Optimal exploitation of hadronic final states played a key role in successes of all recent collider experiment in HEP, and the ability to use hadronic final states will continue to be one of the decisive issues during the analysis phase of the LHC experiments. Monte Carlo programs like Geant4 facilitate the use of hadronic final states, and have been developed for many years.
We give an overview of the Object Oriented frameworks for hadronic generators in Geant4, and illustrate the physics models underlying hadronic shower simulation on examples, including the three basic types of modeling; data-driven, parametrisation-driven, and theory-driven modeling, and their possible realisations in the Object Oriented component system of Geant4. We put particular focus on the level of extendibility that can and has been achieved by our Russian dolls approach to Object Oriented design, and the role and importance of the frameworks in a component system.
The purpose of this section is to explain the implementation frameworks used in and provided by Geant4 for hadronic shower simulation as in the 1.1 release of the program. The implementation frameworks follow the Russian dolls approach to implementation framework design. A top-level, very abstracting implementation framework provides the basic interface to the other Geant4 categories, and fulfils the most general use-case for hadronic shower simulation. It is refined for more specific use-cases by implementing a hierarchy of implementation frameworks, each level implementing the common logic of a particular use-case package in a concrete implementation of the interface specification of one framework level above, this way refining the granularity of abstraction and delegation. This defines the Russian dolls architectural pattern. Abstract classes are used as the delegation mechanism  .
All framework functional requirements were obtained through use-case analysis. In the following we present for each framework level the compressed use-cases, requirements, designs including the flexibility provided, and illustrate the framework functionality with examples. All design patterns cited are to be read as defined in [ Gamma1995 ].
There are two principal use-cases of the level 1 framework. A user will want to choose the processes used for his particular simulation run, and a physicist will want to write code for processes of his own and use these together with the rest of the system in a seamless manner.
Provide a standard interface to be used by process implementations.
Provide registration mechanisms for processes.
Both requirements are implemented in a sub-set of the tracking-physics interface in Geant4}. The class diagram is shown in Figure 3.1.
All processes have a common base-class
from which a set of specialised classes are derived. Three of them are
used as base classes for hadronic processes for particles at rest
G4VRestProcess), for interactions in flight
G4VDiscreteProcess), and for processes like
radioactive decay where the same implementation can represent both these
extreme cases (
Each of these classes declares two types of methods; one for calculating the time to interaction or the physical interaction length, allowing tracking to request the information necessary to decide on the process responsible for final state production, and one to compute the final state. These are pure virtual methods, and have to be implemented in each individual derived class, as enforced by the compiler.
The functionality provided is through the use of process base-class pointers
in the tracking-physics interface, and the
All functionality is implemented in abstract, and registration of derived process
classes with the
G4Process-Manager of an individual particle
allows for arbitrary combination of both Geant4 provided processes, and
user-implemented processes. This registration mechanism is a modification on a
Chain of Responsibility. It is outside the scope of the current paper, and its
description is available from
At the next level of abstraction, only processes that occur for particles in flight are considered. For these, it is easily observed that the sources of cross sections and final state production are rarely the same. Also, different sources will come with different restrictions. The principal use-cases of the framework are addressing these commonalities. A user might want to combine different cross sections and final state or isotope production models as provided by Geant4, and a physicist might want to implement his own model for particular situation, and add cross-section data sets that are relevant for his particular analysis to the system in a seamless manner.
Flexible choice of inclusive scattering cross-sections.
Ability to use different data-sets for different parts of the simulation, depending on the conditions at the point of interaction.
Ability to add user-defined data-sets in a seamless manner.
Flexible, unconstrained choice of final state production models.
Ability to use different final state production codes for different parts of the simulation, depending on the conditions at the point of interaction.
Ability to add user-defined final state production models in a seamless manner.
Flexible choice of isotope production models, to run in parasitic mode to any kind of transport models.
Ability to use different isotope production codes for different parts of the simulation, depending on the conditions at the point of interaction.
Ability to add user-defined isotope production models in a seamless manner.
The above requirements are implemented in three framework components, one for cross-sections, final state production, and isotope production each. The class diagrams are shown in Figure 3.2 for the cross-section aspects, Figure 3.3 for the final state production aspects, and figure Figure 3.4 for the isotope production aspects.
Figure 3.2. Level 2 implementation framework of the hadronic category of Geant4; cross-section aspect.
Figure 3.3. Level 2 implementation framework of the hadronic category of Geant4; final state production aspect.
Figure 3.4. Level 2 implementation framework of the hadronic category of Geant4; isotope production aspect
The three parts are integrated in the
class, that serves as base-class for all hadronic processes of particles in flight.
Each hadronic process is derived from
which holds a list of ``cross section data sets''.
The term ``data set'' is
representing an object that encapsulates methods and data for calculating
total cross sections for a given process in a certain range of validity.
The implementations may take any form. It can be a simple equation as well as
sophisticated parameterisations, or evaluated data.
