Mandatory keys

Init_Multiplicities — map<int,int>, required

Initial multiplicities per particle species. The value of this key shall be a map of PDG number and amount corresponding to it. Use this key to specify how many particles of each species will be initialized. This key can be omitted if Use_Thermal_Multiplicities is true.

Radius — double, required

Radius of the sphere in fm.

Start_Time — double, required

Starting time of sphere calculation in fm.

Temperature — double, required

Temperature in GeV to sample momenta in the sphere.

Optional keys

Account_Resonance_Widths — bool, optional, default = true

This key is considered only in case of thermal initialization and the following two behaviors can be choosen:

true → Account for resonance spectral functions, while computing multiplicities and sampling masses.
false → Simply use pole masses.

Add_Radial_Velocity — double, optional, default = -1.0

This can be used in order to give each particle in the sphere an additional velocity in radial direction of the size \(u_r = u_0 \, \frac{r}{R}\) with \(u_0\) being the parameter of this feature, \(r\) the radius of the particle and \(R\) the total radius of the sphere. \(u_0\) can only take values in \([0, 1]\) and specifying a negative value is equivalent in omitting this key (i.e. not attributing any additional radial velocity).

Baryon_Chemical_Potential — double, optional, default = 0.0

Baryon chemical potential \(\mu_B\) in GeV. This key is used to compute thermal densities \(n_i\) only if Use_Thermal_Multiplicities is true.

Charge_Chemical_Potential — double, optional, default = 0.0

Charge chemical potential \(\mu_Q\) in GeV. This key is used to compute thermal densities \(n_i\) only if Use_Thermal_Multiplicities is true.

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
"thermal momenta quantum" → equilibrium Fermi-Dirac or Bose-Einstein distribution
"IC_ES" → off-equilibrium distribution
"IC_1M" → off-equilibrium distribution
"IC_2M" → off-equilibrium distribution
"IC_Massive" → off-equilibrium distribution

See Bazow:2016oky [2] and Tindall:2016try [15] for further explanations about the different distribution functions.

Strange_Chemical_Potential — double, optional, default = 0.0

Strangeness chemical potential \(\mu_S\) in GeV. This key is used to compute thermal densities \(n_i\) only if Use_Thermal_Multiplicities is true.

Use_Thermal_Multiplicities — bool, optional, default = false

If this option is set to true then Init_Multiplicities are ignored and the system 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 \(\mathrm{Poi}(n_i V)\), where \(n_i\) are the grand-canonical thermal densities of the corresponding species and \(V\) is the system volume. This option simulates the grand-canonical ensemble, where the number of particles is not fixed from event to event.

Specifying jets

The Jet section within the Sphere one is used to put a single high energy particle (a "jet") in the center of the system, on an outbound trajectory along the x-axis. If no PDG code is specified, but the section is given, an error about the missing key is raised.

Jet_Momentum — double, optional, default = 20.0

The initial momentum in GeV to give to the jet particle.

Jet_PDG — int, required

The type of particle to be used as a jet, as given by its PDG code.

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
         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:
     Sphere:
         Radius: 10.0
         Temperature: 0.2
         Use_Thermal_Multiplicities: True

If one wants to simulate a jet in the hadronic medium, this can be done by using the following configuration setup:

 Modi:
     Sphere:
         Radius: 10.0
         Temperature: 0.2
         Use_Thermal_Multiplicities: True
         Jet:
             Jet_PDG: 211
             Jet_Momentum: 100.0

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 the input/sphere folder at the top-level of SMASH codebase. To run SMASH with the provided example configuration for the sphere system, execute

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

where INPUT_DIR needs to be replaced by the path to the input directory at the top-level of SMASH codebase.