## Low-energy QCD Processes

The normal Soft QCD Processes 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.

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 variable-energy beams framework it is possible to smoothly transition from the low-energy processes here to the corresponding higher-energy soft QCD processes.

Technically, the processes here are the same as used in the Hadronic Rescattering 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.

It is possible to switch on either all or a subset of the allowed collisions, as follows.

`flag` 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` 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` LowEnergyQCD:elastic
Elastic scattering A B → A B. Code 152.

`flag` LowEnergyQCD:singleDiffractiveXB
Single diffractive scattering A B → X B. Here X represents a single longitudinal string. Code 153.

`flag` LowEnergyQCD:singleDiffractiveAX
Single diffractive scattering A B → A X. Code 154.

`flag` LowEnergyQCD:doubleDiffractive
Double diffractive scattering A B → X_1 X_2. Code 155.

`flag` LowEnergyQCD:excitation
Only relevant for N N or Nbar Nbar collisions, where N is a p or n. Either or both are excited to higher exclusive states, so it works as a low-mass variant of single or double diffraction, where X is replaced by a single hadron. Code 157.

`flag` LowEnergyQCD:annihilation
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 `probDoubleAnnihilation` below. Code 158.

`flag` LowEnergyQCD:resonant
Scattering A B → X → C D via an intermediate resonance state X. The final state may agree with the initial one, but is distinguished from elastic scattering by a quite different angular decay distribution. Code 159.

There are also some settings that can be used to modify the behaviour or cross section of the processes above.

`parm` LowEnergyQCD:probDoubleAnnihilation (`default = 0.2`; `minimum = 0.`; `maximum = 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` LowEnergyQCD:sEffAQM (`default = 0.6`; `minimum = 0.`; `maximum = 1.0`)
`parm` LowEnergyQCD:cEffAQM (`default = 0.2`; `minimum = 0.`; `maximum = 1.0`)
`parm` LowEnergyQCD:bEffAQM (`default = 0.07`; `minimum = 0.`; `maximum = 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 u and d 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 s, c or b quark contributes, where a u or d is normalized to unity. The assumption in the choice of default values is that these factors scale inversely with the respective quark constituent mass.

`flag` LowEnergyQCD:useSummedResonances (`default = off`)
By default, pi pi and pi K cross sections are calculated using the parameterization of Pelàez et al. When this option is on, these cross sections are instead calculated by summing Breit-Wigner forms for each resonance.