pT Selection
The StringPT class handles the choice of fragmentation
transverse momentum pT. At each string breaking the quark
and antiquark of the produced pair, or the diquark and antidiquark,
are supposed to receive opposite and compensating pT kicks.
How they are distributed depends on the following flag:
In more detail, the quarks receive pT kicks according to a
Gaussian distribution in p_x and p_y separately.
Call sigma_q the width of the p_x and p_y
distributions separately, i.e.
d(Prob) = exp( -(p_x^2 + p_y^2) / 2 sigma_q^2).
Then the total squared width is
<pT^2> = <p_x^2> + <p_y^2> = 2 sigma_q^2 = sigma^2.
It is this latter number that is stored in
parm StringPT:sigma
(default = 0.335; minimum = 0.0; maximum = 1.0)
the width sigma in the fragmentation process.
Since a normal hadron receives pT contributions for two string
breakings, it has a
<p_x^2>_had = <p_y^2>_had = sigma^2,
and thus <pT^2>_had = 2 sigma^2.
Some studies on isolated particles at LEP has indicated the need for
a slightly enhanced rate in the high-pT tail of the above
distribution. This would have to be reviewed in the context of a
complete retune of parton showers and hadronization, but for the
moment we stay with the current recipe, to boost the above pT
by a factor enhancedWidth for a small fraction
enhancedFraction of the breakups, where
parm StringPT:enhancedFraction
(default = 0.01; minimum = 0.0; maximum = 1.)
enhancedFraction,the fraction of string breaks with enhanced
width.
parm StringPT:enhancedWidth
(default = 2.0; minimum = 1.0; maximum = 10.0)
enhancedWidth,the enhancement of the width in this fraction.
In the context of some toy studies [Fis16] the following three
options have also been introduced, but are not part of any recommended
framework.
parm StringPT:widthPreStrange
(default = 1.0; minimum = 1.0; maximum = 10.0)
Prefactor multiplying the Gaussian width for strange quarks.
parm StringPT:widthPreQQ0
(default = 1.0; minimum = 1.0; maximum = 10.0)
Prefactor multiplying the Gaussian width for spin-0 diquarks. In case of
diquarks with one or two strange quarks the prefactor is calculated by
multiplying widthPreQQ0 once or twice respectively with
widthPreStrange.
parm StringPT:widthPreQQ1
(default = 1.0; minimum = 1.0; maximum = 10.0)
Prefactor multiplying the Gaussian width for spin-1 diquarks. In case of
diquarks with one or two strange quarks the prefactor is calculated by
multiplying widthPreQQ1 once or twice respectively with
widthPreStrange.