Heavy Ion Collisions

  1. Heavy Ion beam selection
  2. Top level flags
  3. Nucleus Models - the nuclear geometry
  4. Nucleons and subcollisions
  5. Obtaining event information
  6. Angantyr - the default heavy ion model
PYTHIA includes a facility in which models for heavy ion collisions can be implemented through models which combine several nucleon-nucleon collisions into one heavy ion collision. One such model, called Angantyr, is provided with PYTHIA and is in part inspired by the old Fritiof program from the Lund group [And87] with recent improvements [Bie16a].

In the following documentation, there is a split between the general facility in which a heavy ion model can be implemented, and the concrete implementation of Angantyr which uses the facility to implement a model. In order to run Angantyr, several settings from the base facility must, however, be used, so it is not advisible simply to skip to that section.

The implementation of the heavy ion facility is split between two main classes and a number of helper classes. The main classes are:

The HIUserHooks class is provided to simplify the customization of a model implemented as a HeavyIons subclass. It can be used to eg. change the ImpactParameterGenerator used, in a way similar to how the UserHooks and MergingHooks classes are used.

The HeavyIons class is very simple and flexible and basically only specifies that the HeavyIons::init() and HeavyIons::next() functions are overridden in a subclass. But the additional helper classes should be generic enough to be used by any model implemented.

Heavy Ion beam selection

A heavy ion beam is selected in the same way as any other beams' beam parameters, ie. by specifying the PDG particle id for the two beams. A PDG id code for nuclei is on the form 100ZZZAAAI. To be able to specify a certain nucleus as a beam particle, the particle must be available in the particle data scheme, a number of nuclei are already added, including negative numbers for the corresponding anti-particle:
More can be added by the user, using the function ParticleData::addParticle, documented in the particle data scheme, but be mindful that a suitable model geometry must be available for the nucleus added, see the available nucleus models below. Angantyr supports beam switching if Beams:allowIDAswitch is turned on. In this case, idA is allowed to be any hadron, while idB is allowed to be a proton, nucleon or any ion.

Top level flags

mode  HeavyIon:mode   (default = 1; minimum = 1; maximum = 2)
This is the master switch for heavy ions, and determines the mode of operation of the HeavyIon model.
option 1 : The heavy ion machinery will be used in case of ion beams.
option 2 : Even collisions without ions are treated using the heavy ion machinery. (Typically only used for debugging purposes.)

If HeavyIon:mode is on, the normal initialization in Pythia::init() is early on diverted to an object with the base class HeavyIons which may instantiate secondary Pythia objects needed to generate different types of nucleon-nucleon collisions that can be merged together into a full heavy ion event. This is all done in the virtual HeavyIons::init() function. Subsequent calls to Pythia::next() will then also be diverted to the virtual function HeavyIons::next() which will be responsible for building up the heavy ion collision. The final event will be available in the primary Pythia object.

flag  HeavyIon:showInit   (default = on)
Please note: this flag is not currently in use due to conflicts with the Parallelism framework, but is expected to be reintroduced in the future. Output detailed initialization information from the heavy ion model. Typically there will be several Pythia object initialised in a heavy ion run, this flag can be used to reduce the amount of output. If off, only the output from initialisation of the primary Pythia object will be shown. The impact parameter between two colliding nuclei, is sampled by the code contained in ImpactParameterGenerator. The base class implements a Gaussian sampling, which means that the events produced will always be weighted. Other distributions can be implemented in subclasses.

parm  HeavyIon:bWidth   (default = 0.0; minimum = 0.0)
The width in fermi of the distribution by which the impact parameter is sampled. If zero, a suitable width will be guessed by the ImpactParameterGenerator itself.

