Box

Initial_Condition (string, required, no default):
Controls initial momentum distribution of particles.

• "peaked momenta" - All particles have momentum \(p = 3 \cdot T\), where T is the temperature. Directions of momenta are uniformly distributed.
• "thermal momenta" - Momenta are sampled from a Maxwell-Boltzmann distribution.

Length (double, required):
Length of the cube's edge, in fm.

Temperature (double, required):
Temperature in the box, in GeV.

Start_Time (double, required):
Starting time of the simulation. All particles in the box are initialized with \(x^0\) = Start_Time.

Equilibration_Time (double, optional):
Time after which the output of the box is written out. The first time however will be printed. This is useful if one wants to simulate boxes for very long times and knows at which time the box reaches its thermal and chemical equilibrium.

Init_Multiplicities (int, required):
Map of PDG number and quantity of this PDG number. Controls how many particles of each sort 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 \( \mu_B \) used in case if Use_Thermal_Multiplicities is true to compute thermal densities \( n_i \).

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

Charge_Chemical_Potential (double, optional, default = 0.0):
Charge chemical potential \( \mu_Q \) used in case if Use_Thermal_Multiplicities is true to compute thermal densities \( n_i \).

Account_Resonance_Widths (bool, optional, default = true):
In case of thermal initialization: true – account for resonance spectral functions, while computing multiplicities and sampling masses, false – simply use pole masses.

Normally, one wants this option true. For example for the detailed balance studies it is better to account for spectral functions, because then at t = 0 one has exactly the expected thermal grand-canonical multiplicities, that can be compared to final ones. However, by switching true/false one can observe the effect of spectral function on the multiplicity. This is useful for understanding the implications of different ways of sampling resonances in hydrodynamics.

Jet:
This subset of config values is used to put a single high energy particle (a "jet") in the center of the box, on an trajectory along the x axis; if no pdg is specified no jet is produced.

• Jet_PDG (int, optional): The type of particle to be used as a jet, as given by its PDG code; if none is provided no jet is initialized.

• Jet_Momentum (double, optional, default = 20.): The initial momentum to give to the jet particle (in GeV)

Examples: Configuring a Box Simulation

The following example configures an infinite matter simulation in a Box with 10 fm cube length at a temperature of 200 MeV. The particles are initialized with thermal momenta at a start time of 10 fm. The particle numbers at initialization are 100 \( \pi^+ \), 100 \( \pi^0 \), 100 \( \pi^- \), 50 protons and 50 neutrons.

 Modi:
     Box:
         Length: 10.0
         Temperature: 0.2
         Initial_Condition: "thermal momenta"
         Start_Time: 10.0
         Init_Multiplicities:
             211: 100
             111: 100
             -211: 100
             2212: 50
             2112: 50

On the contrary, it is also possible to initialize a thermal box based on thermal multiplicities. This is done via

 Modi:
     Box:
         Length: 10.0
         Temperature: 0.2
         Use_Thermal_Multiplicities: True
         Initial_Condition: "thermal momenta"
         Baryon_Chemical_Potential: 0.0
         Strange_Chemical_Potential: 0.0
         Charge_Chemical_Potential: 0.0
         Account_Resonance_Widths: True

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

 Modi:
     Box:
         Length: 10.0
         Temperature: 0.2
         Use_Thermal_Multiplicities: True
         Initial_Condition: "thermal momenta"
         Jet:
             Jet_PDG: 211
             Jet_Momentum: 100.0

Note

The box modus is most useful for infinite matter simulations with thermal and chemical equilibration and detailed balance. Detailed balance can however not be conserved if 3-body decays (or higher) are performed. To yield useful results applying a SMASH box simulation, it is therefore necessary to modify the provided default particles.txt and decaymodes.txt by removing 3-body and higher order decays from the decaymodes file and all corresponding particles that can no longer be produced from the particles file. In addtion, strings need to be turned off, since they also break detailed balance due to lacking backreactions.
SMASH is shipped with example files (config.yaml, particles.txt, decaymodes.txt) meeting the above mentioned requirements to set up an infinite matter simulation. They are located in /input/box. To run SMASH with the provided example files, execute

    ./smash -i INPUT_DIR/box/config.yaml -p INPUT_DIR/box/particles.txt -d
 INPUT_DIR/box/decaymodes.txt 

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).