Decays and Particle Properties

Introduction

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 leptons.in, quarks.in, bosons.in, mesons.in, baryons.in, 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

Herwig build

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

do /Herwig/Particles/h0:PrintDecayModes

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 PrintDecayModes command.

Switching off decays of a particle

There are two way to switch decay modes on/off in Herwig.

1. 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 boson

   do /Herwig/Particles/h0:SelectDecayModes
   do /Herwig/Particles/h0:PrintDecayModes
   

   and result in

   h0->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 in

   h0->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.

2. Alternatively specific decay modes can be switched on/off using the OnOff interface of the DecayMode object. For example the \(\tau^+\) mode of the \(W^+\) can be switched off using

   set /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.