<chapter name="Hadron Scattering"> 
 
<h2>Hadron Scattering</h2> 
 
A simple hadron scattering model. It is intended to take into account 
that the overlap of multiple strings at low transverse dimensions 
is likely to lead to some collective effects, not unlike those 
observed in heavy-ion collisions, even if not quite as pronounced. 
Specifically, it is assumed that the hadrons produced can scatter 
against each other on the way out, before the fragmenting system 
has had time to expand enough that the hadrons get free. Thereby 
heavier particles are shifted to higher transverse momenta, at the 
expense of the lighter ones. 
 
<note>Warning:</note> This is still at an experimental level, 
and should not be used unless you know what you are doing. 
 
<flag name="HadronScatter:scatter" default="off"> 
Master flag for hadron scattering. 
</flag> 
 
<flag name="HadronScatter:afterDecay" default="off"> 
Perform hadron scattering before or after first round of decays, 
involving very short-lived particles like the <ei>rho</ei>. 
The default is to perform scattering directly after the 
string fragmentation, before any decays. 
</flag> 
 
<flag name="HadronScatter:allowDecayProd" default="off"> 
Allow two hadrons with same parent hadron to scatter. 
</flag> 
 
<flag name="HadronScatter:scatterRepeat" default="off"> 
Allow hadrons which have already scattered to scatter again. 
Even if switched on, the same pair can not scatter off each 
other twice. 
</flag> 
 
<h3>Hadron selection</h3> 
 
<modepick name="HadronScatter:hadronSelect" default="0" 
min="0" max="0"> 
Probability that a hadron is soft enough to scatter. 
(A high-<ei>pT</ei> hadron presumably being part of a jet, 
and thus produced away from the high-particle-density region 
at small transverse dimensions.) 
<option value="0"> 
<ei>P = N exp(-pT^2 / 2 / sigma^2) / 
  ( (1 - k) exp(-pT^2 / 2 / sigma^2) + k pT0^p / (pT0^2 + pT^2)^(p/2), </ei> 
with <ei>sigma = 2 StringPT:sigma</ei> and <ei>pT0</ei> the same as that 
used in <ei>MultipartonInteractions</ei>. 
</option> 
</modepick> 
 
<parm name="HadronScatter:N" default="1.0" min="0.01" max="1.0"> 
<ei>N</ei> parameter as above. 
</parm> 
<parm name="HadronScatter:k" default="1.0" min="0.01" max="1.0"> 
<ei>k</ei> parameter as above. 
</parm> 
<parm name="HadronScatter:p" default="6" min="2" max="30"> 
<ei>p</ei> parameter as above. 
</parm> 
 
<h3>Scattering probability</h3> 
 
<modepick name="HadronScatter:scatterProb" default="0" 
min="0" max="2"> 
Probability for a pair of hadrons to scatter. 
<option value="0">All hadrons scatter with probability 
<ei>j max(0, 1 - dR^2 / rMax^2)</ei>. Angular distribution 
is picked flat in <ei>cos(theta).</ei></option> 
<option value="1">As option 0, above, but only <ei>pi-pi</ei>, 
<ei>pi-K</ei> and <ei>pi-p</ei> scatterings are considered. 
</option> 
<option value="2">Only <ei>pi-pi</ei>, <ei>pi-K</ei> and 
<ei>pi-p</ei> scatterings are considered, with probability 
given by <ei>(1 - exp(-j sigEl)) max(0, 1 - dR^2 / rMax^2)</ei>. 
The elastic cross sections and angular distributions are taken 
from the partial-wave distributions. 
</option> 
</modepick> 
 
<parm name="HadronScatter:j" default="0.5" min="0.0", max="10.0"> 
<ei>j</ei> parameter as above. 
</parm> 
<parm name="HadronScatter:rMax" default="0.5" min="0.1" max="2.0"> 
<ei>rMax</ei> parameter as above. 
</parm> 
 
<flag name="HadronScatter:tile" default="off"> 
Use tiling in <ei>(eta, phi)</ei> to reduce number of pairwise tests. 
</flag> 
 
</chapter> 
 
<!-- Copyright (C) 2014 Torbjorn Sjostrand --> 
 
