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High Energy Physics Glossary
This glossary provides definitions for terms used elsewhere in this chapter, covering the event data model, the ROOT ecosystem, and derived analysis formats.
Event Data & Processing
Event
A single particle collision (or a fixed time window of one) and everything the detector recorded for it, including tracks, energy deposits, timing information, and reconstructed physics objects.
Reconstruction
The processing stage that turns raw detector signals into physics objects (electrons, muons, jets, and similar) that an analysis can work with directly.
Derivation
A further processing step that reduces reconstructed event data down to a smaller, analysis-focused format (e.g. ATLAS's DAOD derivations), trading generality for size and speed.
The ROOT Ecosystem
ROOT
A self-describing, hierarchical binary file format and the software framework built around it. Developed at CERN and used to store HEP data at essentially every processing stage.
TTree
The columnar data structure historically used to store event data inside a ROOT file, where each variable is stored in its own branch and each entry corresponds to one event.
Branch
A single named column within a TTree or RNTuple, holding the values of one variable across all stored events.
RNTuple
ROOT's next-generation columnar event-data format, designed to succeed TTree with better throughput and compression for the High-Luminosity LHC era.
Ntuple
A generic term for a flat, row-per-event data structure.
Derived Analysis Formats
AOD (Analysis Object Data)
Full reconstructed-event format, storing physics objects in experiment-specific C++ classes.
MiniAOD
A reduced version of CMS's AOD, still based on CMS-specific classes but with less redundant content.
NanoAOD
A flat, TTree-based CMS format using only standard C++ types, roughly 20 times smaller than MiniAOD and 200 times smaller than AOD.
DAOD_PHYSLITE
An ATLAS derivation format from which the flat ROOT ntuples distributed through ATLAS Open Data are produced.
DecayTreeTuple
The LHCb DaVinci tool used to write selected candidate and event information into a ROOT ntuple, the standard output of an LHCb analysis.
Software & Analysis Tools
RDataFrame
ROOT's modern, declarative interface for analyzing TTree and RNTuple data.
uproot
A pure-Python library for reading and writing ROOT files without requiring a ROOT installation.
PyROOT
The Python bindings for the ROOT framework, giving Python access to ROOT's full C++ API.
TBrowser
ROOT's graphical browser for interactively inspecting the contents of a ROOT file, including its trees and branches.
CMSSW
The CMS software framework used to process raw data into AOD/MiniAOD-level formats.
DaVinci
The LHCb analysis software application used to select candidates and produce ROOT ntuples via tools like DecayTreeTuple.
Facilities & Data Sharing
HL-LHC (High-Luminosity LHC)
The upgraded phase of the LHC, expected to begin with Run 4, that will substantially increase collision rates and data volumes.
HEPData
The open-access repository for the digitized data points, tables, and plots underlying HEP publications.
INSPIRE-HEP
The literature database for high energy physics that HEPData records are anchored to, linking datasets to their originating publication.