Decays and Particle Properties¶
In Herwig the particle properties are contained in a
ParticleData object. This has a number of interfaces which can be used to change the properties of the particles. The particles are setup as objects in the files
diquarks.in in the directory
share/Herwig/defaults. These properties can be changed in the input file
for an event generator.
How do I change the mass of a particle?¶
For example to change the mass of the top quark the following lines should be added to the input file for the generator
set /Herwig/Particles/t:NominalMass 170.*GeV set /Herwig/Particles/tbar:NominalMass 170.*GeV
N.B. By default the properties of the particles and antiparticles are forced to be the same, but it is safest to change the mass of both the particle and antiparticle.
This can cause a problem with the calculation of off-shell effects and decays and therefore we would recommend that you also reinitialize the mass and width generators, the limit on how off-shell the particle can be, and the decayers for any important decay modes. For example for a 40 GeV top quark
set /Herwig/Particles/t:NominalMass 40. set /Herwig/Particles/t:WidthCut 4. set /Herwig/Particles/t:Width_generator:Initialize Yes set /Herwig/Particles/t:Mass_generator:Initialize Yes set /Herwig/Decays/Top:Initialize Yes
and initialise any other decays, see
share/Herwig/defaults/*_decays.in for the relevant names.
Similarly for a 40 GeV Higgs only decaying to \(W^+W^-\)
do /Herwig/Particles/h0:SelectDecayModes h0->W+,W-; set /Herwig/Particles/h0:NominalMass 40. set /Herwig/Particles/h0:WidthCut 4. set /Herwig/Particles/h0:Width_generator:Initialize Yes set /Herwig/Particles/h0:Mass_generator:Initialize Yes set /Herwig/Decays/HWW:Initialize Yes
Switch off all decays¶
The following commands can be used to switch off all the decays.
cd /Herwig/EventHandlers set [NameofEventHandler]:DecayHandler NULL # Switch off decay cd /
Of course you can also switch off also the parton shower and hadronization stage, see Parton shower.
List decay modes of a particle¶
The following commands can be used to list the possible and the default decay modes of a particle. Furthermore, the decay of a particle can be limited to a decay mode.
Start Herwig in interactive mode
List the particles defined in Herwig
cd /Herwig/Particles ls
List available decay modes of a particle, e.g., listing the available decay modes of the Higgs boson
will show that all decay modes are activated.
h0->b,bbar; 0.569369 h0->W+,W-; 0.224 h0->g,g; 0.085 h0->tau-,tau+; 0.0628 h0->Z0,Z0; 0.0279 h0->c,cbar; 0.0264 h0->gamma,gamma; 0.00229 h0->Z0,gamma; 0.00159 h0->s,sbar; 0.000433 h0->mu-,mu+; 0.000218 h0->t,tbar; 0
If you switch off decay modes it is essential that the decay products are in the order given usign the
Switching off decays of a particle¶
There are two way to switch decay modes on/off in Herwig.
The first is easiest if you only want to consider a small number of decay modes.
Using the following command will switch off all decay modes of the Higgs bosondo /Herwig/Particles/h0:SelectDecayModes do /Herwig/Particles/h0:PrintDecayModes
and result inh0->b,bbar; (off) 0 h0->W+,W-; (off) 0 h0->tau-,tau+; (off) 0 h0->g,g; (off) 0 h0->c,cbar; (off) 0 h0->Z0,Z0; (off) 0 h0->Z0,gamma; (off) 0 h0->gamma,gamma; (off) 0 h0->s,sbar; (off) 0 h0->mu-,mu+; (off) 0 h0->t,tbar; (off) 0
To choose a specific decay mode of a particle, e.g. to allow only the \(h^0\to\gamma\gamma\) decay use the following command in your Herwig input file:do /Herwig/Particles/h0:SelectDecayModes h0->gamma,gamma; do /Herwig/Particles/h0:PrintDecayModes
This results inh0->g,g; (off) 0 h0->b,bbar; (off) 0 h0->W+,W-; (off) 0 h0->tau-,tau+; (off) 0 h0->c,cbar; (off) 0 h0->Z0,Z0; (off) 0 h0->Z0,gamma; (off) 0 h0->gamma,gamma; 0.00229 h0->s,sbar; (off) 0 h0->mu-,mu+; (off) 0 h0->t,tbar; (off) 0
The options All for all the modes or None for no modes are also supported.
Alternatively specific decay modes can be switched on/off using the
OnOffinterface of the
DecayModeobject. For example the \(\tau^+\) mode of the \(W^+\) can be switched off usingset /Herwig/Particles/W+/W+->nu_tau,tau+;:OnOff Off
Set a particle as stable¶
Setting a particle as stable can be done by using the following command:
set /Herwig/Particle/[ParticleName]:Stable Stable
For example to make the \(\pi^0\) stable
set /Herwig/Particles/pi0:Stable Stable
N.B. As by default the properties of the particles and antiparticles are forced to be the same it is safest to change the properties of both the particle and antiparticle.
How do I make a group of particles stable?¶
All particles with a lifetime larger than 10 mm can be set stable with:
set /Herwig/Decays/DecayHandler:MaxLifeTime 10*mm
To apply this to the nominal lifetimes (the default), use
set /Herwig/Decays/DecayHandler:LifeTimeOption Average
to apply the cut on the actual lifetime of the generated particle, use
set /Herwig/Decays/DecayHandler:LifeTimeOption Real
I’ve switched some W decay modes off but the W still decays via the switched off modes¶
In addition to appearing as an external particle, the W boson often appears as an intermediate state, for example in three-body top quark decays. The W decay modes only affect the decay of external W bosons and so if for example the Ws you are interested in come from top quark decays, they won’t be effected and you’ll need to switch off the relevant top quark modes as well. The same applies to hard production processes, e.g. W + jets, where the decay is part of the \(2\to3\) production matrix element and either controlled from the MatrixElement class or in the Process command. Similarly in some cases the same applies to \(Z^0\) boson decays.