<chapter name="Low-energy QCD Processes"> 
 
<h2>Low-energy QCD Processes</h2> 
 
The normal <aloc href="QCDSoftProcesses">Soft QCD Processes</aloc> assume 
sufficiently high energies that a perturbative treatment is possible, 
e.g. that the nondiffractive event class is defined by an MPI machinery 
involving QCD matrix elements and PDFs. This will not work at arbitrary 
low collision energies, and also not for incoming hadrons for which no 
PDFs are available. 
 
<p/> 
The processes in this section offer an alternative, where a simplified 
nonperturbative description is used. No QCD matrix elements or PDFs are 
needed, making the framework more flexible. The lack of hard collisions 
and MPIs do imply that it is only relevant for low energies, and will 
undershoot e.g. the expected charged multiplicity at higher energies. 
Within the <aloc href="BeamParameters">variable-energy beams</aloc> 
framework it is possible to smoothly transition from the low-energy 
processes here to the corresponding higher-energy soft QCD processes. 
 
<p/> 
Technically, the processes here are the same as used in the 
<aloc href="HadronicRescattering">Hadronic Rescattering</aloc> framework, 
i.e. it is the same partial cross sections and the same hadronization 
setup that is used in both cases. The two play different functions, 
however, either representing the primary collision as here or secondary 
rescattering collisions as there. 
 
<p/> 
It is possible to switch on either all or a subset of the allowed 
collisions, as follows. 
 
<flag name="LowEnergyQCD:all" default="off"> 
Common switch for the group of all low-energy nonperturbative 
QCD processes. If this one is off, it is still possible to enable a 
subset from the ones below. 
</flag> 
 
<flag name="LowEnergyQCD:nonDiffractive" default="off"> 
Inelastic nondiffractive processes, simulated as a single gluon 
exchange giving two longitudinal strings stretched between the beam 
remnants. Simplifications will be introduced if the energy is not 
sufficient. Code 151. 
</flag> 
 
<flag name="LowEnergyQCD:elastic" default = "off"> 
Elastic scattering <ei>A B &rarr; A B</ei>. Code 152. 
</flag> 
 
<flag name="LowEnergyQCD:singleDiffractiveXB" default = "off"> 
Single diffractive scattering <ei>A B &rarr; X B</ei>. Here 
<ei>X</ei> represents a single longitudinal string. Code 153. 
</flag> 
 
<flag name="LowEnergyQCD:singleDiffractiveAX" default = "off"> 
Single diffractive scattering <ei>A B &rarr; A X</ei>. Code 154. 
</flag> 
 
<flag name="LowEnergyQCD:doubleDiffractive" default = "off"> 
Double diffractive scattering <ei>A B &rarr; X_1 X_2</ei>. Code 155. 
</flag> 
 
<flag name="LowEnergyQCD:excitation" default = "off"> 
Only relevant for <ei>N N</ei> or <ei>Nbar Nbar</ei> collisions, where 
<ei>N</ei> is a <ei>p</ei> or <ei>n</ei>. Either or both are excited 
to higher exclusive states, so it works as a low-mass variant of 
single or double diffraction, where <ei>X</ei> is replaced by a single 
hadron. Code 157. 
</flag> 
 
<flag name="LowEnergyQCD:annihilation" default = "off"> 
Main application for baryon-antibaryon collisions, where it requires 
at least one matching valence quark-antiquark pair to annihilate, 
with colour flow between the leftovers such that the baryon numbers 
are annihilated. See further <code>probDoubleAnnihilation</code> 
below. Code 158. 
</flag> 
 
<flag name="LowEnergyQCD:resonant" default = "off"> 
Scattering <ei>A B &rarr; X &rarr; C D</ei> via an intermediate 
resonance state <ei>X</ei>. The final state may agree with the initial 
one, but is distinguished from elastic scattering by a quite different 
angular decay distribution. Code 159. 
</flag> 
 
<flag name="LowEnergyQCD:inelastic" default = "off"> 
All the processes above, except for elastic scattering. 
</flag> 
 
<p/> 
There are also some settings that can be used to modify the behaviour 
or cross section of the processes above. 
 
<parm name="LowEnergyQCD:probDoubleAnnihilation" default="0.2" 
min="0." max="1.0"> 
If only one pair can annihilate, then the remnants of a baryon-antibaryon 
collision are combined into two quark-antiquark strings. If two pairs 
can annihilate, then with this probability they do, leaving only a single 
quark-antiquark string. For the rest two strings are formed as before. 
</parm> 
 
<parm name="LowEnergyQCD:sEffAQM" default="0.6" min="0." max="1.0"> 
</parm> 
<parmmore name="LowEnergyQCD:cEffAQM" default="0.2" min="0." max="1.0"> 
</parmmore> 
<parmmore name="LowEnergyQCD:bEffAQM" default="0.07" min="0." max="1.0"> 
Cross sections for many strange hadrons and all charm and bottom ones 
are unmeasured. In such cases the Additive Quark Model offers a simple 
approximate recipe to extrapolate unknown cross sections from known 
ones, where the colliding hadrons only consist of <ei>u</ei> and 
<ei>d</ei> quarks. To this end, cross sections are assumed to be 
proportional to the product of the effective number of quarks in each 
of the two colliding hadrons. The numbers above indicate how much an 
<ei>s</ei>, <ei>c</ei> or <ei>b</ei> quark contributes, where a 
<ei>u</ei> or <ei>d</ei> is normalized to unity. The assumption in the 
choice of default values  is that these factors scale inversely with 
the respective quark constituent mass. 
</parmmore> 
 
<flag name="LowEnergyQCD:useSummedResonances" default="off"> 
By default, <ei>pi pi</ei> and <ei>pi K</ei> cross sections are 
calculated using the parameterization of Pel&agrave;ez et al. When 
this option is on, these cross sections are instead calculated by 
summing Breit-Wigner forms for each resonance. 
</flag> 
 
</chapter> 
 
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