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 + delta
"Strangeness_exchange" - processes with strangeness exchange
"NNbar" - annihilation processes, when NNbar_treatment is set to resonances; this is superseded if NNbar_treatment is set to anything else
"PiDeuteron_to_NN" - deuteron + pion <--> nucleon + nucleon and its CPT-conjugate
"PiDeuteron_to_pidprime" - deuteron + pion <--> d' + pion
"NDeuteron_to_Ndprime" - deuteron + (anti-)nucleon <--> d' + (anti-)nucleon, and their CPT-conjugates
"All" - include all binary processes, no necessity to list each single category

Detailed balance is preserved by these reaction switches: if a forward reaction is off then the reverse is automatically off too.

Multi_Particle_Reactions (list of reactions with more than 2 in- or outgoing particles, optional, default = [])
List that contains all possible multi-particle process categories. Multi particle reactions only work with the stochastic collison criterion. See also example below. Possible categories are:

"Meson_3to1" - Mesonic 3-to-1 reactions: \(\pi^0\pi^+\pi^-\leftrightarrow\omega\), \(\pi^0\pi^+\pi^-\leftrightarrow\phi\), \(\eta\pi^+\pi^-\leftrightarrow\eta'\) and \(\eta\pi^0\pi^0\leftrightarrow\eta'\). Since detailed balance is enforced, the corresponding decays also have to be added in decaymodes.txt to enable the reactions.
"Deuteron_3to2" - Deuteron 3-to-2 reactions: \(\pi pn\leftrightarrow\pi d\), \(Npn\leftrightarrow Nd\) and \(\bar{N}pn\leftrightarrow \bar{N}d\). The deuteron has to be uncommented in particles.txt, too. 2-body reactions involving the d' have to be exluded (no "PiDeuteron_to_pidprime" and "NDeuteron_to_Ndprime" in Included_2to2), since they effectively yield the same reaction. (The same cross section is used as for the d' reactions, therefore the d' in particles.txt and its decay in decaymodest.txt also need to be uncommented.)

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. This option requires "NNbar" to be enabled in Included_2to2.
"strings" - Annihilation through 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 [4])

Resonance_Lifetime_Modifier (double, optional, default = 1.0)
Multiplicative factor by which to scale the resonance lifetimes up or down. This additionally has the effect of modifying the initial densities by the same factor in the case of a box initialized with thermal multiplicities (see Use_Thermal_Multiplicities). WARNING: This option is not fully physically consistent with some of the other asssumptions used in SMASH; notably, modifying this value WILL break detailed balance in any gas which allows resonances to collide inelastically, as this option breaks the relationship between the width and lifetime of resonances. Note as well that in such gases, using a value of 0.0 is known to make SMASH hang; it is recommended to use a small non-zero value instead in these cases.

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.

Maximum_Cross_Section (double, optional, default = 200 mb or 2000 mb in case d' is present)
The maximal cross section for which it is guaranteed that all collisions with this cross section will be found. This means that all particle pairs, where the transverse distance is smaller or equal to \( \sqrt{\sigma_{max}/\pi} \), will be checked for collions. The maximum around 200 mb occurs in the Delta peak of the pi+p cross section. Many SMASH cross sections diverge close at the threshold, these divergent parts are effectively cut off. If deuteron production via d' is considered, then the default should be increased to 2000 mb to function correctly (see Oliinychenko:2018ugs [37]). The maximal cross section is scaled with Cross_Section_Scaling factor.

Cross_Section_Scaling (double, optional, default = 1.0)
Scale all cross sections by a global factor. WARNING: Most cross sections are constrained by experimental data. Scaling them will therefore lead to unphysical results and is only meant for explorative studies.

Additional_Elastic_Cross_Section (double, optional, default = 0.0 mb)
Add an additional constant contribution to the elastic cross section. WARNING: Most elastic cross sections are constrained by experimental data. Adding an additional contribution to them will therefore lead to unphysical results and is only meant for explorative studies.

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

Collision_Criterion (string, optional, default = "Covariant"):
Choose collision criterion. For more information see Collision_Criterion

Only_Warn_For_High_Probability (bool, optional, default = false):
Only warn and not error for reaction probabilities higher than 1. This switch is meant for very long production runs with the stochastic criterion. It has no effect on the other criteria. If enabled the users for themself have to make sure that the warning, that the probability has slipped above 1, is printed very rarely.

For information about more configuration options see the following subpages

Examples: 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 adding

    No_Collisions: True

in the configuration file.

Examples: Configuring deuteron multi-particle reactions
The following example configures SMASH to includes deuteron multi-particle reactions scatterings. Note, that all 2-to-2 reactions, in particular "PiDeuteron_to_NN", are included except the d' reactions, since they effectively yield the same reaction. Before using the example, if important, check for completeness of all 2-to-2 reactions. The list of 2-to-2 reactions might have grown and the example is therefore potentially out of date.

Collision_Term:
    Collision_Criterion: Stochastic
    # All (check for completeness, if important) 2-to-2 reactions except d'
    Included_2to2: ["Elastic","NN_to_NR", "NN_to_DR", "KN_to_KN",
                    "Strangeness_exchange", "NNbar", "PiDeuteron_to_NN"]
    Multi_Particle_Reactions: ["Deuteron_3to2"]