Flesh out Gen/ee: WHIZARD mumuH card + Pythia8 (k4Gen) shower step - #2
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kjvbrt wants to merge 6 commits into
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Flesh out Gen/ee: WHIZARD mumuH card + Pythia8 (k4Gen) shower step#2kjvbrt wants to merge 6 commits into
kjvbrt wants to merge 6 commits into
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Adapts the FCC-config golden card (winter2023 branch) for e+e- -> mu mu H, stripping its internal legacy-PYTHIA6 shower/decay to keep the two-step WHIZARD (LHE) -> Pythia8 (k4Gen Gaudi, H -> b b) chain explicit for students, and adds the runnable mumuH.sin, mumuH_Hbb.cmd, and pythia_mumuH.py files alongside the README.
mumuH_Hbb.cmd now bases its Beams:/PartonLevel settings on FCC-config's actual LHE-reading card (p8_ee_default.cmd) instead of an unrelated production card, turning off Pythia8's own ISR/FSR since WHIZARD already applies it. Vertex/time smearing moves to the Gaudi GaussSmearVertex tool in pythia_mumuH.py to avoid double-smearing against Pythia8's own Beams:allowVertexSpread. Also documents the golden card's PYTHIA6 settings (Higgs mass/width, fragmentation tune, Bose-Einstein correlations, long-lived particle stability) that still need porting, as open TODOs.
Ran WHIZARD + Pythia8 (k4Gen) against the real Key4hep stack: WHIZARD step works and gives a cross section matching the known sigma(ee->ZH)*BR(Z->mumu) value, but the Pythia8 step crashes inside Pythia8's own resonanceShowers/iBotCopyId machinery when decaying and showering the bare Higgs resonance from the LHE file. Documents the crash, backtrace, and ruled-out fixes as a blocking Open TODO. Also recommends VSCodium over VSCode in the top-level README, and adds Open TODOs for background-sample generation, a jupytext-based Jupyter export, and a custom TextMate grammar for WHIZARD/Pythia8 syntax highlighting.
The Step 2 (Pythia8) crash was caused by WHIZARD's ?keep_beams = true, which FCC-config's own (unheeded) maintainer comment on a sibling card already warns about. Fixed by no longer setting ?keep_beams at all in mumuH.sin, relying on its default (false). Verified with a standalone, zero-Gaudi Pythia8 reproducer and a full 1000-event k4run pipeline run. Also corrects PartonLevel:FSR in mumuH_Hbb.cmd: it must stay on (Pythia8 default) since it's what showers the H -> b b decay products before hadronization, not just a source of double-counted radiation as previously assumed - verified by dumping the Pythia8 event record with it on and off. PartonLevel:ISR stays off, confirmed correct the same way. Also removes explicit defaults (?keep_beams, ?keep_remnants) from mumuH.sin and documents mH, ISR/FSR, and the ISR-photon-visibility caveat in Gen/ee/README.md.
A small fraction of events (~1 in a few thousand) hit an unrecoverable Pythia8-level failure that retrying doesn't fix. Without raising Gaudi's default per-algorithm ErrorMax = 1, a single such event aborts the entire run - only shows up at scale (a 1000-event test never hits it), confirmed by generating and processing 10,000 events. ErrorMax = 20 lets a handful be skipped instead, verified with a clean 10,000/10,000-event run.
kjvbrt
marked this pull request as ready for review
August 21, 2026 11:45
For each PYTHIA6 setting the golden card had enabled, checked whether Pythia8's own default already covers it rather than porting the tuned numeric values (risky - the two codes don't always define equivalent- sounding parameters identically). Most items needed no change: Bowler fragmentation for b/c quarks is already Pythia8's default, and Higgs mass/width already closely match. Bose-Einstein correlations were checked and deliberately left off - they don't affect the mu mu recoil mass and only marginally affect the H -> b b dijet mass, not worth the added complexity here. The long-lived-particle setting went through a wrong attempt (mapped to a proper-lifetime cutoff) before checking the actual PYTHIA 6.4 manual, which shows MSTJ(22)=4 is a geometric decay-vertex-position cutoff instead - Pythia8's default already matches its practical outcome, confirmed on real generated events (K_S0/Lambda decay behavior), so no setting is needed there either.
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Summary
wzp6_ee_mumuH_Hbb_ecm240.sin,winter2023) for e+e- -> mu mu H, keeping its physics but splitting out an explicit WHIZARD (LHE) -> Pythia8 (k4Gen Gaudi, H -> b b) two-step chain for students, instead of the golden card's internal legacy-PYTHIA6 shower.mumuH.sin,mumuH_Hbb.cmd,pythia_mumuH.pyalongsideGen/ee/README.md.Test plan
whizard mumuH.sinand confirmmumuH.lheis producedk4run pythia_mumuH.pyand confirmmumuH_Hbb.rootis produced with a saneMCParticlescollection