Decay modes

All possible decays and resonance formations in SMASH are provided by the decaymodes.txt file, which is located in '$SMASH_SRC_DIRECTORY/input'. If you want to modify and use this file to set up SMASH, execute

./smash -d $SMASH_SRC_DIRECTORY/input/decaymodes.txt

in the '$SMASH_SRC_DIRECTORY/build' directory.
The decaymodes are formatted in blocks of the following format:

<name of decaying particle>
<branching ratio> <angular momentum L> <names of decay products>
<branching ratio> <angular momentum L> <names of decay products>
...

The blocks have to be separated by at least one empty line.

The names have to be the ones defined in particles.txt (see Particles). If multiplet names are used, the other branching ratios are generated by SMASH assuming isospin symmetry. Note that currently decay channels can only be specified for whole multiplets; individual particles can however still be used in a decay channel as specific daughters.

The branching ratios are given as a floating point number. If the branching ratios in one block do not add up to 1, they are automatically normalized by SMASH.

The angular momentum of the decay channel has to be specified as an integer.

The names of two or three decay products have to be given for each channel. Note that the SMASH defaults avoid three-body decays, because they break detailed balance due to the lack of 3-to-1 reactions in SMASH.

For example, the following lines are enough to specify all possible decays of the N(1440) resonance multiplet:

N(1440)
0.60   1  N π
0.24   1  Δ π
0.16   0  N σ

For decays violating isospin symmetry, it is possible to specify the members of the multiplets in the final state explicitely:

φ
0.489   1  K⁺ K̅⁻
0.342   1  K⁰ K̅⁰

It is possible to add comments to decaymodes.txt using the # character. Everything after # until the end of the line is ignored.

Note

Note that SMASH has an internal width cutoff (currently 10 keV), below which particles cannot decay, even if decays are specified in decaymodes.txt. This only changes when Ignore_Minimum_Decay_Width_For_Decays_At_The_End is enabled, then all decays are considered in the final decays, no matter their decay width.

Note further, that the decay modes file will need to be modified when running a box simulation in which detailed balance is expected to be conserved. See Box for further information.