Nucleus Models - the nuclear geometry

Several models and parametrizations exist to describe the distribution in the radial density of nucleons in a nucleus. The NucleusModel class is a base class for implementing such models. There are five ready-made models implemented already. When using Angantyr, the nuclear models can be selected using the following settings.

mode  Angantyr:NucleusModelA   (default = 1; minimum = 1; maximum = 5)

mode  Angantyr:NucleusModelB   (default = 1; minimum = 1; maximum = 5)
Select the model for nuclear geometry for beam A and B respectively. Models and associated parameters are documented below.
option 1 : GLISSANDO Woods-Saxon. This default model is suitable for large nuclei.
option 2 : Standard Woods-Saxon.
option 3 : Harmonic Oscillator Shell model.
option 4 : Gaussian model.
option 5 : Hulthen model.

For large nuclei, the Woods-Saxon potential is suitable. The WoodsSaxonModel, implements a standard Woods-Saxon distribution with parameters to be input by the user, and the GLISSANDOModel, implements the more advanced model from [Bro09,Ryb14]. For light nuclei (16O and lighter), several options are implemented. The HulthenModel implements the Hulthen potential suitable for deuterons, the HOShellModel a harmonic oscillator shell model density and GaussianModel a Gaussian radial distribution. Finally it is possible to read in externally generated nuclear configurations from a file, in the ExternalNucleusModel.

If a user wishes to specify own parameters for the NucleusModels, parameters must be specified for beam A and B separately, using prefix HeavyIonA: or HeavyIonB: as specified below.

Standard Woods-Saxon

For sufficiently large nuclei, the Woods-Saxon distribution:
rho(r) = rho_0/(1 + exp((r-R)/a))
is a suitable choice. The model has two parameters, which can be set by:

parm  HeavyIonA:WSR   (default = 0.0; minimum = 0.0)

parm  HeavyIonB:WSR   (default = 0.0; minimum = 0.0)
The radius of a nucleon in units of fermi in the default Woods-Saxon model for nucleon distributions. If zero, the size is given by the formulae [Ryb14], based on the number of nucleons in the nuclei and whether a hard core is used or not.

parm  HeavyIonA:WSa   (default = 0.0; minimum = 0.0)

parm  HeavyIonB:WSa   (default = 0.0; minimum = 0.0)
The skin width of a nucleus in units of fermi in the default Woods-Saxon model for nucleon distributions. If zero, the size is given by the numbers in [Ryb14], based on the number of nucleons in the nucleus and whether a hard core is used or not.


In [Ryb14] it was shown that for a=0.54 fm, and sufficiently heavy nuclei, the Woods-Saxon radius parameter is well approximated by:
R = (1.1 A^(1/3) - 0.656 A^(-1/3)).
These values are used for all nuclei when this model is selected.

Harmonic Oscillator Shell

For lighter nuclei, a two-parameter Harmonic Oscillator Shell model is implemented. Here:
rho(r) = 4/(pi^(3/2)C^3)(1 + (A - 4)/6 * (r/C)^2)exp(-r^2/C^2) with:
C = (5/2 - 4/A)^(-1) (< r^2>_A - <r^2>_p).
The model parameters can be set by:

parm  HeavyIonA:HONuclearChargeRadius   (default = 0.0; minimum = 0.0)

parm  HeavyIonB:HONuclearChargeRadius   (default = 0.0; minimum = 0.0)
Use custom nuclear charge radius squared. If nothing is selected, the program includes reasonable defaults for a number of often used nuclei. Units of fm^2.

parm  HeavyIonA:HOProtonChargeRadius   (default = 0.7714; minimum = 0.0)

parm  HeavyIonB:HOProtonChargeRadius   (default = 0.7714; minimum = 0.0)
The proton charge radius squared. Units of fm^2.

Gaussian model

The Gaussian model parametrizes the nuclear radial density as a Gaussian distribution. While this may not be a fully adequate physics based model, it provides a starting point for parametrization of some light nuclei. The model has a single parameter, the with of the distribution, intepreted as a nuclear charge radius.

parm  HeavyIonA:GaussianChargeRadius   (default = 7.7; minimum = 0.0)

parm  HeavyIonB:GaussianChargeRadius   (default = 7.7; minimum = 0.0)
The charge radius of the Gaussian parametrization. Units of fm. The default value gives a reasonable description of O16.