All cross section data set classes are derived from the abstract class
G4VCrossSection-DataSet}, which declares methods that allow
the process inquire, about the applicability of an individual data-set through
IsApplicable(const G4DynamicParticle*, const G4Element*),
and to delegate the calculation of the actual cross-section value through
GetCrossSection(const G4DynamicParticle*, const G4Element*).
G4HadronicInteraction base class is provided for final state
generation. It declares a minimal interface of only one pure virtual method:
G4VParticleChange* ApplyYourself(const G4Track &, G4Nucleus &)}.
G4HadronicProcess provides a registry for final state
production models inheriting from
Again, final state production model is meant in very general terms. This can
be an implementation of a quark gluon string model
a sampling code for ENDF/B data formats
or a parametrisation describing only
neutron elastic scattering off Silicon up to 300~MeV.
For isotope production, a base class (
is provided. It declares a method
G4IsoResult * GetIsotope(const G4Track &, const G4Nucleus &)
that calculates and returns the isotope production information. Any concrete
isotope production model will inherit from this class, and implement the
method. Again, the modeling possibilities are not limited, and the
implementation of concrete production models is not restricted in any way.
By convention, the
GetIsotope method returns NULL, if the model
is not applicable for the current projectile target combination.
G4HadronicProcess provides registering possibilities
for data sets. A default is provided covering all possible
conditions to some approximation. The process stores and retrieves
the data sets through a data store that acts like a FILO stack
(a Chain of Responsibility with a
First In Last Out decision strategy). This allows the user to map out
the entire parameter space by overlaying cross section data sets to optimise
the overall result. Examples are the cross sections for low energy neutron
transport. If these are registered last by the user, they will be used
whenever low energy neutrons are encountered. In all other conditions the
system falls back on the default, or data sets with earlier registration dates.
The fact that the registration is done through abstract base classes with no
side-effects allows the user to implement and use his own cross sections.
Examples are special reaction cross sections of
that might be used for ∊/∊' analysis at LHC to control the
G4HadronicProcess class provides a
registration service for classes deriving from
G4Hadronic-Interaction, and delegates final state
production to the applicable model.
G4Hadronic-Interactionprovides the functionality needed
to define and enforce the applicability of a particular model.
Models inheriting from
can be restricted in applicability in projectile
type and energy, and can be activated/deactivated for individual materials and
elements. This allows a user to use final state production models in
arbitrary combinations, and to write his own models for
final state production. The design is a variant of a Chain of Responsibility.
An example would be the likely CMS scenario - the combination of low energy
neutron transport with a quantum molecular dynamics
invariant phase space decay
and fast parametrised models for calorimeter materials, with user defined
modeling of interactions of spallation nucleons with the most abundant
tracker and calorimeter materials.
G4HadronicProcess by default calculates the isotope production
information from the final state given by the transport model. In addition, it
provides a registering mechanism for isotope production models that run in
parasitic mode to the transport models and inherit from
G4VIsotope-Production. The registering mechanism behaves like a FILO
stack, again based on Chain of Responsibility. The models will be asked for
isotope production information in inverse order of registration. The first
model that returns a non-NULL value will be applied. In addition, the
G4Hadronic-Process provides the basic infrastructure for accessing and
steering of isotope-production information. It allows to enable and disable
the calculation of isotope production information globally, or for individual
processes, and to retrieve the isotope production information through the
G4IsoParticleChange * GetIsotopeProductionInfo()}
method at the end of each step. The
G4HadronicProcess is a finite state
machine that will ensure the
GetIsotope-ProductionInfo returns a
non-zero value only at the first call after isotope production occurred. An
example of the use of this functionality is the study of activation of a
Germanium detector in a high precision, low background experiment.
Figure 3.5. Level 3 implementation framework of the hadronic category of Geant4; theoretical model aspect.
Geant4 provides at present one implementation framework for theory driven models. The main use-case is that of a user wishing to use theoretical models in general, and to use various intra-nuclear transport or pre-compound models together with models simulating the initial interactions at very high energies.
Allow to use or adapt any string-parton or parton transport [VNI],
Allow to adapt event generators, ex. [PYTHIA7], state production in shower simulation.
Allow for combination of the above with any intra-nuclear transport (INT).
Allow stand-alone use of intra-nuclear transport.
Allow for combination of the above with any pre-compound model.
Allow stand-alone use of any pre-compound model.
Allow for use of any evaporation code.
Allow for seamless integration of user defined components for any of the above.
To provide the above flexibility, the following abstract base classes have been implemented:
In addition, the class
G4TheoFS-Generator is provided to orchestrate
interactions between these classes. The class diagram is shown in
G4VHighEnergy-Generator serves as base class for parton transport or
parton string models, and for Adapters to event generators. This class
declares two methods,
The base class for INT inherits from
making any concrete implementation usable as a stand-alone model. In doing so, it
ApplyYourself interface of
and adds a second interface,
Propagate, for further propagation
after high energy interactions.
Propagate takes as arguments a
three-dimensional model of a wounded nucleus, and a set of secondaries with
energies and positions.
