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    Key4hep Discussion

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    Key4hep Discussion
    Zoom Meeting ID
    95444210901
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    Andre Sailer
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    # Key4hep Live Notes
     
    Date: October 06, 2026
     
    Agenda: https://indico.cern.ch/event/1738194/
     
    Connected: Markus, Ianna, Joseph, Andre, Leonhard, Joshua, Andrea, Thomas, Benedikt, Andreas, Juraj, Sanghyun, David, Mahmoud, Juan, Wouter
    Apologies: Brieuc
     
    ## Github project
     
    - https://github.com/issues?q=is%3Aopen+is%3Aissue+user%3Akey4hep
     
    ## Discussions
    [Github Project Discussions](https://github.com/orgs/key4hep/projects/2/views/1?visibleFields=%5B%22Repository%22%2C%22Title%22%2C3970156%2C%22Status%22%2C%22Assignees%22%5D)
     
    ## Presentations
     
    https://github.com/orgs/key4hep/projects/4/views/1
     
    * Idea of having a Key4hep workshop first week of December (Mon 7th) at CERN
        * Second half of the week has analysis tools workshop
        * [ ] Blockers for anyone?
     
    ## Communities Round Table
    * Status of the release?
        * Need the release for production and testing of workflows etc.
    * Upstreaming of `PropertyChoices` to k4FWCore (see https://github.com/key4hep/k4RecTracker/pull/107)
        * Make it a Util header in k4FWCore
     
     
     
    ## LCContent and k4GaudiPandora
    - Introduce `LCCaloHit` customization point in `LCContent`: [LCContent#45](https://github.com/PandoraPFAOrg/LCContent/pull/45)
        - Corresponding factory update in `k4GaudiPandora`: [k4GaudiPandora#55](https://github.com/key4hep/k4GaudiPandora/pull/55)
        - **Validated with CLD that this doesn't change the behavior of Pandora**
        - [ ] Review & merge
    - Add some *status bits* to the `LCCaloHit` and reserve one of them for *isPossibleBIB*: [LCContent#46](https://github.com/PandoraPFAOrg/LCContent/pull/46)
        - Requested by MuonCollider
        - **Validated with CLD that this doesn't change the behavior of existing algorithms**
        - [ ] Review & merge
    - From MuonCollider: Algorithms that could profit from additional flags in `CaloHit` (see [LCContent#43](https://github.com/PandoraPFAOrg/LCContent/pull/43))
        - Review once the two above are merged
        
    ## EDM4hep & podio vs ROOT
    - ROOT migrated from `cppyy` to `cppjit` [root#21261](https://github.com/root-project/root/pull/21261)
        - Breaks a few things in `dev3` based CI workflows
        - Not yet entirely clear what needs fixing and what doesn't -> should all be transparent
        - Hope to get answers via https://root-forum.cern.ch/t/question-s-about-the-migration-to-cppjit/65010
        - Some temporary fixes in [podio#1031](https://github.com/AIDASoft/podio/pull/1031), [EDM4hep#532](https://github.com/key4hep/EDM4hep/pull/532)
        - [ ] Clarify what should be merged
        
    ## EDM4hep
     
    * Add a `timeError` to all tracker hit types (and interface): [EDM4hep#528](https://github.com/key4hep/EDM4hep/pull/528)
        * Triggered by 4D tracking work in [k4ActsTracking#72](https://github.com/key4hep/k4ActsTracking/pull/72)
        * Will come with a `schema_version` bump for EDM4hep
        * [ ] Schema evolution PoV: Existing 1.0 files remain readable? What is the value for old files?
    * Revival of discussion about how to have "raw hits" attached to a `TrackerHit`: [EDM4hep#382](https://github.com/key4hep/EDM4hep/issues/382)
        - Context now: `k4GaudiPandora` does track extrapolation and for ILD *SET* hits are composite space points that would like to point to two 1D measurements
        - A bit further into the future: Need to clarify what a *TrackerHit* actually represents (also in light of slightly different ACTS nomenclature)
    * Handling / storage of *track only vertices* [EDM4hep#526](https://github.com/key4hep/EDM4hep/issues/526)
        * Triggered by vertex finder work in [k4ActsTracking#109](https://github.com/key4hep/k4ActsTracking/pull/109)
        * Should not block the release in any case
    * Feedback from DELPHI open data conversion to EDM4hep [EDM4hep#530](https://github.com/key4hep/EDM4hep/issues/530)
        * Expect a similar report from ALEPH and potentially OPAL
        * Should have a look to see what we can (and want to) accommodate in EDM4hep
     
