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      =========================================================
      Geant4 - an Object-Oriented Toolkit for Simulation in HEP
      =========================================================
 
                           MCTRUTH using HepMC
                           -------------------
 
 This example demonstrates a mechanism for Monte Carlo truth handling
 using HepMC as the event record. The user does not interact directly
 with the HepMC classes but with the MCTruthManager class which takes
 care with storing all the necessary information about particles,
 vertices and relations between them. A specialized tracking action is
 used to test whether given particle is to be stored or not. The
 decision criteria for storing particle are configurable via the
 MCTruthConfig class. 
 
 HOW TO BUILD THE EXAMPLE ?
 
 - if you do not have it yet, install HepMC event record (tested with version 2.06.08)
 
 - set HEPMC_ROOT_DIR variable to point to the directory where HepMC is installed; 
   if the HepMC is installed in your system directory (/usr/local) you do not need to set anything
 
 - run the CMake configuration and build mctruthex target in your build directory
       
 - execute the application:
         % your_binary_directory/mctruthex
 
 DESCRIPTION OF THE MCTRUTH HANDLING MECHANISM
  
 The main element of the MC truth handling machinery is the
 MCTruthManager class. This class is responsible for all the
 interaction with the HepMC event and does not depend on Geant4. It is
 a singleton, therefore it is guaranteed to be instanciated only once
 and the static 'GetInstance' method allows to access it from anywhere
 in the code. It contains methods like 'NewEvent' to start a new event,
 'AddParticle' to add particle to the current event, as well as
 'PrintEvent' for the purpose of the debugging.  The core of the
 algorithm which deals with building up the MC truth event tree within
 the HepMC event is implemented in AddParticle method.
  
 The AddParticle method is called with the following arguments:
 four-momentum, production position and 'end' position of the particle,
 PDG code of the particle, as well as the particle ID (unique identifier,
 as we will see later, corresponding to Geant4 TrackID) and the ID of
 the mother. Finally, there is a boolean flag specifying whether the
 direct mother of the given particle has been stored, or not.
  
 The first step, which always takes place, is to instanciate a new
 HepMC::GenParticle with the barcode corresponding to particle ID, as
 well as to instanciate a new HepMC::GenVertex which will represent the
 'end' vertex of the particle. The barcode of the 'end vertex' is equal
 to minus the barcode of the particle.
  
 We can now distinguish several cases:
  
 1) the particle is a primary in the Geant4 language, i.e. its
    mother ID is 0
  
    This is the simplest case, we just instanciate a new 'primary'
    (without any incoming particles) GenVertex, we add to it the
    particle and we put it all in the event. Additionally we store the
    ID of the particle in a special vector, where all the IDs of
    primary particles will be stored, allowing quick access to each of
    the main 'branches' of the event. We return from the method.
  
 2) the particle is not a primary
 
    We use the 'event->barcode_to_particle(motherID)' method to get the
    pointer to its mother.
     
    We check if the 'end vertex' of the mother corresponds to the
    'production vertex' of the particle in question.
  
    2.1) If the two vertices do match, we attach the new particle to
         the 'end vertex' of the mother. We return from the method.
  
    2.2) If the two vertices do not match, i.e. the new particle is not
         a product of the 'end vertex' of the mother particle, we can
         have two cases:
                 
         2.2.1) The boolean flag says that the direct mother of the
                particle has _not_ been stored. This means that the
                particle has been 'adopted' by one of its ancestors, or
                in other words, the mother ID of the particle does not
                correspond to its direct mother (so clearly the
                vertices cannot match). This for instance could happen
                if we decided not to store gamma coming from pi0 decay
                but did decide to store e+/- coming from the gamma
                conversion (so the gamma between pi0 and e+/- was
                missing). In such a case we instanciate (or use one of
                the existing ones, if vertices match) a 'dummy'
                particle (with pdg = -999999) which then acts as the
                link between the 'adopted' particle and the
                (non-direct) mother. In such a way, the navigability up
                in the event is still possible, but in the same time,
                we can clearly see that the link is not a direct
                one. We return from the method.
  
         2.2.2) The boolean flag says that direct mother of the
                particle _has_ been stored. Taking into account that
                the vertices do not match, it can mean only one
                thing. The new particle has been produced 'on the
                flight', i.e. somewhere 'before' the 'end vertex' of
                the mother. This can be the case, for instace, for
                delta electrons, bremsstrahlung gammas, etc.  In such a
                situation, we 'split' the mother particle in two
                particles and create a new vertex from which the
                secondary will be going out. The complication, however,
                arises when we have more than one generated 'on the
                flight' particle attached to the same mother. In such a
                case, for each secondary we need to locate the right
                'segment' of the mother particle (i.e. we need to find
                between which two vertices we need to add a new
                one). To keep track of those segmentations we introduce
                a map where each particle ID we map into the number of
                existing segments (in the normal case one). Each new
                'segment' gets barcode equal to the barcode of the
                original particle + N*10000000, where N is the segment
                number. In such a way, one can easily follow the
                'segmentation' (if any) of each particle.  We return
                from the method.
  
 This concludes the description of MCTruthManager. The MCTruthConfig
 class is a collection of criteria (minimal energy, PDG, creator
 process, etc) that we want to apply when deciding whether to store or
 not given particle. These values are used by the
 'MCTruthTrackingAction' which we describe below. This class can
 certainly be extended with other members.
  
 The actual Geant4-dependent part of the MCTruth handling machinery
 consists of a few 'G4 user actions' as well as an implementation of
 G4VUserTrackInformation. The later one is, for the moment, used only
 to store one boolean flag indicating whether the direct mother of the
 given track has been stored or not.
  
 The first user action is MCTruthEventAction which is only reponsible
 for calling MCTruthManager::GetInstance()->NewEvent() at the beginning
 of each event. It can also be used for printing out events for the
 purpose of debugging.
  
 The actual 'decision making' concerning which particle to store is
 done in MCTruthTrackingAction. At the end of each track the method
 trackToBeStored(track) is called to check for various characteristics
 of the particle. These, for instance can be energy, particle ID,
 creator process, etc.
  
 If the particle satisfies the conditions the
 MCTruthManager::GetInstance()->AddParticle is called and all the
 procedure described above is performed. The important element here is
 that the Geant4 TrackID is used as the unique particle ID in
 MCTruthManager and eventually as the barcode of the
 HepMC::GenParticle.
  
 If the particle does not qualify to be stored, there are two actions
 performed. First the 'ParentID' of the _daughters_ is set to the
 'ParentID' of the currenly processed particle. In other words, the
 'ParentID' of the daughters is set to the ID of the last stored
 particle. Second, the 'directParent' flag from MCTruthTrackInformation
 of the __daughters__ is set to FALSE. In such a way, one is still able
 to navigate up in the event (to get the ancestors of the particle),
 but in the same time, the particle is flagged as 'not having direct
 parent'.

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