Running HEJ 2

Quick start

In order to run HEJ 2, you need a configuration file and a file containing fixed-order events. A sample configuration is given by the config.yml file distributed together with HEJ 2. Events in the Les Houches Event File format can be generated with standard Monte Carlo generators like MadGraph5_aMC@NLO or Sherpa. HEJ 2 assumes that the cross section is given by the sum of the event weights. Depending on the fixed-order generator it may be necessary to adjust the weights in the Les Houches Event File accordingly.

The processes supported by HEJ 2 are

• Pure multijet production

• Production of a Higgs boson with jets

where at least two jets are required in each case. For the time being, only leading-order events are supported.

After generating an event file events.lhe adjust the parameters under the fixed order jets setting in config.yml to the settings in the fixed-order generation. Resummation can then be added by running:

HEJ config.yml events.lhe

Using the default settings, this will produce an output event file HEJ.lhe with events including high-energy resummation.

When using the Docker image, HEJ can be run with

docker run -v $PWD:$PWD -w $PWD hejdock/hej HEJ config.yml events.lhe

Settings

HEJ 2 configuration files follow the YAML format. The following configuration parameters are supported:

trials

High-energy resummation is performed by generating a number of resummation phase space configurations corresponding to an input fixed-order event. This parameter specifies how many such configurations HEJ 2 should try to generate for each input event. Typical values vary between 10 and 100.

min extparton pt

Specifies the minimum transverse momentum in GeV of the most forward and the most backward parton. This setting is needed to regulate an otherwise uncancelled divergence. Its value should be slightly below the minimum transverse momentum of jets specified by resummation jets: min pt. See also the max ext soft pt fraction setting.

max ext soft pt fraction

Specifies the maximum fraction that soft radiation can contribute to the transverse momentum of each the most forward and the most backward jet. Values between around 0.05 and 0.1 are recommended. See also the min extparton pt setting.

fixed order jets

This tag collects a number of settings specifying the jet definition in the event input. The settings should correspond to the ones used in the fixed-order Monte Carlo that generated the input events.

min pt

Minimum transverse momentum in GeV of fixed-order jets.

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, roughly corresponding to the jet radius in the plane spanned by the rapidity and the azimuthal angle.

resummation jets

This tag collects a number of settings specifying the jet definition in the observed, i.e. resummed events. These settings are optional, by default the same values as for the fixed order jets are assumed.

min pt

Minimum transverse momentum in GeV of resummation jets. This should be between 25% and 50% larger than the minimum transverse momentum of fixed order jets set by fixed order jets: min pt.

algorithm

The algorithm used to define jets. The HEJ 2 approach to resummation relies on properties of antikt jets, so this value is strongly recommended. For a list of possible other values, see the fixed order jets: algorithm setting.

R

The R parameter used in the jet algorithm.

FKL

Specifies how to treat events respecting FKL rapidity ordering. These configurations are dominant in the high-energy limit. The possible values are reweight to enable resummation, keep to keep the events as they are up to a possible change of renormalisation and factorisation scale, and discard to discard these events.

unordered

Specifies how to treat events with one emission that does not respect FKL ordering. In the high-energy limit, such configurations are logarithmically suppressed compared to FKL configurations. The possible values are the same as for the FKL setting, but reweight is currently only supported for Higgs boson plus jets production.

non-HEJ

Specifies how to treat events where no resummation is possible. The allowed values are keep to keep the events as they are up to a possible change of renormalisation and factorisation scale and discard to discard these events.

scales

Specifies the renormalisation and factorisation scales for the output events. This can either be a single entry or a list [scale1, scale2, ...]. For the case of a list the first entry defines the central scale. Possible values are fixed numbers to set the scale in GeV or the following:

• H_T: The sum of the scalar transverse momenta of all final-state particles

• max jet pperp: The maximum transverse momentum of all jets

• jet invariant mass: Sum of the invariant masses of all jets

• m_j1j2: Invariant mass between the two hardest jets.

Scales can be multiplied or divided by an overall factor, e.g. H_T/2.

It is also possible to import scales from an external library, see Custom scales

scale factors

A list of numeric factors by which each of the scales should be multiplied. Renormalisation and factorisation scales are varied independently. For example, a list with entries [0.5, 2] would give the four scale choices (0.5μ_r, 0.5μ_f); (0.5μ_r, 2μ_f); (2μ_r, 0.5μ_f); (2μ_r, 2μ_f) in this order. The ordering corresponds to the order of the final event weights.

max scale ratio

Specifies the maximum factor by which renormalisation and factorisation scales may difer. For a value of 2 and the example given for the scale factors the scale choices (0.5μ_r, 2μ_f) and (2μ_r, 0.5μ_f) will be discarded.

log correction

Whether to include corrections due to the evolution of the strong coupling constant in the virtual corrections. Allowed values are true and false.

event output

Specifies the name of a single event output file or a list of such files. The file format is either specified explicitly or derived from the suffix. For example, events.lhe or, equivalently Les Houches: events.lhe generates an output event file events.lhe in the Les Houches format. The supported formats are

• file.lhe or Les Houches: file: The Les Houches event file format.

• file.hepmc or HepMC: file: The HepMC format.

random generator

Sets parameters for random number generation.

name

Which random number generator to use. Currently, mixmax and ranlux64 are supported. Mixmax is recommended. See the CLHEP documentation for details on the generators.

seed

The seed for random generation. This should be a single number for mixmax and the name of a state file for ranlux64.

analysis

Name and Setting for the event analyses; either a custom analysis plugin or Rivet. For the first the plugin sub-entry should be set to the analysis file path. All further entries are passed on to the analysis. To use Rivet a list of Rivet analyses have to be given in rivet and prefix for the yoda file has to be set through output. See Writing custom analyses for details.

Higgs coupling

This collects a number of settings concerning the effective coupling of the Higgs boson to gluons. This is only relevant for the production process of a Higgs boson with jets and only supported if HEJ 2 was compiled with QCDLoop support.

use impact factors

Whether to use impact factors for the coupling to the most forward and most backward partons. Impact factors imply the infinite top-quark mass limit.

mt

The value of the top-quark mass in GeV. If this is not specified, the limit of an infinite mass is taken.

include bottom

Whether to include the Higgs coupling to bottom quarks.

mb

The value of the bottom-quark mass in GeV. Only used for the Higgs coupling, external bottom-quarks are always assumed to be massless.