How the particles file is used

The particles available to SMASH are defined in the input/particles.txt file. The content of this file is internally copied by CMake to the build directory when running the cmake command for the first time to set up SMASH. If you want to modify the particles file, you are encouraged to copy the provided one to a wished location, which has then to be passed to SMASH via the -p option. For example, assuming to have a custom_particles.txt file in the build folder, the SMASH executable can be run from there and instructed to use the own particles file via

./smash -p custom_particles.txt

The particle file format

Particles are specified as a table with particles properties in different columns, which may be separated by an arbitrary number of spaces:

The name has to be a unique UTF-8 string. Conventionally, unicode names are used in SMASH to make the file more readable and generate prettier output. It is possible to only specify the isospin multiplet and SMASH will fill in the properties of the components of the multiplet assuming isospin symmetry. The names generated this way will have the charges appended to the multiplet name using the unicode characters ⁻, ⁰ and ⁺. This is appropriate for almost all particles. Anti particles do not have to be specified explicitly.

The pole mass and the on-shell width of the particle or multiplet have to be specified as floating point numbers in GeV.

The parity has to be either + or -.

The PDG codes are following the numbering scheme specified by the PDG, which depends on the quantum numbers of the particles. For SMASH, it is important that the quark content in the PDG code is correctly specified. Other than that, deviations from the numbering scheme have no effect in SMASH. If the name represents a multiplet, there has to be a PDG code for all multiplet members, except for anti particles.

For example, to define all three pions (π⁻, π⁰, π⁺), it is sufficient to specify the π multiplet using the following line in particles.txt, where the 4th column contains the PDG number of the neutral and the 5th PDG number of the charged state:

π 0.138 7.7e-9 111 211

It is also possible to only specify a specific member of the multiplet. In this case, the charge has to be given as a suffix in the name using the UTF-8 unicode characters ⁻, ⁰ and ⁺. For example, the properties of the electron can be specified like this:

e⁻ 0.000511 0 11

Comments can be added to the particles file using the # character. Everything after # until the end of the line is ignored.

Attention

If you specify an incorrect value, SMASH will print an error similar to the following:
Failed to convert the input string to the expected data types.
SMASH validates (up to some small numeric precision) the mass of some particles (e.g. N, π, K, ω, Δ, D). Therefore, totally nonphysical mass values cannot be used and SMASH will abort with a message error like e.g. the following
Nucleon mass in input file different from 0.938000

if a mass discrepancy for one of the validated particle is detected. This constraint is due to internal assumptions in SMASH and is needed to guarantee the correctness of the results. If you really need to use SMASH with nonphysical mass values, feel free to contact us or open an issue.
Related to the previous point, it is important to mention that all hadrons belonging to the same isospin multiplet must have the same mass and this is enforced by SMASH, which will fail otherwise. Feel free to get in touch with us, if this restriction represents a problem for you.
Some reactions in SMASH are parametrized and require specific particles in the final state. When such a reaction happens and the required particle is not defined, SMASH will crash.
When running a box simulation in which detailed balance is expected to be conserved, the particles file will need to be modified. See this remark about the box modus for further information.