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Version: SMASH-3.4
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Version: SMASH-3.4
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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 = 0.04 — Conditions on key value: \(0\leq x\leq 1\)
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 — Conditions on key value: \(x>0\)
Factor to be multiplied with the formation time of string fragments from the soft string routine.
Formation_Time — double, optional, default = 1.0 — Conditions on key value: \(x>0\)
Parameter for formation time in string fragmentation, in fm.
Gluon_Beta — double, optional, default = 0.5 — Conditions on key value: \(x>0\)
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 — Conditions on key value: \(x>0\)
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 — Conditions on key value: None
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 — Conditions on key value: \(x>0\)
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 — Conditions on key value: \(x>0\)
Parameter \(\beta\) in PDF for quarks shown in Quark_Alpha.
Popcorn_Rate — double, optional, default = 0.5 — Conditions on key value: \(0\leq x\leq 2\)
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).
Damp_Popcorn — double, optional, default = 0.5 — Conditions on key value: \(0\leq x\leq 1\)
Controls whether a diquark endpoint may hadronize via the popcorn mechanism into a leading meson before producing the baryon.
A value of \(1\) corresponds to normal popcorn production, while \(0\) suppresses popcorn completely such that the diquark always fragments directly into a leading baryon. Intermediate values interpolate between these two limits.
Corresponds to Pythia's BeamRemnants:dampPopcorn parameter.
Power_Particle_Formation — double, optional, default = ±1 — Conditions on key value: None
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 — Conditions on key value: \(0<x\leq 1\)
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 — Conditions on key value: None
Whether to use a separate fragmentation function for leading baryons in non-diffractive string processes.
Sigma_Perp — double, optional, default = 0.42 — Conditions on key value: \(x>0\)
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 — Conditions on key value: \(0\leq x\leq 1\)
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 — Conditions on key value: \(0\le x\le 1\)
Standard deviation in GeV in Gaussian for transverse momentum distributed to string fragments during fragmentation.
String_Tension — double, optional, default = 1.0 — Conditions on key value: \(x\geq 0\)
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 = 1.0 — Conditions on key value: \(0\leq x\leq 2\)
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 = 0.0 — Conditions on key value: \(0\leq x\leq 2\)
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 = 0.3 — Conditions on key value: \(0\leq x\leq 2\)
Parameter \(b\) in 1/GeV² in Pythia fragmentation function shown in StringZ_A.
StringZ_B_Leading — double, optional, default = 3.0 — Conditions on key value: \(0.2\leq x\leq 5\)
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 = false — Conditions on key value: None
Whether to use the Monash tune Skands:2014pea [61] for all string processes. By default, the Monash tune is disabled.
Unformed_Xsec_Suppression — double, optional, default = 0.7 — Conditions on key value: \(0 \le x \le 1\)
Applies an additional suppression factor to the interaction cross sections of unformed hadrons.
This parameter rescales the effective cross sections of hadrons during their formation time and can be used to tune the interaction strength of unformed hadrons in dense environments.
Notes:
Pythia_Settings — list of strings, optional, default = [] — Conditions on key value: None
Additional Pythia 8 settings passed directly to the internal Pythia instances used for string fragmentation.
In the YAML configuration file, provide each Pythia 8 setting as a separate string:
In the future, this option will replace SMASH input parameters that map one-to-one to individual Pythia 8 settings.
The settings are applied after the corresponding SMASH string parameters. Therefore, if the same Pythia 8 setting is configured both through a SMASH input key and through Pythia_Settings, the value specified in Pythia_Settings takes precedence.
Invalid Pythia 8 settings cause SMASH to terminate during initialization.
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