Output ROOT file

Author: DraTeots

analysis/edm4hep_mcpart_lambda/README.md

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+# edm4hep_mcpart_lambda
+
+MC truth Lambda decay analysis directly from EDM4HEP/EDM4EIC files.
+Produces a ROOT file with histograms and a flat `TTree`, plus PNG images of every histogram — no CSV step required.
+
+This is the ROOT-native counterpart of [`csv_convert/csv_mcpart_lambda.cxx`](../../csv_convert/csv_mcpart_lambda.cxx) + [`analysis/csv_mcpart_lambda/lambda_decay.py`](../csv_mcpart_lambda/lambda_decay.py).
+
+## Output
+
+All output goes into a single directory:
+
+```
+<output_dir>/
+  mcpart_lambda.root   # TTree (lambda_decays) + all histograms
+  h_lam_p.png
+  h_lam_pt.png
+  h_lam_decay_rz.png
+  ...                  # ~30 PNGs total
+```
+
+### TTree columns
+
+Same schema as `*.mcpart_lambda.csv`. One row per Lambda found in MCParticles.
+Columns: `event`, `lam_is_first`, `lam_decay`, then 15 fields per particle with prefixes
+`lam`, `prot`, `pimin`, `neut`, `pizero`, `gamone`, `gamtwo`
+(fields: `_id _pdg _gen _sim _px _py _pz _vx _vy _vz _epx _epy _epz _time _nd`).
+
+`lam_decay` values: `0` = no decay, `1` = p+π⁻, `2` = n+π⁰, `3` = shower/recharge, `4` = other.
+
+### Histograms
+
+| Group | Histograms |
+|---|---|
+| Λ kinematics | \|p\|, pT, pz, η, φ, mass |
+| Λ decay | decay type, N daughters |
+| Λ decay vertex | z, r, r-vs-z (2D), x-vs-y (2D) |
+| Λ correlations (2D) | \|p\| vs η, pT vs η, pz vs pT |
+| Proton (p+π⁻) | \|p\|, pT, η |
+| π⁻ (p+π⁻) | \|p\|, pT, η |
+| Neutron (n+π⁰) | \|p\|, pT, η |
+| π⁰ (n+π⁰) | \|p\|, pT |
+| γ from π⁰ | energy, η |
+| p+π⁻ derived | opening angle, invariant mass (peaks at Λ mass) |
+| Λ generator status | status code distribution |
+
+## Usage
+
+### ROOT macro (no compilation needed)
+
+```bash
+# single file
+root -x -l -b -q 'mcpart_lambda.cxx("input.edm4hep.root","output_dir")'
+
+# multiple files, comma-separated, limit to 5000 events
+root -x -l -b -q 'mcpart_lambda.cxx("f1.root,f2.root","output_dir",5000)'
+```
+
+### Compiled
+
+```bash
+mkdir build && cd build
+cmake .. && make
+./mcpart_lambda -o output_dir input1.root input2.root
+./mcpart_lambda -n 5000 -o output_dir input1.root
+```
+
+### Dependencies
+
+`ROOT`, `podio`, `EDM4HEP`, `EDM4EIC`, `fmt` — all available in `eicweb/eic_xl:nightly`.