Dipole shower

Using the dipole shower

Herwig also includes a dipole shower which may be used instead of the angular-ordered shower by adding the line:

set LHCHandler:CascadeHandler /Herwig/DipoleShower/DipoleShowerHandler

to the input file. Straightforward input file snippets to use the dipole shower together with Matchbox generated processes are avaialable in the *-Matchbox.in input files, while input files for builtin matrix elements require in addition to this, that the following line has also been included:

read snippets/DipoleShowerFiveFlavours.in

to read tune parameters specific to the dipole shower. If a four flavour scheme with massive outgoing b quark is intented, instead please use:

read snippets/DipoleShowerFourFlavours.in

keeping in mind that the builtin matrix elements may require adjustment to the processes they generate.

Evaluating uncertainties

As with the angular-ordered shower, the scales using in the dipole shower may be varied using:

cd /Herwig/DipoleShower
set DipoleShowerHandler:RenormalizationScaleFactor x
set DipoleShowerHandler:FactorizationScaleFactor x
set DipoleShowerHandler:HardScaleFactor x

On-the-fly reweighting for scale variations

On-the-fly reweighting for scale variations can be performed using:

cd /Herwig/DipoleShower
do DipoleShowerHandler:AddVariation NameOfVariation xiR xiF {Hard,Secondary,All}

where NameOfVariation is a string tag identifying the weight in the extended HepMC record, and xiF and xiR are the repsective factors multiplying the arguments of the strong coupling and the PDF in the shower. The last variable identifies whether the reweighting is to be applied to showering of the hard process (Hard), the showering of secondary interactions (Secondary) or both of them (All).

The convergence of reweighted distributions can be enhanced by using a detuining factor e.g.:

set DipoleShowerHandler:Detuning 2.0

at a slight expense in overall run time. Please cite arXiv:1605.08256 when using the shower reweighting feature.

Current limitations of the dipole shower

Currently, coloured particle decays are not generated within the dipole shower. As such, it should not be used when producing unstable top quarks. Furthermore, only the splitting kernels relevant to Standard Model processes are implemented at present.