<chapter name="PTSelection"> 
 
<h2>pT Selection</h2> 
 
The <code>StringPT</code> class handles the choice of fragmentation 
transverse momentum <ei>pT</ei>. At each string breaking the quark 
and antiquark of the produced pair, or the diquark and antidiquark, 
are supposed to receive opposite and compensating <ei>pT</ei> kicks. 
How they are distributed depends on the following flag: 
 
<p/> 
In more detail, the quarks receive <ei>pT</ei> kicks according to a 
Gaussian distribution in <ei>p_x</ei> and <ei>p_y</ei> separately. 
Call <ei>sigma_q</ei> the width of the <ei>p_x</ei> and  <ei>p_y</ei> 
distributions separately, i.e. 
<eq> 
    d(Prob) = exp( -(p_x^2 + p_y^2) / 2 sigma_q^2). 
</eq> 
Then the total squared width is 
<eq> 
    &lt;pT^2> = &lt;p_x^2> +  &lt;p_y^2> = 2 sigma_q^2 = sigma^2. 
</eq> 
It is this latter number that is stored in 
 
<parm name="StringPT:sigma" default="0.335" min="0.0" max="1.0"> 
the width <ei>sigma</ei> in the fragmentation process. 
</parm> 
 
<p/> 
Since a normal hadron receives <ei>pT</ei> contributions for two string 
breakings, it has a 
<eq> 
    &lt;p_x^2>_had = &lt;p_y^2>_had = sigma^2, 
</eq> 
and thus <ei>&lt;pT^2>_had = 2 sigma^2</ei>. 
 
<p/> 
Some studies on isolated particles at LEP has indicated the need for 
a slightly enhanced rate in the high-<ei>pT</ei> 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 <ei>pT</ei> 
by a factor <ei>enhancedWidth</ei> for a small fraction 
<ei>enhancedFraction</ei> of the breakups, where 
 
<parm name="StringPT:enhancedFraction" default="0.01" min="0.0" max="1."> 
<ei>enhancedFraction</ei>,the fraction of string breaks with enhanced 
width. 
</parm> 
 
<parm name="StringPT:enhancedWidth" default="2.0" min="1.0" max="10.0"> 
<ei>enhancedWidth</ei>,the enhancement of the width in this fraction. 
</parm> 
 
<p/> 
In the context of some toy studies <ref>Fis16</ref> the following three 
options have also been introduced, but are not part of any recommended 
framework. 
 
<parm name="StringPT:widthPreStrange" default="1.0" min="1.0" max="10.0"> 
Prefactor multiplying the Gaussian width for strange quarks. 
</parm> 
 
<parm name="StringPT:widthPreQQ0" default="1.0" min="1.0" max="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 <ei>widthPreQQ0</ei> once or twice respectively with 
<ei>widthPreStrange</ei>. 
</parm> 
 
<parm name="StringPT:widthPreQQ1" default="1.0" min="1.0" max="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 <ei>widthPreQQ1</ei> once or twice respectively with 
<ei>widthPreStrange</ei>. 
</parm> 
 
</chapter> 
 
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