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
parm 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.
parm 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.
parm 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.
parm 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
parm 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.
parm 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.
parm 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.
parm 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)
parm 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.)
parm 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
parm name="StringFlav:suppressEta" default="1.0" min="0." max="1."
the additional suppression of eta production, multiplying the
normal production probability.
parm 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
parm name="StringFlav:suppressDecuplet" default="1.0"
min="0.0" max="1.0"
the suppression, relative to default SU(6) factors, of decuplet
baryon production.