Hulthén model

The Hulthén distribution [Hul42] is given by:
rho(r) = rho_0(exp(-a r) - exp(-b r))
where b term regulates the a term at small distances so that rho(0) = 0. The a term can be written as sqrt(m E), where m is the mass of the nucleon and E is the binding energy. The b parameter can be written in terms of a using the triplet effective range parameter as roughly 3a [Adl75].

parm  HeavyIonA:HulthenA   (default = 0.4; minimum = 0.0)

parm  HeavyIonB:HulthenA   (default = 0.4; minimum = 0.0)
Hulthen a-parameter, units of fm^-1. Note that b > a > 0 always.

parm  HeavyIonA:HulthenB   (default = 1.2; minimum = 0.0)

parm  HeavyIonB:HulthenB   (default = 1.2; minimum = 0.0)
Hulthen b-parameter, units of fm^-1. Note that b > a > 0 always.

Externally generated configuration

For certain purposes it might be appropriate for a user to read in nuclear configurations from a file stored on disk, produced by another program. The ExternalNucleusModel acts as an interface for this operation. The file must be of the following format:
Isospin information cannot currently by added, and is assigned randomly.
When the configurations are read in, they will be shuffled randomly, in order to avoid collisions between identical configurations. When the list of configurations is exhausted, elements will be reshuffled again.
Note: All configurations in the given file will be read in when Pythia is initialized. If your file is very large, this will consume a large amount of memory, so this is typically only advised for a few thousand configurations, or at least not orders of magnitude more.

word  HeavyIonA:externalNucleusFile  

word  HeavyIonB:externalNucleusFile  
The name of the file to be read in. Must be accessible, no default provided.

flag  HeavyIonA:externalShuffle   (default = on)

flag  HeavyIonB:externalShuffle   (default = on)
Randomly shuffle configurations when read in, and when the list is exhausted. Turning shuffling off is allowed mostly for debug purposes.

Nucleon hard core: Common parameters

As an attempt to model multi-nucleon correlations inside nuclei, nucleons have a "hard core", which cannot overlap with other nucleons' hard cores, when nucleons are sampled. All models except for the Hulthen model (where the same behaviour is directly encoded into the density function) have the possibility for either a fixed-size hard core of a given width, or a Gaussian hard core, where the hard core radius is sampled from a Gaussian at the per-nucleon level. The hard core repulsion is switched on by default.

flag  HeavyIonA:HardCore   (default = on)

flag  HeavyIonB:HardCore   (default = on)
Assume that there is a minimum distance between nucleons defined by a hard core radius in HeavyIon:HardCoreRadius.

parm  HeavyIonA:HardCoreRadius   (default = 0.9; minimum = 0.0)

parm  HeavyIonB:HardCoreRadius   (default = 0.9; minimum = 0.0)
The hard core radius in units of fm, defining the minimum distance between nucleons in a nucleus.

flag  HeavyIonA:GaussHardCore   (default = off)

flag  HeavyIonB:GaussHardCore   (default = off)
Option to use a Gaussian profile of the hard core instead of a sharp cut-off, inspired by [Bay95].

Nucleons and subcollisions

The geometry models above, wil generate two lists of Nucleons. Collisions between individual projectile and target nucleons are generated by SubCollisionModels, outputting individual, potential nucleon-nucleon SubCollisions.


The Nucleon class represents a single nucleon in a nucleus. The class contains several methods for accessing information and modifying state, here only the ones deemed useful for a user are mentioned.

int Nucleon::id()  
The nucleon particle id. Currently limited to protons and neutrons, ie. no hypernuclei possible.

const Vec4& Nucleon::nPos()  
The nucleon position relative to the nucleus center.

const Vec4& Nucleon::bPos()  
The nucleon absolute position in impact parameter space.

void Nucleon::bShift(const Vec4& bvec)  
Shift the nucleon absolute position in impact parameter space.