    ## k4MarlinWrapper
     
    ## Gaudi
     
    ## k4CEDViewer
     
    ## k4Clue
     
    Recap of the past few months (on top of writing the PhD thesis):
    * few PRs merged
        * #79 to have different clustering strategies and different coordinates --> improve generality
        * #80, #81 and #82 by Juan to fix bugs and clean the repo
        * #83 by Sangh Yun to fix the position computation
    * comparison with Pandora in terms of physics and computing performance
        * the full Pandora chain is better but slower, only the topological clustering is worse (and slower) than k4Clue
    * while testing the computing performance I ran into a crash due to the Conformal Tracking algo finding too many tracks
        * sample with 50 photons (tracks probably from pair production)
        * I suspect the reason is that the algo branched on every parent cell (instead of only the longest-chain one), with the combinatorial exploding.
        * Tracks in the event that crashes (after putting a break in the while loop when >15000 tracks are found):
          ```
       --- Event 17 ---
       n tracks       : 9
         track 0: chi2/ndf=0.78  nHits=22  nHoles=0
         track 1: chi2/ndf=7.84  nHits=20  nHoles=0
         track 2: chi2/ndf=1.19  nHits=20  nHoles=0
         track 3: chi2/ndf=12.48  nHits=7  nHoles=0
         track 4: chi2/ndf=31.87  nHits=3  nHoles=0
         track 5: chi2/ndf=56.96  nHits=3  nHoles=0
         track 6: chi2/ndf=49.08  nHits=3  nHoles=0
         track 7: chi2/ndf=47.25  nHits=3  nHoles=0
         track 8: chi2/ndf=62.44  nHits=3  nHoles=0
           ```
        * my temporary fix was to run with the truth tracking, I also [changed the logic](https://github.com/AuroraPerego/k4Reco/tree/fixConformal) by following only the longest branch, it runs without crashing (but the results have to be validated) 
     
     
    ## (Re-)organization of repository content
    * Started to move interfaces from k4FWCore to downstream where they are defined
        * See discusion from last time
        * https://github.com/key4hep/k4FWCore/pull/386 and https://github.com/key4hep/k4SimGeant4/pull/90
        * https://github.com/key4hep/k4FWCore/pull/387 and https://github.com/HEP-FCC/k4RecCalorimeter/pull/224
    * [ ] Moving of packages / algorithms
        * [x] Overlay -> k4FWCore
        * [ ] DDPlanarDigi -> k4RecTracker
        * [ ] GaudiLumiCalClustering -> k4RecCalorimeter
        * [ ] DDCaloDigi, DDSimpleMuonDigi, DDScintillatorPpdDigi -> k4RecCalorimeter
        * Three components for Tracking: ConformalTracking, GaudiTrkUtils, Other (RefitFinal, TruthTrackFinder, ClonesAndSplitTracksFinder)
            - ConformalTracking, RefitFinal, TruthTrackFinder depend on GaudiTrkUtils
            - GaudiTrkUtils depends on KalTest, DDKalTest (iLCSoft)
            - ClonesAndSplitTracksFinder does not, it could be moved
    * **Is someone actively working on this?**
        * Not at the moment
     
    ## Documentation of existing algorithms
    - [ ] Versioning
        - Key4hep releases?
     
    ### ACTS integration
     
    ### Podio
     
     
    ## GSoC
     
    ### Documentation
     
    - [ ] Add page with logos on documentation page
        - E.g. add an archive / tarball that is easy to download
        - LICENSE, allowed usage (also wrt altering logo), copyright, credits...
        - CERN Design Office doesn't care about attribution
        - [x] Confirm License and standard CERN rules
        - e.g. CERN logo usage guidelines: https://design-guidelines.web.cern.ch/guidelines/logo
     
    ## [k4Reco](https://github.com/key4hep/k4Reco)
    * Migration / port of LCFIPlus
        * https://github.com/key4hep/k4Reco/pull/63
        * Split this into different parts? (Vertexing, FlavorTagging, PID)
        * Try to figure out what is still necessary and if there are things that could be let go
     
     
    ## k4FWCore
            
    * Code generator for algorithm boilerplate and a key4hep tutorial:
        * https://github.com/key4hep/k4FWCore/pull/372
            * fully functional with compiled examples and documentation. Please review
        * https://github.com/key4hep/key4hep-tutorials/pull/33
     
    ## k4Geo
     
    ## CEPCSW
     
    ## k4SimDelphes
     
    ## ML-based Flavourtagger
     
    ## k4SimGeant4
     
    ## Tooling in Key4hep
     
    ## FCC-Analyses
     
    ## Analysers
     
    ## Build system / Target compiler / dependencies
     
    ## k4EDM4hep2LcioConv
     
    ## Distributed Computing, Workload Management, Data Management
     
    ## Validation system
     
    ## FCC Calo reconstruction
     
    ## AOB
     
    * https://github.com/AIDASoft/DD4hep/pull/1689
        * Bigger topic question: expectations on generator - simulation interface
        * See also k4GeneratorsConfig for another conversion
        * Do we need / want to always be compatible with HepMC3? E.g. for some MDI studies the HepMC3 format might be too tight (e.g. needs a beam particle at some point).
    * (Long term) storage of minutes (Archiv IV has become full today)
        * Preference for archive vs. attaching minutes to indico agenda? -> Can just do both
     
    ## Next meetings
     
    * Oct 13, 2026, 9:00 CEST
    There are minutes attached to this event. Show them.
      • 16:00 → 16:10
        Introduction 10m
      • 16:10 → 17:00
        Discussion 50m
        Speaker: Everyone