Radius (double, required):
Radius of the Sphere, in fm.

Sphere_Temperature (double, required):
Temperature to sample momenta in the sphere, in GeV.

Start_Time (double, required):
Starting time of sphere calculation.

Init_Multiplicities (int int, required):
Initial multiplicities per particle species. Map of PDG number and quantity of this PDG number. Controls how many particles of each species will be initialized.

Use_Thermal_Multiplicities (bool, optional, default = false):
If this option is set to true then Init_Multiplicities are ignored and the box is initialized with all particle species of the particle table that belong to the hadron gas equation of state (see HadronGasEos::is_eos_particle()). The multiplicities are sampled from Poisson distributions \( Poi(n_i V) \), where \( n_i \) are the grand-canonical thermal densities of the corresponding species and \( V \) is the box volume. This option simulates the grand-canonical ensemble, where the number of particles is not fixed from event to event.

Baryon_Chemical_Potential (double, optional, default = 0.0):
Baryon chemical potential \( \mu_B \) only used if Use_Thermal_Multiplicities is true to compute thermal densities \( n_i \).

Strange_Chemical_Potential (double, optional, default = 0.0):
Strangeness chemical potential \( \mu_S \) only used if Use_Thermal_Multiplicities is true to compute thermal densities \( n_i \).

Initial_Condition (string, optional, default = "thermal momenta")
Initial distribution to use for momenta of particles. Mainly used in the expanding universe scenario, options are:

thermal momenta - equilibrium Boltzmann distribution
IC_ES - off-equilibrium distribution
IC_1M - off-equilibrium distribution
IC_2M - off-equilibrium distribution
IC_Massive - off-equilibrium distribution

See Bazow:2016oky and Tindall:2016try for further explanations about the different distribution functions.

Examples: Configuring a Sphere Simulation

The following example configures an expanding sphere with a radius of 5 fm at a temperature of 200 MeV. The particles are initialized with thermal momenta at a start time of 0 fm. The particle numbers at initialization are 100 \( \pi^+ \), 100 \( \pi^0 \), 100 \( \pi^- \), 50 protons and 50 neutrons.

Modi:
    Sphere:
        Radius: 5.0
        Sphere_Temperature: 0.2
        Initial_Condition: "thermal momenta"
        Start_Time: 0.0
        Init_Multiplicities:
            211: 100
            111: 100
            -211: 100
            2212: 50
            2112: 50

It is also possible to initialize a sphere based on thermal multiplicities. This is done via

Modi:
    Box:
        Length: 10.0
        Temperature: 0.2
        Use_Thermal_Multiplicities: True

Note

SMASH is shipped with an example configuration file to set up an expanding sphere simulation initialized with predefined initial particle multiplicities. This file is located in /input/sphere. To run SMASH with the provided example configuration for the sphere, execute

   ./smash -i INPUT_DIR/sphere/config.yaml

Where 'INPUT_DIR' needs to be replaced by the path to the input directory ('../input', if the build directory is located in the smash folder).