Status Nucleon::status()  
The nucleon status, meaning whether and how it is participating. It can take the following values:


The SubCollision class represents a potential nucleon-nucleon collision between a projectile and a target Nucleon. From a subcollision, pointers to projectile and target nucleons participating can be accessed:

Nucleon* SubCollision::proj  
Nucleon* SubCollision::targ  
Pointers to projectile and target nucleons are public members of the SubCollision class The impact parameter distance between nucleons participating in a given subcollision can also be accessed directly.

double SubCollision::b  
double SubCollision::bp  
The impact parameter distance between the two participating nuclei, respectively in units of fm and scaled like in Pythia to have unit average for non-diffractive collisions. Finally the type of subcollision can be accessed by a method:

CollisionType SubCollision::type()  
The subcollision type, meaning how the two nucleons interacted. It can take the following values:

Subcollision models

Presently three SubCollisionModels are available. The BlackSubCollisionModel, which is the simplest black disk model possible, the NaiveSubCollisionModel which on top of a black disk model, allows less central subcollisions to be diffractive or elastic, and a more elaborate treatment called DoubleStrikman, described below.

The main idea behind currently implemented SubCollisionModels, is that parameters should be fitted from semi-inclusive cross sections. Thus, the code includes a fitting machinery, based on a simple genetic algorithm. Cross sections are taken form the standard Pythia total cross sections. Parameters for the models are therefore not set in the normal sense, but initial values for the fit are set instead, an example for the default model given below.

The default model for nucleon fluctuations has three parameters, the general fitting machinery in SubCollisionModel allows for up to eight parameters.

pvec  HeavyIon:SigFitDefPar   (default = {})
These are the default values of the parameters of the SubCollisionModel in Angantyr. They will be used as starting point when fitting to the inclusive nucleon cross sections. If left empty, the values will be set to the model default.

The fitting procedure in SubCollisionModel is a kind of genetic algorith where a population of parameter values are allowed to evolve for a number of generations. In the end the the parameter set in the final population which gives the best inclusive cross sections is picked. Eight different cross sections may be fitted to but it is possible to select only some of them:

pvec  HeavyIon:SigFitErr   (default = {0.02,0.02,0.1,0.05,0.05,0.0,0.1,0.15}; minimum = 0.0; maximum = 1.0)
The relative error assumed in the calculation of goodness of fit corresponding to the different cross sections fitted to. The cross sections are obtained from the SigmaTotal and are given as (in order) total, non-diffractive, double diffractive, wounded target, wounded projectile, central diffractive, and elastic cross sections, and in addition the elastic slope parameter. A relative error of zero for one of these cross sections means the corresponding cross section not be used in the fit.

mode  HeavyIon:SigFitNInt   (default = 100000; minimum = 0)
The number of integration points used for each parameter setting to calculate the cross sections.

mode  HeavyIon:SigFitNPop   (default = 20; minimum = 1)
The number individuals (parameter settings) in a population in each generation.

mode  HeavyIon:SigFitNGen   (default = 20; minimum = 0)
The number of generation used in the genetic algorithm. If set to zero, no fitting will be performed and the values in HeavyIon:SigFitDefPar will be used.

parm  HeavyIon:SigFitFuzz   (default = 0.2; minimum = 0.0; maximum = 0.5)
A parameter determining the probability that an individual parameter setting will evolves further away from the best parameter set in each generation.

flag  HeavyIon:SigFitPrint   (default = on)
Controls the output from the fitting procedure. If on, extensive information about the fitting will be printed.

Obtaining event information

The HIInfo class contains information related to the generated heavy ion events. In a run, it can be obtained from the usual Info object, by calling pythia.info.hiInfo(), which returns a pointer to the current HIInfo object. The methods of this class are:

double HIInfo::b()  
The impact parameter distance of the heavy ion event.

double HIInfo::sigmaTot()  
The integrated total cross section in the current run (in millibarn).

double HIInfo::sigmaTotErr()  
The estimated statistical error on sigmaTot().

double HIInfo::sigmaND()  
The integrated non-diffractive cross section in the current run.

double HIInfo::sigmaNDErr()  
The estimated error on sigmaND().

int HIInfo::nAttempts()  
The number of attempted impact parameter points.

int HIInfo::nAccepted()  
The number of produced events. Several methods exist to extract information from the Glauber calculation. Caution should be taken when using this information directly, under the assumption that obtained numbers correspond one-to-one to numbers obtained from another calculation, as performed eg. by an experiment. Since they are all model dependent quantities, a direct comparison can often not be made.

