A set of parameters with which the string fragmentation can be modified.

String_Tension (double, optional, default = 1.0 GeV/fm)
String tension \(\kappa\) 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).

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 GeV)
Smallest possible scale for gluon lightcone momentum. This is divided by sqrts to get the minimum fraction to be sampled from PDF shown above.

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 above.

Strange_Supp (double, optional, default = 0.16)
Strangeness suppression factor \(\lambda\):

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

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

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.

Sigma_Perp (double, optional, default = 0.42 GeV)
Parameter \(\sigma_\perp\) in distribution for transverse momentum transfer between colliding hadrons \(p_\perp\) and string mass \(M_X\):

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

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_B (double, optional, default = 0.55 1/GeV²)
Parameter \(b\) in pythia fragmentation function shown above.

Separate_Fragment_Baryon (bool, optional, default = True)
Whether to use a separate fragmentation function for leading baryons in non-diffractive string processes.

StringZ_A_Leading (double, optional, default = 0.2)
Parameter a in Lund fragmentation function used to sample the light cone momentum fraction of leading baryons in non-diffractive string processes.

StringZ_B_Leading (double, optional, default = 2.0 1/GeV²)
Parameter b in Lund fraghmentation function used to sample the light cone momentum fraction of leading baryons in non-diffractive string processes.

String_Sigma_T (double, optional, default = 0.5 GeV)
Standard deviation in Gaussian for transverse momentum distributed to string fragments during fragmentation.

Form_Time_Factor (double, optional, default = 1.0)
Factor to be multiplied with the formation time of string fragments from the soft string routine.

Power_Particle_Formation (double, optional, default = 1.0 or -1 for collisions at sqrt(s) = 200 GeV or higher)
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.

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

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 \( \tau = \sqrt{2}\frac{m}{\kappa} \).

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.

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).

Examples: Configuring the String Paramters
String fragmentation is activated and if desired, the string parameters can be altered as shown below.

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