Decay calculation with SPheno

Decays calculated by SPheno

The calculation of decays widths and branching ratios can be done by using the interface between SARAH and SPheno. SPheno modules created by SARAH calculate all two-body decays for SUSY and Higgs states as well as for additional gauge bosons. In addition, the three-body decays of a fermion into three other fermions and of a scalar into another scalar and two fermions are included. In the Higgs sector, possible decays into two SUSY particles, leptons and massive gauge bosons are calculated at tree-level. For two quarks in the final state the dominant QCD corrections due to gluons are included . The loop induced decays into two photons and gluons are fully calculated at leading-order (LO) with the higher order corrections known from the SM/MSSM. For the LO contributions all charged and coloured states in the given model are included in the loop, i.e. new contributions rising in a model beyond the MSSM are fully covered at one-loop. In addition, in the Higgs decays also final states with off-shell gauge bosons (Z**Z^*, W**W^*) are included. The only missing piece is the γ**Z channel. The corresponding loops are not yet derived by SARAH and the partial width is set to zero.

Setting up the list of decaying particles

One can set the lists of particles for which the two- and three-body decays shall be calculated via

ListDecayParticles=...
ListDecayParticles3B=...

with the following possibilities and conventions

ListDecayParticles defines the list of particles for which the two-body decays are to be calculated. There are two possibilities:
- A list of particles is given
- The keyword Automatic is used. In that case, the widths of all particles not defined as standard model particles as well as for the top quark are calculated. Note, Automatic is not possible for a low scale SPheno version.
ListDecayParticles3B defines the list for three body decays. Also here two options are possible:
- A list of particles and the corresponding file name is given
- The keyword Automatic is used. In that case, the widths of all particles not defined as standard model particles as well as for the top quark are calculated. Note, Automatic is not possible for a low scale SPheno version.

Example

All two-body decays for SUSY states, the Higgs states and the top in the MSSM are included via ListDecayParticles = {Sd,Su,Se,hh,Ah,Hpm,Chi,Cha,Glu};

This would give the same output as just
```
ListDecayParticles=Automatic;
```
The three body decays of SUSY fermions in the MSSM are included via ListDecayParticles3B = {{Chi,"Neutralino.f90"}, {Cha,"Chargino.f90"}, {Glu,"Gluino.f90"}};

If instead
```
ListDecayParticles3B=Automatic;
```
would be used, also three-body decays of scalars are included. The auto generated file names are the OutputName of each field.

The parameters used in the decays

In contrast to other spectrum generators, SPheno modules by SARAH can perform a RGE running to the mass scale of the decaying particle. This should give a more accurate prediction for the decay width and branching ratios. However, the user can also turn off this running and use always the parameters as calculated at the renormalisation scale Q in all decays as this is done by other codes.

Important flags in the Les Houches file

11 if 1, the branching ratios of the SUSY and Higgs particles are calculated; default is 1
12 defines minimum value for a branching ratios to be included in output; default is 10⁻⁴
13 adjusts the three-body decays: 0: no three-body decays are calculated; 1 only three-body decays of fermions are calculated; 2 only three-body decays of scalars are calculated; 3 three-body decays of fermions and scalars are calculated; default is 1
14 if 1, the running parameters at the mass scale of the decaying particle are calculated. Otherwise, the parameters at the standard renormalization scale are used; default is 1

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