A user is often better off by extracting the full set of subcollisions, and extracting the relevant information from that.

multiset<SubCollision>* HIInfo::subCollisionsPtr()  
A (pointer to) a multiset containing all SubCollisions of the current event. Can be modified by the user.

void HIInfo::subCollisionsPtr(multiset<SubCollision> * sPtrIn)  
Sets this event's subcollision. Starting point for a user who wishes to interface their own or an external Glauber calculation.

int HIInfo::nCollTot()  
The total number of separate subcollisions in the current event.

int HIInfo::nCollND()  
The number of separate non-diffractive subcollisions in the current event.

int HIInfo::nCollNDTot()  
The total number of non-diffractive subcollisions in the current event.

int HIInfo::nCollSDP()  
The number of separate single diffractive projectile excitation subcollisions in the current event.

int HIInfo::nCollSDT()  
The number of separate single diffractive target excitation subcollisions in the current event.

int HIInfo::nCollDD()  
The number of separate double diffractive subcollisions in the current event.

int HIInfo::nCollCD()  
The number of separate central diffractive subcollisions in the current event.

int HIInfo::nCollEL  
The number of separate elastic subcollisions in the current event.

int HIInfo::nPartProj()  
int HIInfo::nPartTarg()  
The number of interacting projectile/target nucleons in the current event (number of participants).

int HIInfo::nAbsProj()  
int HIInfo::nAbsTarg()  
The number of absorptively interacting projectile/target nucleons in the current event (number of absorptive participants).

int HIInfo::nDiffProj()  
int HIInfo::nDiffTarg()  
The number of diffractively interacting projectile/target nucleons in the current event (number of diffractive participants).

int HIInfo::nElProj()  
int HIInfo::nElTarg()  
The number of elastically interacting projectile/target nucleons in the current event (number of elastic participants).

double HIInfo::weight()  
The weight for this collision. Note that heavy ion weights are propagated to the normal weights system, and a call to pythia.info.weight() contains the heavy ion weight as well. Normally a user would have no reason to access this method directly.

double HIInfo::weightSum()  
The sum of weights for all produced events up to this point. Note that heavy ion weights are propagated to the normal weights system, and a call to pythia.info.weightSum() contains the heavy ion weights as well. Normally a user would have no reason to access this method directly.

Angantyr - the default heavy ion model

The default model in PYTHIA is called Angantyr and is inspired by the old Fritiof model [And86] with improvements described in [Bie16a]. The main idea is to stack parton level events, corresponding to individual nucleon-nucleon sub-collisions, on top of each other and hadronize them together.

Please note:

To determine which projectile nucleon interacts with which target nucleon, special care is taken to determine in which way the nucleons interact. In a standard Glauber calculations one typicaly only cares about if a sub-collision is inelastic or not, but in Angantyr this is divided up, so that each inelastic sub-collision can either be single-diffractive, double-diffractive or absorptive (ie. non-diffractive). To achieve this, Angantyr uses a model with fluctuating radii of the nucleons resulting in a fluctuating nucleon-nucleon cross section inspired by the model by Strikman et al. [Alv13], called DoubleStrikman. The model for this includes a number of parameters which should be fitted to reproduce inclusive nucleon-nucleon cross sections. To be consistent, the values used comes from PYTHIA's internal model of total cross sections, see above for further documentation of this routine.

mode  Angantyr:CollisionModel   (default = 1; minimum = 0; maximum = 5)
The Angantyr model has a couple of option for the SubCollisionModel.
option 0 : A simplified model with fixed nucleon radii.
option 1 : The DoubleStrikman default model with fluctuating radii and cross sections.
option 2 : Fluctuating radii and cross sections but different treatment of opacity.
option 3 : Black disks with no fluctuations, ie. no diffraction.
option 4 : Model with fluctuations where the projectile and target can have different parameters, relevant for hadron-nucleon collisions. Note that this option requires much longer initialization time than option 1. Our studies indicate that option 1 is sufficiently precise in most cases.
option 5 : As option 4, but different treatment of opacity.

flag  Angantyr:GlauberOnly   (default = off)
If switched on, the event generation will stop after SubCollisions has been determined, allowing the user to read out the nucleon configuration only.

