FCCee-TreeProducer

Description

This script converts an LHE file into a ROOT file with a structure inspired by NanoAOD, storing only branches analogous to the LHEPart collection.

On top of that, it classifies each event of a Vector Boson Scattering (VBS) sample at an e+e- collider and stores the result in a kind_event branch. The classification works for purely leptonic as well as semileptonic/hadronic final states.

Requirements

• pylhe
• awkward
• uproot
• numpy

Usage

Run the script by providing an input LHE file and a name for the output ROOT file:

python3 lhe2root.py input.lhe output.root

While running, the script prints a check of the cross section (the sum of the normalised event weights must reproduce the cross section stored in the LHE file) and a summary of the number of events per kind_event code.

Output

The output file contains a TTree named Events with the following branches:

Branch	Content
LHEPart	eta, firstMotherIdx, lastMotherIdx, incomingpz, mass, pdgId, phi, pt, spin, status for every particle of the event
Weight	weight (event weight); if the LHE file contains reweighting weights, also weights
kind_event	event classification code (see below)

Event classification (kind_event)

The classification only uses the PDG ids and the status of the mothers of the final-state particles, never the type of the final-state particles themselves. It therefore works unchanged when the daughters of the vector bosons are leptons or quarks.

An event is classified as VBS if it has exactly 6 final-state particles (status == 1) such that:

• 2 of them are electrons or electron neutrinos whose mother is an incoming e+/e- (the tag particles);
• the other 4 are daughters of two vector bosons (W/Z) in intermediate state (status == 2), whose mother is in turn an incoming particle: 2 daughters per boson.

The events are then assigned to WW, WZ or ZZ according to the bosons. Events with a Higgs in intermediate state, with the tags and a Higgs decaying via ZZ* or WW*, are classified as ZZ+H or WW+H. All the other events are background.

The set of codes is chosen event by event from the final state:

• purely leptonic final state (no quarks or gluons): first column;
• semileptonic / hadronic final state (at least one quark or gluon): second column.

Process	Leptonic	Semileptonic / hadronic
WW	-1	-11
WZ	-2	-12
ZZ	-3	-13
ZZ with Higgs	-4	-14
WW with Higgs	-5	-15
Background	+1	+11
Background with Higgs	+2	+12

Negative codes are signal, positive codes are background. The code 0 flags a VBS-like event that matches none of the categories (it should not occur).

The codes are defined at the top of the script (CODES_LEP, CODES_HAD) and can be changed there.

Notes and limitations

• W, Z and H must appear as intermediate particles (status == 2) in the LHE file. This is the case when the decays are done with MadSpin or with the MadGraph decay-chain syntax (e.g. , (w+ > j j)). If the final state is generated directly, without intermediate resonances, the VBS events cannot be recognised and end up in the background category.
• The classification needs exactly 6 final-state particles in every event. If at least one event has a different number (for example an extra gluon), the script prints a warning, does not classify any event of the file, and writes the TTree without the kind_event branch.
• The tag particles are identified only as electrons or electron neutrinos coming from the incoming particles. The script does not check that the sample contains only fusion diagrams: this depends on how the process is defined in MadGraph.