Les Houches Event Format

The Les Houches Event (LHE) format uses an XML-like structure, but the content within the <init>` and ``<event> blocks consists of whitespace-separated values designed for straightforward parsing in Fortran. Its first version was defined in [1]. The <header> block can contain arbitrary XML content, usually metadata or comments explaining how the events were generated. The following skeleton example illustrates the overall structure of an LHE file using the pylhe naming of the attributes:

<LesHouchesEvents version="1.0">
<header></header>
<init>
beamA beamB energyA energyB PDFgroupA PDFgroupB PDFsetA PDFsetB weightingStrategy numProcesses
xSection error unitWeight procId
...
# additional hash-commented information can go here
</init>
<event>
nparticles pid weight scale aqed aqcd
id status mother1 mother2 color1 color2 px py pz e m lifetime spin
...
# additional hash-commented information can go here
</event>
...
</LesHouchesEvents>

The table below summarizes the main parameters found in LHE files grouped by their dataclass representation in pylhe.

Parameter	Type	Description	Unit
pylhe.LHEInitInfo
beamA	int	PDG ID of first beam particle
beamB	int	PDG ID of second beam particle
energyA	float	Energy of first beam particle	GeV
energyB	float	Energy of second beam particle	GeV
PDFgroupA	int	PDF group ID for first beam
PDFgroupB	int	PDF group ID for second beam
PDFsetA	int	PDF set ID for first beam
PDFsetB	int	PDF set ID for second beam
weightingStrategy	int	Weighting strategy
numProcesses	int	Number of processes
pylhe.LHEProcInfo
xSection	float	Cross section	pb
error	float	Cross section uncertainty	pb
unitWeight	float	Maximum cross section	pb
procId	int	Process ID
pylhe.LHEEventInfo
nparticles	int	Number of particles in event
pid	int	Process ID for this event
weight	float	Event weight
scale	float	Factorization/renormalization scale	GeV
aqed	float	QED coupling constant
aqcd	float	QCD coupling constant
pylhe.LHEParticle
id	int	PDG particle ID
status	int	Particle status code
mother1	int	Index of first mother particle
mother2	int	Index of second mother particle
color1	int	First color line index
color2	int	Second color line index
px	float	x-component of momentum	GeV
py	float	y-component of momentum	GeV
pz	float	z-component of momentum	GeV
e	float	Energy	GeV
m	float	Mass	GeV
lifetime	float	Proper lifetime	mm
spin	float	Spin information. 9.0 for unpolarized

Further details can be found in the original definition of the LHE file standard. Besides the original publication there were two extensions to the LHE format, version 2.0 in 2009 [2] and version 3.0 in 2013 [3]. However, pylhe currently only implements the widely adopted extension from version 1.0, that is the addition of multiple weights via <initrwgt>, <rwgt>, <weight>, <weights>, <wgt>, and <weightgroup>. If in the future there is a demand for <scales>, <generator>, <pdfinfo>, or <clustering>, support for these can be added as well.