25 micrometres for high-momentum tracks. The detector achieves rapid time stamping compatible with the LHCb upgrade 40 MHz readout architecture, feeding tracking information to the High Level Trigger and offline reconstruction chains developed by software groups at CERN IT and university partners. Performance metrics such as hit efficiency, cluster size, spatial resolution, and material budget are monitored continuously using track-based algorithms shared with other detector teams like VELO R&D collaborations and the LHCb tracking consortium.
Alignment combines hardware surveys, laser metrology, and track-based algorithms to determine module positions with micron precision relative to the LHC beam and the global LHCb coordinate system. Calibration tasks include time-walk corrections, threshold tuning, and charge calibration performed in runs coordinated with CERN machine cycles and detector control systems developed with RAL and IN2P3 groups. Dedicated alignment campaigns occur after major interventions or thermal cycles, and software tools incorporate techniques from Millepede and other alignment frameworks used by ATLAS and CMS to propagate uncertainties into physics analyses.
Located close to the beam, the detector endures high fluences of ionising particles and non-ionising energy loss from collision debris, challenging silicon sensor longevity and front-end electronics. Radiation effects observed include increased leakage current, depletion voltage shifts, charge collection degradation, and damage to bump-bond interfaces, comparable to experiences in CERN radiation test facilities and irradiation campaigns conducted at institutes such as Sandia National Laboratories and Paul Scherrer Institute. Mitigation strategies implemented include operation at low temperatures using CO2 cooling, sensor bias tuning, annealing protocols, use of radiation-hard ASIC design practices originating from RD53 efforts, and the adoption of thinner sensors and 3D architectures in R&D studies led by University of Glasgow and INFN teams.
The upgrade deployed for LHC Run 3 replaced strip sensors with pixelated modules read out at 40 MHz, incorporating the VeloPix ASIC and novel low-mass supports to reduce material while improving rate capability. Future development paths studied by the collaboration include monolithic active pixel sensors (MAPS), 3D-integrated detectors, enhanced cooling techniques, and machine-learning-based real-time reconstruction algorithms pursued in partnerships with CERN OpenLab and computing groups at EPFL and University of Cambridge. Planning is coordinated with upgrade timelines of the High-Luminosity Large Hadron Collider and broader LHC programme to maintain vertexing performance at higher luminosity and pile-up conditions.
The Vertex Locator has been central to LHCb discoveries and precision measurements, enabling time-dependent CP violation studies in B meson systems, measurements of mixing parameters, and reconstruction of rare decay topologies such as B0 -> mu+ mu- searches and studies of exotic hadron candidates like tetraquark and pentaquark states reported by the collaboration. Its vertex resolution and impact-parameter determination underpin lifetime measurements, flavor-tagging calibration, and searches for displaced signatures from long-lived particles relevant to beyond-Standard Model scenarios explored in joint analyses with ATLAS and CMS. The detector’s performance continues to enable LHCb’s physics programme and informs detector R&D across the high-energy-physics community.
Category:LHCb detectors