Event Reconstruction Physicist (CMS/FCC)

About the role

CERN is looking for an Event Reconstruction Physicist to turn a prototype event-level graph into a detector-independent, experiment-independent library used by both CMS and the Future Circular Collider (FCC). The work covers graph algorithms and modern machine learning for clustering, particle flow and particle identification, and reliable matching between reconstructed objects and simulation truth. The role sits in the EP-FCC group, works 40 hours per week on a hybrid basis, starts on 1 January 2027 and lasts 24 months, extendable to 36 months.

Key responsibilities

• Generalise the CMS MC-truth graph into a detector- and experiment-independent library, separating the graph data model, navigation API and hit index from experiment bindings.
• Keep the CMSSW binding working and add a binding for the FCC stack (Key4hep, EDM4hep, Gaudi).
• Extend the data model with multi-surface boundary crossings, sparse association storage and a reduced hit-index layout, handling pileup consistently.
• Develop the reconstruction-to-truth association, extending the adaptive branch associator and validating it on merged topologies such as converted photons, bremsstrahlung and early hadronic interactions.
• Define what is reconstructible from the truth graph and detector description, using cell granularity, energy and time resolution, readout thresholds and two-object separation.
• Apply portable graph algorithms and machine learning (e.g. graph neural networks and transformers) to clustering, linking, particle flow and particle identification in CMS and FCC detectors.
• Build training datasets and labels, define metrics, calibrate model outputs and check models before they feed reconstruction.
• Contribute code to the official CMS and FCC repositories with unit tests, documentation and code review, and present results in meetings, forums and publications.
• The role includes team supervision responsibilities.

Required profile

• Master's degree or PhD in Physics, Computer Science, Applied Mathematics or a related field.
• Demonstrated experience with modern C++ in a large, shared, peer-reviewed codebase (CMSSW, Gaudi or Key4hep).
• Demonstrated experience with time-aware event reconstruction and detector simulation in a high-energy physics experiment.
• National of a CERN Member or Associate Member State, with a professional background in Physics (or related), and no previous CERN fellow or graduate contract.
• Spoken and written English, with a commitment to learn French.

Required skills

• C++20, clustering and event reconstruction, detector simulation, particle flow, CMSSW, Key4hep, heterogeneous computing.
• Python and at least one machine-learning framework, preferably PyTorch.
• Graph and clustering algorithms, git, code review and continuous integration.

What we offer

• A monthly net stipend of 6,372 or 7,004 Swiss Francs depending on degree.
• 30 days of paid leave plus 2 weeks annual closure.
• Health insurance coverage for yourself, spouse and children, and membership of the CERN Pension Fund.
• Family, child and infant allowances and a relocation package depending on circumstances.
• Possibility to extend the contract to 36 months, and on-the-job and formal training including language classes.