Within Collision_Term section, the String_Parameters section can be used to modify a series of parameters which affect the string fragmentation.

Diquark_Supp — double, optional, default = 0.036

Diquark suppression factor. Defines the probability to produce a diquark antidiquark pair relative to producing a qurk antiquark pair.

Form_Time_Factor — double, optional, default = 1.0

Factor to be multiplied with the formation time of string fragments from the soft string routine.

Formation_Time — double, optional, default = 1.0

Parameter for formation time in string fragmentation, in fm.

Gluon_Beta — double, optional, default = 0.5

Parameter \(\beta\) in parton distribution function for gluons,

\[\mathrm{PDF}_g(x) \propto \frac{1}{x}(1-x)^{\beta+1}\;.\]

Gluon_Pmin — double, optional, default = 0.001

Smallest possible scale for gluon lightcone momentum in GeV. This is divided by \(\sqrt{s}\) to get the minimum fraction to be sampled from PDF shown in Gluon_Beta.

Mass_Dependent_Formation_Times — bool, optional, default = false

Whether the formation time of string fragments should depend on their mass. If it is set to true, the formation time is calculated as \(\tau = \sqrt{2}\frac{m}{\kappa} \).

Quark_Alpha — double, optional, default = 2.0

Parameter \(\alpha\) in parton distribution function for quarks,

\[\mathrm{PDF}_q\propto x^{\alpha-1}(1-x)^{\beta-1}\;.\]

Quark_Beta — double, optional, default = 7.0

Parameter \(\beta\) in PDF for quarks shown in Quark_Alpha.

Popcorn_Rate — double, optional, default = 0.15

Parameter StringFlav:popcornRate, which determines production rate of popcorn mesons in string fragmentation. It is possible to produce a popcorn meson from the diquark end of a string with certain probability (i.e., diquark to meson + diquark).

Power_Particle_Formation — double, optional, default = ±1

The default value of this parameter is +1 if \(\sqrt{s}<200\,\mathrm{GeV}\) and -1 otherwise. If positive, the power with which the cross section scaling factor of string fragments grows in time until it reaches 1. If negative, the scaling factor will be constant and jump to 1 once the particle forms.

Prob_proton_to_d_uu — double, optional, default = 1./3

Probability of splitting an (anti)nucleon into the quark it has only once and the diquark it contains twice in terms of flavour in the soft string routine.

Separate_Fragment_Baryon — bool, optional, default = true

Whether to use a separate fragmentation function for leading baryons in non-diffractive string processes.

Sigma_Perp — double, optional, default = 0.42

Parameter \(\sigma_\perp\) in GeV in the distribution for transverse momentum transfer between colliding hadrons \(p_\perp\) and string mass \(M_X\),

\[ \frac{d^3N}{dM^2_Xd^2\mathbf{p_\perp}}\propto \frac{1}{M_X^2} \exp\left(-\frac{p_\perp^2}{\sigma_\perp^2}\right)\;. \]

Strange_Supp — double, optional, default = 0.16

Strangeness suppression factor \(\lambda\),

\[\lambda= \frac{P(s\bar{s})}{P(u\bar{u})\vphantom{\bar{d}}}= \frac{P(s\bar{s})}{P(d\bar{d})}\;. \]

Defines the probability to produce a \(s\bar{s}\) pair relative to producing a light \(q\bar{q}\) pair.

String_Sigma_T — double, optional, default = 0.5

Standard deviation in GeV in Gaussian for transverse momentum distributed to string fragments during fragmentation.

String_Tension — double, optional, default = 1.0

String tension \(\kappa\) in GeV/fm connecting massless quarks in Hamiltonian,

\[H=|p_1|+|p_2|+\kappa |x_1-x_2|\;.\]

This parameter is only used to determine particles' formation times according to the yo-yo formalism (in the soft string routine for now).

StringZ_A — double, optional, default = 2.0

Parameter \(a\) in Pythia fragmentation function \(f(z)\),

\[f(z) = \frac{1}{z} (1-z)^a \exp\left(-b\frac{m_T^2}{z}\right)\;.\]

StringZ_A_Leading — double, optional, default = 0.2

Parameter \(a\) in Lund fragmentation function (see StringZ_A) used to sample the light cone momentum fraction of leading baryons in non-diffractive string processes.

StringZ_B — double, optional, default = 0.55

Parameter \(b\) in 1/GeV² in Pythia fragmentation function shown in StringZ_A.

StringZ_B_Leading — double, optional, default = 2.0

Parameter \(b\) in 1/GeV² in Lund fraghmentation function (see StringZ_B) used to sample the light cone momentum fraction of leading baryons in non-diffractive string processes.

Use_Monash_Tune — bool, optional, default = (Modus == "Collider" && Sqrtsnn >= 200)

Whether to use the monash tune Skands:2014pea [53] for all string processes. If nothing is specified, this option will be generally false unless SMASH is run in the collider modus with an energy per nucleon pair \(\mathtt{Sqrtsnn}\geq 200\,\mathrm{GeV}\).

Example of string parameters customization

Collision_Term:
    Strings: True
    String_Parameters:
        String_Tension: 1.0
        Gluon_Beta: 0.5
        Gluon_Pmin: 0.001
        Quark_Alpha: 2.0
        Quark_Beta: 7.0
        Strange_Supp: 0.16
        Diquark_Supp: 0.036
        Sigma_Perp: 0.42
        StringZ_A_Leading: 0.2
        StringZ_B_Leading: 2.0
        StringZ_A: 2.0
        StringZ_B: 0.55
        String_Sigma_T: 0.5
        Prob_proton_to_d_uu: 0.33
        Separate_Fragment_Baryon: True
        Popcorn_Rate: 0.15