Parton shower

Switch off parton shower, hadronization and decay

The following commands can be used to switch off the parton shower, the hadronization and the subsequent decay.

cd /Herwig/EventHandlers
set [NameofEventHandler]:CascadeHandler        NULL     # Switch off parton shower
set [NameofEventHandler]:HadronizationHandler  NULL     # Switch off hadronization
set [NameofEventHandler]:DecayHandler          NULL     # Switch off decay
cd /

Of course you can also switch off only the decay stage or the hadronization and decay stage.

Switch off underlying event

The following commands can be used to switch off the underlying event.

cd /Herwig/EventHandlers
set /Herwig/Shower/ShowerHandler:MPIHandler    NULL
cd /

Define the parton shower tune

A description of different tunes can be found here.

UE-EE-5-CTEQ6L1 tune from Herwig++ 2.7.1

The following commands can be used to set the UE-EE-5-CTEQ6L1 tune which was defined in Herwig++ 2.7.1.

set /Herwig/UnderlyingEvent/MPIHandler:ReferenceScale 7000.*GeV
set /Herwig/UnderlyingEvent/MPIHandler:Power 0.33
set /Herwig/UnderlyingEvent/MPIHandler:pTmin0 3.91*GeV
set /Herwig/UnderlyingEvent/MPIHandler:InvRadius 2.30
set /Herwig/UnderlyingEvent/MPIHandler:softInt Yes
set /Herwig/UnderlyingEvent/MPIHandler:twoComp Yes
set /Herwig/UnderlyingEvent/MPIHandler:DLmode 2

set /Herwig/Hadronization/ColourReconnector:ColourReconnection Yes
set /Herwig/Hadronization/ColourReconnector:ReconnectionProbability 0.49
set /Herwig/Partons/RemnantDecayer:colourDisrupt 0.80

UE-EE-5-MRST tune from Herwig++ 2.7.1

The following commands can be used to set the UE-EE-5-MRST tune which was defined in Herwig++ 2.7.1.

set /Herwig/UnderlyingEvent/MPIHandler:ReferenceScale 7000.*GeV
set /Herwig/UnderlyingEvent/MPIHandler:Power 0.314
set /Herwig/UnderlyingEvent/MPIHandler:pTmin0 4.620*GeV
set /Herwig/UnderlyingEvent/MPIHandler:InvRadius 2.240
set /Herwig/UnderlyingEvent/MPIHandler:softInt Yes
set /Herwig/UnderlyingEvent/MPIHandler:twoComp Yes
set /Herwig/UnderlyingEvent/MPIHandler:DLmode 2

set /Herwig/Hadronization/ColourReconnector:ColourReconnection Yes
set /Herwig/Hadronization/ColourReconnector:ReconnectionProbability 0.420
set /Herwig/Partons/RemnantDecayer:colourDisrupt 0.860

How do I turn on the UA5 Underlying Event model?

By default Herwig uses a multiple scattering model of the underlying event. If you wish to use the old UA5 model, which we do not recommend for realistic physics studies, you should first turn off the multiple scattering model and then enable the UA5 model.

set /Herwig/Shower/ShowerHandler:MPIHandler NULL
set /Herwig/Hadronization/ClusterHadHandler:UnderlyingEventHandler /Herwig/Hadronization/UA5

How to I use the Non-Perutrbative Gluon emission model?

The model of non-perturbative gluon emission in an initial-state parton shower implemented in Herwig is described in arXiv:0712.1199. The last tuning of its parameters was presented in arXiv:0905.3455.

In order to use the parameters of the model from the tune from the publication arXiv:0905.3455 the following lines have to be added to the input file

##################################################################
cd /Herwig/Shower
set SplittingGenerator:FSR No
set Evolver:IntrinsicPtGaussian 0.4*GeV
####
set AlphaQCD:NPAlphaS 5
set AlphaQCD:Qmin 2.5*GeV
set AlphaQCD:AlphaMaxNP 4.0
##################################################################

Here, AlphaQCD:NPAlphaS 5 selects the quadratic non–perturbative model. The flat model would correspond to setting this parameter to 6. AlphaQCD:Qmin sets the value of \(p_{\perp_0}\) and AlphaQCD:AlphaMaxNP directly sets the value \(\alpha_S(0)\).