The base class for pre-equilibrium decay models,
G4Hadronic-Interaction, again making any concrete
implementation usable as stand-alone model. It adds a pure virtual
DeExcite method for further evolution of the system when
intra-nuclear transport assumptions break down.
DeExcite takes a
the Geant4 representation of an excited nucleus, as argument.
The base class for evaporation phases,
declares an abstract method,
BreakItUP(), for compound decay.
G4TheoFSGenerator class inherits from
and hence can be registered as a model for final state production with a
hadronic process. It allows a concrete implementation of
G4VHighEnergy-Generator to be
registered, and delegates initial interactions, and intra-nuclear transport
of the corresponding secondaries to the respective classes. The design is a
complex variant of a Strategy. The most spectacular application of this
pattern is the use of parton-string models for string excitation, quark
molecular dynamics for correlated string decay, and quantum molecular dynamics
for transport, a combination which promises to result in a coherent
description of quark gluon plasma in high energy nucleus-nucleus interactions.
registering mechanisms for concrete implementations of
G4VPreCompound-Model, and provides
concrete intra-nuclear transports with the possibility of delegating
pre-compound decay to these models.
G4VPreCompoundModel provides a registering mechanism
for compound decay through the
G4VExcitation-Handler interface, and provides concrete
implementations with the possibility of delegating the decay of a compound
The concrete scenario of
G4TheoFS-Generator using a
dual parton model
and a classical cascade, which in turn uses an exciton pre-compound model that
delegates to an evaporation phase, would be the following:
G4TheoFS-Generator receives the conditions of the interaction;
a primary particle and a nucleus. It asks the dual parton model to perform the
initial scatterings, and return the final state, along with the by then
damaged nucleus. The nucleus records all information on the damage sustained.
G4TheoFS-Generator forwards all information to the classical cascade,
that propagates the particles in the already damaged nucleus, keeping track of
interactions, further damage to the nucleus, etc.. Once the cascade assumptions
break down, the cascade will collect the information of the current state of
the hadronic system, like excitation energy and number of excited particles,
and interpret it as a pre-compound system. It delegates the decay of this to
the exciton model. The exciton model will take the information provided, and
transitions and emissions, until the number of excitons in the system equals
the mean number of excitons expected in equilibrium for the current excitation
energy. Then it hands over to the
evaporation phase. The evaporation phase decays the residual nucleus, and the Chain of
Command rolls back to
G4TheoFS-Generator, accumulating the
information produced in the various levels of delegation.
Figure 3.6. Level 4 implementation framework of the hadronic category of Geant4; parton string aspect.
Figure 3.7. Level 4 implementation framework of the hadronic category of Geant4; intra-nuclear transport aspect.
The use-cases of this level are related to commonalities and detailed choices in string-parton models and cascade models. They are use-cases of an expert user wishing to alter details of a model, or a theoretical physicist, wishing to study details of a particular model.
Allow to select string decay algorithm
Allow to select string excitation.
Allow the selection of concrete implementations of three-dimensional models of the nucleus
Allow the selection of concrete implementations of final state and cross sections in intra-nuclear scattering.
To fulfil the requirements on string models, two abstract classes are provided,
G4VParton-StringModel and the
The base class for parton string models,
GetStrings() pure virtual method.
G4VString-Fragmentation, the pure abstract base class for string
fragmentation, declares the interface for string fragmentation.
To fulfill the requirements on intra-nuclear transport, two abstract classes
At this point in time, the usage of these intra-nuclear transport related classes
by concrete codes is not enforced by designs, as the details of the
cascade loop are still model dependent, and more experience has to be gathered to achieve
standardisation. It is within the responsibility of the implementers of concrete
codes to use the abstract interfaces as defined in these classes.
Again variants of Strategy, Bridge and Chain of Responsibility are used.
G4VParton-StringModel implements the initialisation of a
three-dimensional model of a nucleus, and the logic of scattering. It
delegates secondary production to string fragmentation through a
G4VString-Fragmentation pointer. It provides a registering service for
the concrete string fragmentation, and delegates the string excitation to
derived classes. Selection of string excitation is through selection of
derived class. Selection of string fragmentation is through registration.
Figure 3.8. Level 5 implementation framework of the hadronic category of Geant4; string fragmentation aspect.
The use-case of this level is that of a user or theoretical physicist wishing to understand the systematic effects involved in combining various fragmentation functions with individual types of string fragmentation. Note that this framework level is meeting the current state of the art, making extensions and changes of interfaces in subsequent releases likely.
Allow the selection of fragmentation function.
A base class for fragmentation functions,
provided. It declares the
The design is a basic Strategy. The class diagram is shown in
At this point in time, the usage of the
G4VFragmentation-Function is not enforced by design,
but made available from the
G4VString-Fragmentation to an implementer of a concrete string
G4VString-Fragmentation provides a registering
mechanism for the
concrete fragmentation function. It delegates the calculation of
zf of the
hadron to split of the string to the concrete implementation. Standardisation
in this area is expected.
 The same can be achieved with template specialisations with slightly improved CPU performance but at the cost of significantly more complex designs and, with present compilers, significantly reduced portability.