D3PD Converter

Convert D3PDs to object oriented files for skimming, thinning, and slimming and for more object-oriented analysis.

The typical D3PD is a flat tree structure, consisting of blocks of objects by related variable names. For example, electrons may have branches "electron_cl_eta" and "el_cl_phi" which are both std::vector<float> containers. This code instead chooses to turn these related containers into a single container of objects. Instead of electron_cl_eta->at(0), one would be able to use electrons.at(0).cl_eta or, better, to iterate through the container and use my_electron->cl_eta.

Requirements

This project uses ROOT6, which also requires c++1y support. The Makefile contained in this project uses -std=c++14, however. Compilation has only been tested on Scientific Linux 6, though any Unix-like system which can build ROOT6 should work.

Type Safety

One potential issue with ROOT files in this format is that pointers to std::vector objects containing one type may be confused with a different type if the two types happen to have the same size. For example, a std::vector<int>* and a std::vector<float>* often have this confusion. By default, ROOT doesn't do any checking because the branch location one typically passes into TTree::SetBranchAddress is taken via void*. In this project, D3PD::Branches::Tree is careful to distinguish between different kinds of pointers and will throw a std::invalid_argument if the type being used to hold the branch data is different from what the branch truly contains.

Using

Once checked out, one only needs to run make to build the project and produce the provided executable. To add the library to the LD_LIBRARY_PATH environmental variable one can also use source add_library.sh prior to trying to invoke the executable. The expected input is ./convert input_filename output_filename where output_filename defaults to "out.root" if not provided.

Selection criteria

Event selection criteria could be added into converter.cxx and selection criteria for each object in a module can be modified. Using the electrons example, this would be in D3PD::Branches::Electron::GetElectrons().

Extending

The general structure of the project is that the D3PD::Reader object inherits from numerous submodules (eg, one for electrons, one for muons, one for EM jets, etc.) which in turn inhert from D3PD::Branches::Tree virtually. This parent type is responsible for owning and managing the TFile and TTree. Using a TChain would also be possible. In order to add or remove branches to an existing module (example below using electrons), one needs to modify four things:

1. The branch in the D3PD::Branches::Electron class (include/D3PD/branches/electron.h), with a default member initializer if adding.
2. The field in the D3PD::Objects::Electron class (include/D3PD/objects/electron.h), also with a default member initalizer
3. Attach/remove the branch in the constructor D3PD::Branches::Electron::Electron() (in src/D3PD/branches/electron.cxx) using the SetBranchAddress method provided by D3PD::Branches::Tree
4. Modify the conversion from the branch data to the object in D3PD::Branches::Electron::GetElectron(int)

In order to add a new module entirely, similar classes (with an example of photons, D3PD::Branches::Photon and D3PD::Objects::Photon) would be required and in addition three modifications need to be made to D3PD::Reader, and D3PD::Event:

1. To D3PD::Reader, one must also include the header for the new module and inherit from the new module
2. To D3PD::Event, one needs to
   1. Add in the correct object type as a new private data member (eg, D3PD::Objects::Photon::Vector photons;)
   2. Modify D3PD::Event::Attach(TTree*) to add a new branch for that data member (eg, tree->Branch("photons", &photons);)
   3. Add new Get and Set methods
3. Add the new class to the ROOT dictionary. To dict/headers.h one would need to include the new object's header (eg, #include "D3PD/objects/photon.h")

One last note about the header files in D3PD/objects is that there are no corresponding files in src/ since no methods are defined on those classes.