Flavour Selection

The StringFlav class handles the choice of a new flavour in the fragmentation process, and the production of a new hadron from a set of input flavours. It is mainly used by the string fragmentation machinery (including ministrings), but also e.g. in some particle decays and for some beam-remnant cases. The basic concepts are in agreement with [And83].

Currently only simple diquark production of baryons is included; the more sophisticated popcorn scenario remains to be implemented. Therefore the number of parameters on this page will be expanded later on.

New flavours

The main parameters of the selection of a new flavour are

parameter name="StringFlav:probQQtoQ" default="0.10" min="0.0" max="1.0"
the suppression of diquark production relative to quark production, i.e. of baryon relative to meson production.

parameter name="StringFlav:probStoU" default="0.30" min="0.0" max="1.0"
the suppression of s quark production relative to ordinary u or d one.

parameter name="StringFlav:probSQtoQQ" default="0.4" min="0.0" max="1.0"
the suppression of strange diquark production relative to light diquark production, over and above the one already given by probStoU.

parameter name="StringFlav:probQQ1toQQ0" default="0.05" min="0.0" max="1.0"
the suppression of spin 1 diquark production relative to spin 0 one, apart from the factor of 3 enhancement of spin 0 from counting the number of states.

Meson production

For a given set of flavours,the main parameters for meson production are

parameter name="StringFlav:mesonUspin1" default="0.50" min="0." max="1."
the relative fraction of light (u, d) mesons that are chosen to have spin 1, i.e. be vectors, with the remainder having spin 0, i.e. pseudoscalars.

parameter name="StringFlav:mesonSspin1" default="0.60" min="0." max="1."
the relative fraction of strange mesons that are chosen to have spin 1, i.e. be vectors, with the remainder having spin 0, i.e. pseudoscalars.

parameter name="StringFlav:mesonCspin1" default="0.70" min="0." max="1."
the relative fraction of charm mesons that are chosen to have spin 1, i.e. be vectors, with the remainder having spin 0, i.e. pseudoscalars.

parameter name="StringFlav:mesonBspin1" default="0.75" min="0." max="1."
the relative fraction of bottom mesons that are chosen to have spin 1, i.e. be vectors, with the remainder having spin 0, i.e. pseudoscalars.

Inside each light-quark meson nonet, an octet-singlet mixing angle describes the mixing of the two flavour-diagonal isoscalar = 0 states. (For terminology and details see [Eid04], chapter 14 on the quark model.) This angle is needed to specify the probability for such a q qbar state to project onto a specific meson. More transparent formuale are obtained by introducing the angle alpha = theta + 54.7 degrees:
f = (uubar + ddbar)/sqrt(2) * sin(alpha) + ssbar * cos(alpha)
f' = (uubar + ddbar)/sqrt(2) * cos(alpha) - ssbar * sin(alpha)

parameter name="StringFlav:thetaPS" default="-15." min="-90." max="90."
gives the mixing angle theta_PS in the pseudoscalar meson sector (which is rather poorly determined), expressed in degrees. Here f is associated with eta' and f' with eta. (This standard but counterintuitive choice is fixed up in the code by replacing alpha -> 90^0 - alpha so that eta <-> eta'; relative signs do not matter since we are interested in probabilities only.)

parameter name="StringFlav:thetaV" default="36." min="-90." max="90."
gives the mixing angle theta_V in the vector meson sector (which is somewhat better determined), expressed in degrees. Here f is associated with omega and f' with phi.

Further, the simple model overestimates the production of eta and, in particular, eta' mesons, which can be rectified by

parameter name="StringFlav:suppressEta" default="1.0" min="0." max="1."
the additional suppression of eta production, multiplying the normal production probability.

parameter name="StringFlav:suppressEtaPrime" default="0.4" min="0." max="1."
the additional suppression of eta production, multiplying the normal production probability.

Baryon production

The relative rate of baryon production is mainly given by the quark and diquark production parameters plus SU(6) Clebsch-Gordans. The only extra modifiable parameter is

parameter name="StringFlav:suppressDecuplet" default="1.0" min="0.0" max="1.0"
the suppression, relative to default SU(6) factors, of decuplet baryon production.