The Truth Modules

There are three modules in an eventAnalysis file that store truth information - the ND::TTruthVerticesModule, ND::TTruthTrajectoriesModule and ND::TRooTrackerVtxModuleBase, which is the base class of both the ND::TGRooTrackerVtxModule and ND::TNRooTrackerVtxModule.

The first two modules will store the truth information that is present in the reconstruction file being processed and contain TClonesArrays of ND::TTruthVerticesModule::TTruthVertex and ND::TTruthTrajectoriesModule::TTruthTrajectory with counters to track the number of vertices and trajectories. The RooTracker modules then store the pass-through information from the NEUT or GENIE neutrino interaction generators.

What does each module store?

The RooTracker module will store all the information presented by the neutrino generator for the neutrino interaction. You can find information on the target nuclei, the incoming neutrino and all the final state particles produced from the interaction. There is a single entry for each interaction that holds this information in StdHep arrays, which the documentation describe.

The ND::TTruthVerticesModule and ND::TTruthTrajectoriesModule modules store information about the neutrino interaction vertex and trajectory information about the truth particles produced respectively.

The ND::TTruthVerticesModule::TTruthVertex holds a condensed version of the RooTracker module information, and should have everything you want for an analysis, unless you need to re-weight the Monte Carlo.

A ND::TTruthTrajectoriesModule::TTruthTrajectory is created for each truth particle that passes through the ND280 basket or ECal, and for any particle whose daughter passes through the basket or ECal. It stores basic information such as particle type, momentum and a number of trace vectors, storing the entry and exit positions and momenta where the particle enters a new sub-detector.

When is truth information stored?

The RooTracker and ND::TTruthVerticesModule modules stores information about every vertex generated and the primary particles produced at the vertex. The ND::TTruthTrajectoriesModule will only store the information of truth trajectories that pass through the basket or ECal, or whose daughters do. If, for example, a trajectory is completely contained within the SMRD and has no daughters that penetrate an ECal, it is not saved in the eventAnalysis file. This selection helps to limit the size of eventAnalysis files to a manageable level.

Also, to save a ND::TTruthTrajectoriesModule::TTruthTrajectory it must be present in the reconstruction file being processed. ND280GEANT4SIM will not save any truth trajectory that does not travel at least 2cm or any neutron or gamma that does not have more than 50MeV of energy, since these particles are not observable in the detector.

This can lead to some confusion. For example, a RooTracker vertex or TruthVertex can say a number of very low energy particles are produced, but they do not have corresponding truth trajectories. In this case, any information you need should be in the RooTracker or ND::TTruthVerticesModule modules. In other instances, a reconstructed object in the SMRD can be truth matched to a trajectory that is not saved by eventAnalysis. If this is an issue it can be addressed in future productions.

Other issues

If there is a problem that is not described above, please contact the eventAnalysis package manager or raise it on Bugzilla.