The HEJ Configuration Generator

HEJ has to be used in conjunction with fixed-order generators. To this end, the HEJ Configuration Generator HEJC generates a set of consistent configuration files.

Installation

The HEJC comes bundled together with HEJ 2 and the installation is very similar. After downloading HEJ 2 and installing the prerequisites as described in Installation the HEJC can be installed with:

cmake /path/to/config_generator -DCMAKE_INSTALL_PREFIX=target/directory
make install

where /path/to/config_generator refers to the config_generator subdirectory in the HEJ 2 directory. If HEJ 2 was installed to a non-standard location, it may be necessary to specify the directory containing HEJ-config.cmake. If the base installation directory is /path/to/HEJ, HEJ-config.cmake should be found in /path/to/HEJ/lib/cmake/HEJ and the commands for installing the HEJC would read:

cmake /path/to/config_generator -DHEJ_DIR=/path/to/HEJ/lib/cmake/HEJ -DCMAKE_INSTALL_PREFIX=target/directory
make install

Using the configuration generator

Usage examples for all processes implemented in HEJ are given in the examples subdirectory. The general usage pattern is:

HEJC --runs RANGE [OPTIONS] CONFIGS.YML... TARGETDIR

which takes one or more input configuration files in YAML format and produces a number of configuration files for HEJ and the specified fixed-order generators.

The value of the --runs option specifies the statistically independent runs for which configuration files should be generated. It is a comma-separated list of numbers, where additionally inclusive ranges can be indicated with an ellipsis. For example --runs 1,3,...,6,9 is equivalent to --runs 1,3,4,5,6,9. For each run, the configuration files will be written to a subdirectory run_XXX of TARGETDIR, where XXX is the run number.

When using the HEJ docker image, the corresponding command is:

docker run -v $PWD:$PWD -w $PWD hejdock/hej HEJC --runs N [OPTIONS] CONFIGS.YML... TARGETDIR

Options

• --runs RANGE specifies the runs for which configuration files should be generated.

• --copy-dir DIR copies the given directory as a subdirectory into each generated set of configuration files. This can be useful to add external information to a run, e.g. precomputed integration grids, custom analyses, or scale setting code.

• --help prints a short help message.

Settings

The HEJC supports the following settings. Note that not all generators accept all settings. For example, when using The HEJ Fixed Order Generator, the beam particles have to be either protons or antiprotons. To facilitate organising the large number of configuration options, it is possible to distribute them over more than one configuration file.

process

The scattering process for which events are being generated. The format is

in1 in2 => out1 out2 ...

Particles can either be identified by their name or their Monte Carlo status code. The process is always considered as inclusive in the number of jets. To indicate this, the keyword jets can be added.

jets

Defines the properties of the generated jets.

min pt

Minimum jet transverse momentum in GeV.

algorithm

The algorithm used to define jets. Allowed settings are kt, cambridge, antikt, cambridge for passive. See the FastJet documentation for a description of these algorithms.

R

The R parameter used in the jet algorithm.

max rapidity

Maximum absolute value of the jet rapidity.

beam

Defines various properties of the collider beam.

energy

The beam energy in GeV. For example, the 13 TeV LHC corresponds to a value of 6500.

particles

A list [p1, p2] of two beam particles.

pdf

The LHAPDF number or name of the PDF set. For example, 230000 or NNPDF23_nlo_as_0119 corresponds to an NNPDF 2.3 NLO PDF set.

low pt event fraction

The fraction of the nominal number of events that should be generated in separate low transverse momentum runs.

low pt fraction

The fraction of the minimum jet transverse momentum to be used for separate low transverse momentum runs. Defaults to 0.8 if omitted.

generators

A sequence of generators for which configuration files should be generated.

generator

The name of the generator. Should be the name of one of the Supported Generators. If it is set to HEJ, then the remaining keys apart from jets will overwrite the general settings and additional specific configuration files for the given jet multiplicities will be generated. This can be useful if certain HEJ settings should only apply to certain multiplicities, e.g. for matching.

jets

The number of jets to which this configuration should apply. This can either be a single number or an inclusive range N-M. In the latter case a separate configuration file will be written for each jet multiplicity from N to M .

settings

Generator-specific settings that should be added to the generated file. See Supported Generators for the supported entries.

HEJ settings

Settings that should be added to the HEJ configuration file generated for the current generator and number of jets. In case of conflicts, these specific HEJ entries overwrite the globally defines ones. This entry is optional.

In addition, the following HEJ 2 Settings as well as all advanced settings are supported and directly added to the generated HEJ configuration file:

• trials

• event treatment

• scales

• scale factors

• max scale ratio

• import scales

• unweight

• event output

• analyses

• random generator

• log correction

• vev

• particle properties

• Higgs coupling

• off-shell tolerance

Supported Generators

HEJC can generate configuration files for HEJ as well as for the following generators

Sherpa2

Version 2.2 of the Sherpa event generator.

The settings should be given in YAML format using the run, selector, and analysis keys. run should be a YAML map, while selector and analysis should be sequences. Sherpa tags can be set inside run: by adding a YAML map under the key tags. The following entries in run are generated and should be omitted:

• PDF_LIBRARY

• PDF_SET

• USE_PDF_ALPHAS

• BEAM_1

• BEAM_2

• FRAGMENTATION

• YFS_MODE

• MI_HANDLER

• CSS_MAXEM

• BEAM_REMNANTS

• MASSIVE[4]

• MASS[4]

• MASSIVE[5]

• MASS[5]

• MASS[23]

• MASS[23]

• WIDTH[23]

• MASS[24]

• WIDTH[24]

• MASS[25]

• WIDTH[25]

There is also experimental support for beam, csv, me, integration, shower, mi, fragmentation as YAML maps, including support for Sherpa tags.

Sherpa2NLO

NLO run card for Sherpa. The format is the same as for Sherpa2

HEJFOG

The HEJ Fixed Order Generator

The following settings are generated and should be omitted:

• jets

• process

• vev

• particle properties

• Higgs coupling

• pdf

• beam