After all possible nucleon-nucleon sub-collisions has been determined, they are ordered in increasing nucleon-nucleon impact parameter. This list is then gone through in order several time. First all absorptive sub-collisions are treated. One full nucleon-nucleon non-diffractive minimum bias event is generated for each possible absorptive sub-colision. These are also ordered in impact parameter. Note that one target nucleon can interact absorptively with several target nucleons, in a first round only those absorptive sub-collisions involving nucleons that have not already interacted absorptively are are assigned a non-diffractive event.

If PYTHIA is not set up to generate minimum bias events, one or more of the generated non-diffractive events will be replaced by events generated with the selected signal process, and the cross section reported will be modified accordingly.

In a second round only those potential absorptive sub-collisions are considered where one nucleon has already been assinged a full non-diffractive event. In the Angantyr model it is then assumed that the other nuclean will contribute to the final state as if it had just been diffractively excited. Therefore a corresponding single-diffractive event is generated, the elastically scattered beam particle is discarded and the rest is added to the previous non-diffractive event, shuffling a bit with the kinematics so that the total emergy and momentum is conserved.

To generate these single-diffraction events to emulate multiple absorptive sub-collisions a special Pythia object is used. To allow flexibility this object need not have exactly the same settings as the the one generating events for normal single-diffraction sub-collisions. To manipulate this Pythia object a special form of settings can be used. All settings available for Diffraction, MultipartonInteractions, PDF, SigmaDiffractive and PDF can be set separately for this Pythia object by prefixing their names with HI. As an example, setting HISigmaDiffractive:PomFlux and HIPDF:PomSet will set the SigmaDiffractive:PomFlux and PDF:PomSet options for this Pythia object.

mode  Angantyr:SASDmode   (default = 4; minimum = 0; maximum = 4)
Determines how to generate single-diffraction events as secondary absorptive (SASD) sub-collisions.
option 0 : Standard singel-diffraction events as speicfied by HIDiffraction settings above.
option 1 : Always use HIPDF:PomSet = 11 and use the same initial HIMultipartonInteractions:pT0Ref as for non-diffractive events for the total nucleon-nucleon collision energy, independent of the mass of the diffractive system.
option 2 : (Experimental) As for option 1 but also rescale the pomeron proton non-diffractive cross section to match the pp non-diffractive one.
option 3 : (Experimental) As for option 1 but use the full nucleon-nucleon cross section for the non-diffractive nucleon-Pomeron in the multiple interaction machinery. Also rescale the Pomeron PDF with the log of the ratio of maximum and minimum Pomeron-nucleon collision energy.
option 4 : As for option 3 but no rescaling of the Pomeron PDF.

mode  Angantyr:impactMode   (default = 2; minimum = 0; maximum = 2)
Determines how to bias non-diffractive minimum-bias sub-collisions in PYTHIA to be appropriately central.
option 0 : If we have N primary sub-collisions and Na secondary sub-collisions, generate N+Na non-diffractive events and pick the N most central.
option 1 : Use UserHooks to force Pythia to produce events with a particular impact parameter for the N primary sub collisions according to the generated impact parameter in the SubCollisionModel.
option 2 : As for option 1 but also the secondary absorptive sub-collisions have their impact parameter set.

parm  Angantyr:impactFudge   (default = 0.85; minimum = 0.0; maximum = 4.0)
Multiplicative factor used to compensate for the fact that the SubColllisionModel in Angantyr may have a different impact parameter profile than what is assumed in the MPI overlap calculation in Pythia.

