gputtley / lhe_ntuples

Convert LHE event information into a ROOT NTuple for easy access and data plotting

LHE_NTuples

Convert LHE event information into a ROOT NTuple for easy access and data plotting

Les Houches Event Files (LHEF) are a common output format from Monte Carlo event generators. A full simulation will take the LHEF and apply a parton shower, followed by hadronization and finally a simulation of the resulting particles interacting with a detector in order to generate the kinds of signals we expect to see at the LHC. When we are testing a new physics model however, it is useful to look at the kinds of partons and their properties that get generated in the final state of our hard scattering.

This software will parse the LHE file, extract the relevant particle information, and fill a ROOT style NTuple with the meta and kinematic information.

Event information includes the following values (as detailed in the original paper)

• Number of particles
• Event weight
• Event scale
• EM coupling α_em
• Strong coupling α_s
• Particle identity from the Particle Data Group (pdg.lbl.gov/2007/reviews/montecarlopp.pdf)
• Particle status
• Mother particles
• Color(s)
• 4-Momentum and mass with the four-vector given as [Px,Py,Pz,E]
• Proper lifetime
• Spin

Note that not all of these values will contain information from the event generator.

Output File Format

The output file will be a standard .root file, containing one tree named mytree. Each of the above data are stored as individual branches inside the tree. Branch names and data types are:

Branch Name	Data Type
numParticles	int
eventWeight	float
eventScale	float
alphaEM	float
alphaS	float
pdgID	std::vector<int>
pdgStatus	std::vector<int>
mother1	std::vector<int>
mother2	std::vector<int>
color1	std::vector<int>
color2	std::vector<int>
px	std::vector<float>
py	std::vector<float>
pz	std::vector<float>
E	std::vector<float>
Mass	std::vector<float>
Tau	std::vector<float>
Spin	std::vector<float>

Any single vector index value corresponds to one particle in the event, so that you may iterate over all data vectors at the same time in order to collect relevant particle data. That is, for any index n, the values pdgID[n] and px[n] will give information for the same particle.

CAUTION! Using the same output file location more than once will overwrite the existing file. Consider this your only warning!

Requires ROOT library!

Example Use

Command line execution:

python ~/Path/to/LHEConverter.py -i ~/Path/to/generated/events.lhe -o output_file_name.root

if no output file name is given, the default will be <events>.root, where <events> is the name of the input file.