.ROOT

Introduction

A .root file is the data file of ROOT, the data analysis framework developed at CERN since 1995. Particle physics experiments, including those at the Large Hadron Collider, store their collision data, histograms and analysis results in ROOT files, which together amount to a vast quantity of data.

A ROOT file can hold any C++ object that ROOT knows how to describe. The most important is the tree (TTree), a table of events with many columns, which is stored column by column so that an analysis reads only the values it needs. A newer column layout, RNTuple, is replacing it.

Structure

The file starts with the four bytes root, followed by the format version and the positions of the first data record, the end of the file, the list of free space and the streamer information, which describes the layout of every class stored in the file so that it can be read without the original code.

The rest of the file is a series of records, each led by a key (TKey) giving the object’s class, name, title, sizes and position. Objects are usually compressed, with zlib, LZMA, LZ4 or Zstandard. Keys are grouped into directories, which work like folders inside the file.

Links