mode  Angantyr:SDTries   (default = 1; minimum = 1)
When adding single diffractive sub-collisions to other sub-collisions, there might not be enough energy for the diffractive mass. One option here is to say that the diffractive sub-event simply fails, but setting this larger than unity allows for regenerating the single diffractive sub-event a number of times to see if a small enough diffractive system is produced.

mode  Angantyr:SDRecoil   (default = 1; minimum = 1; maximum = 2)
Determines which particles in a primary sub-collision will take the recoil when adding single diffractive sub-collisions to other sub-collisions. The choice may be overridded by a user-defined HIUserHooks::findRecoilers function.
option 1 : Only elastically scattered nucleons and nucleon remnants will take recoil.
option 2 : All particles outside the added diffractive system's rapidity range are considered.

flag  Angantyr:SDTest   (default = off)
Used in conjunction with HeavyIon:mode = 2 and proton beams to generate single diffractive events that would be used as secondary non-diffractive scatterings in the Angantyr heavy ion model for the given nucleon energies. Used for tuning special HI-prefixed parameters of the secondary absorptive sub-collisions.

parm  Angantyr:SDTestB   (default = -1.0)
In conjunction with Angantyr:SDTest = on and Angantyr:impactMode = 2 only pick diffractive events with a particular impact parameter (as defined by the scaled value given in Info::bMPI()). If negative, the standard impact parameter distribution is used.

After all absorptive sub-collisions have been dealt with, the diffractive and elastic sub-collisions are dealt with in a similar way. In the end there will be a number of parton level events which are finally stacked together, and then hadronized. Finally nucleus remnants constructed from the non-interacting nucleans, are added to complete the full nucleaus-nucleus collision.

It is possible to initialize Angantyr to allow changing the beam energy on an event-by-event basis. This is done by setting Beams:allowVariableEnergy = on, then changing the energy using the Pythia::setKinematics methods. When initializing this way, the SubCollisionModel fitting procedure calculates the parameter values at several different energies, using interpolation to get values between these energy points. The following settings are used when variable energies are allowed.

parm  HeavyIon:varECMMin   (default = 100; minimum = 10)
The minimum allowed energy. Note that Angantyr is currently not designed to properly handle energies below around 100 GeV.

parm  HeavyIon:varECMMax   (default = 0)
The maximum allowed energy. If it is set to 0, then the largest possible value will be given by the CM energy at which Pythia is initialized.

mode  HeavyIon:varECMSigFitNPts   (default = 5; minimum = 2)
Number of eCM points where the SubCollisionModel are calculated. The points are logarithmically spaced.

flag  HeavyIon:varECMStepwiseEvolve   (default = on)
If on, at each evolution point, the algorithm will use the generated parameters from the previous point. If off, it start from the default parameter set given by HeavyIon:SigFitDefPar at each point.

mode  HeavyIon:SigFitReuseInit   (default = 0; minimum = 0; maximum = 4)
If set, the parameters calculated from the evolutionary algorithm can be saved/loaded to disk, for faster initialization in subsequent runs.
option 0 : Initialize from scratch every time.
option 1 : Initialize from scratch, then save configuration.
option 2 : Load configuration from file. If the file is not found, initialization fails.
option 3 : Load configuration from file if it exists, otherwise initialize from scratch and save the file.

word  HeavyIon:SigFitInitFile   (default = Angantyr.sigfit)
The file where the sigfit configuration is stored.

mode  HeavyIon:SasdMpiReuseInit   (default = 0; minimum = 0; maximum = 3)
This setting is similar to MultipartonInteractions:reuseInit, but pertains to the SASD Pythia subobject used internally by Angantyr.
option 0 : current run is self-contained.
option 1 : MPI initialization is done as usual, but afterwards the results of this initialization are saved on file.
option 2 : initialization data is read in from a file, saved from a previous initialization, thereby saving time. If the file is not found, initialization fails.
option 3 : as option 2, but if the file is not found, it will be generated and saved after normal initialization.

word  HeavyIon:SasdMpiInitFile   (default = Angantyr.sasd.mpi)
The file name used to store or read MPI initialization data for the SASD Pythia object. It is up to the user to pick a suitable name (including path if relevant) for the case at hand.