Two_to_One (bool, optional, default = true)
Enable 2 <–> 1 processes (resonance formation and decays).

Included_2to2 (list of 2 <–> 2 reactions, optional, default = ["All"])
List that contains all possible 2 <–> 2 process categories. Each process of the listed category can be performed within the simulation. Possible categories are:

"Elastic" - elastic binary scatterings
"NN_to_NR" - nucleon + nucleon <–> nucleon + resonance
"NN_to_DR" - nucleon + nucleon <–> delta + resonance
"KN_to_KN" - kaon + nucleon <–> kaon + nucleon
"KN_to_KDelta" - kaon + nucleon <–> kaon + dela
"Strangeness_exchange" - processes with strangeness exchange
"All" - include all binary processes, no necessity to list each single category

Force_Decays_At_End (bool, optional, default = true):

true - Force all resonances to decay after last timestep
false - Don't force decays (final output can contain resonances)

No_Collisions (bool, optional, default = false)
Disable all possible collisions, only allow decays to occur if not forbidden by other options. Useful for running SMASH as a decay afterburner, but not recommended in general, because it breaks the detailed balance.

NNbar_Treatment (string, optional, default = "strings")

"no annihilation" - No annihilation of NNbar is performed.
"resonances" - Annhilation through NNbar → ρh₁(1170); combined with ρ → ππ and h₁(1170) → πρ, which gives 5 pions on average.
"strings" - Annihilation throug string fragmentation.

Use_AQM (bool, optional, default = true)
Turn on AQM cross-sections for exotic combination of particles (baryon-baryon cross-sections are scaled from proton-proton high energy parametrization, for example). This includes both elastic and non-elastic contributions; non-elastic contributions go through string fragmentation. Turning off strings or elastic collisions while leaving this on will result in the corresponding part of the AQM cross-sections to also be off. Cross-sections parametrization are scaled according to

\[ \frac{\sigma^{AQM}_{\mathrm{process}}}{\sigma^{AQM}_\mathrm{ref\_process}} \sigma^{param}_\mathrm{ref\_process}\]

where \( \sigma^{AQM}_x = 40 \left( \frac{2}{3} \right)^{n_{meson}} (1 - 0.4 x^s_1) (1 - 0.4 x^s_2) \), with \(n_{meson}\) being the number of mesons in the process, \(x^s_{1,2}\) the fraction of strange quarks in the participant. "process" is then a generic process and "ref_process" a reference process such as PP for which solid parametrizations exist. (Bass:1998ca)

Strings_with_Probability (bool, optional, default = true):

true - String processes are triggered according to a probability increasing smoothly with the collisional energy from 0 to 1 in a certain energy window. At energies beyond that window, all the inelastic scatterings are via strings, while at the energies below that window, all the scatterings are via non-string processes. One should be careful that in this approach, the scatterings via resoances are also suppressed in the intermediate energy region, and vanishes at high energies, e.g. pπ→Δ→ΣK can't happen at a collisional energy beyond 2.2 GeV in this approach. Therefore, the cross sections of the scatterings to the certain final states, which might be crucial for the production of the rare species, will be reduced at the high energies.
false - String processes always happen as long as the collisional energy exceeds the threshold value by 0.9 GeV, and the parametrized total cross section is larger than the sum of cross sections contributed by the non-string processes. The string cross section is thus obtained by taking the difference between them.

Elastic_Cross_Section (double, optional, default = -1.0 [mb])
If a non-negative value is given, it will override the parametrized elastic cross sections (which are energy-dependent) with a constant value. This constant elastic cross section is used for all collisions.

Isotropic (bool, optional, default = false)
Do all collisions isotropically.

Elastic_NN_Cutoff_Sqrts (double, optional, default = 1.98):
The elastic collisions betwen two nucleons with sqrt_s below Elastic_NN_Cutoff_Sqrts, in GeV, cannot happen.

Elastic_NN_Cutoff_Sqrts < 1.88 - Below the threshold energy of the elastic collsion, no effect
Elastic_NN_Cutoff_Sqrts > 2.02 - Beyond the threshold energy of the inelastic collision NN->NNpi, not suggested

Strings (bool, optional, default = true for each setup except box):

true - String excitation is enabled
false - String excitation is disabled

String_Formation_Time (double, optional, default = 1.0):
Parameter for formation time in string fragmentation, in fm/c.

To further configure Pauli blocking and the string parameters, see

Example: Configuring the Collision Term

The following example configures SMASH to include all but strangeness exchange involving 2 <–> 2 scatterings, to treat N + Nbar processes as resonance formations and to not force decays at the end of the simulation. The elastic cross section is globally set to 30 mbarn and the \( \sqrt{s} \) cutoff for elastic nucleon + nucleon collisions is 1.93 GeV. All collisions are performed isotropically and 2 <–> 1 processes are forbidden.

Collision_Term:
    Included_2to2:    ["Elastic", "NN_to_NR", "NN_to_DR", "KN_to_KN",
"KN_to_KDelta"] Two_to_One: True Force_Decays_At_End: False NNbar_Treatment:
"resonances" Elastic_Cross_Section: 30.0 Elastic_NN_Cutoff_Sqrts: 1.93
    Isotropic: True

If necessary, all collisions can be turned off by inserting

    No_Collisions: True

in the configuration file.

Additionally, string fragmentation can be activated. If desired, the user can also configure the string parameters.

    Strings: True
    String_Parameters:
        String_Tension: 1.0
        Gluon_Beta: 0.5
        Gluon_Pmin: 0.001
        Quark_Alpha: 1.0
        Quark_Beta: 2.5
        Strange_Supp: 0.12
        Diquark_Supp: 0.03
        Sigma_Perp: 0.42
        Leading_Frag_Mean: 1.0
        Leading_Frag_Width: 0.6
        StringZ_A: 0.68
        StringZ_B: 0.3
        String_Sigma_T: 0.5
        Prob_proton_to_d_uu: 0.33

Pauli Blocking can further be activated by means of the following subsection

    Pauli_Blocking:
        Spatial_Averaging_Radius: 1.86
        Momentum_Averaging_Radius: 0.08
        Gaussian_Cutoff: 2.2