Definitions and Additional Information

• nTuple is a colloquial term for ROOT TTree objects, which are designed to store an array of arbitrary (well, almost) collections of C++ objects (abstractly, tuples) in a way that allows fast (de)serialization to disk. The history of nTuple is interesting: one can still see the legacy TNtuple, which was a class designed to be more familiar to users of PAW, the predecessor to ROOT. To see how much worse your life could be, check out these NTUPLEs.
• flat nTuples refer to TTree branches that are simple data types, e.g. float, int, etc. rather than predefined C++ objects. CMSSW allows ROOT to access complex data types at the expense of loading a Reflex dictionary into ROOT at runtime, and a reduced toolset to operate on them.
• Combinatorial analysis is a common pattern where individual final state particles reconstructed from the event are combined in each possible way, and the resulting composite object information is stored along with the daughter particles in one tuple per combination, rather than one tuple per event record.