The ATLAS experiments presented on 4 July 2012 the status of the Standard Model Higgs search. All the plots presented that day were created in ROOT.
A key feature of ROOT is a data container called tree, with its substructures branches and leaves. A tree can be seen as a sliding window to the raw data, as stored in a file. Data from the next entry in the file can be retrieved by advancing the index in the tree. This avoids memory allocation problems associated with object creation, and allows the tree to act as a lightweight container while handling buffering invisibly.
The inclusion of a C++ interpreter (CINT until version 5.34, Cling from version 6.00) makes this package very versatile as it can be used in interactive, scripted and compiled modes in a manner similar to commercial products like MATLAB.
Several particle physics collaborations have written software based on ROOT, often in favor of using more generic solutions (e.g. using ROOT containers instead of STL).
Some of the running particle physics experiments using software based on ROOT
Criticisms of ROOT include its difficulty for beginners, as well as various aspects of its design and implementation. Frequent causes of frustration include extreme code bloat, heavy use of global variables,[4] and an overcomplicated class hierarchy. From time to time these issues are discussed on the ROOT users mailing list.[5][6] While scientists dissatisfied with ROOT have in the past managed to work around its flaws,[7] some of the shortcomings are regularly addressed by the ROOT team. The CINT interpreter, for example, has been replaced by the Cling interpreter,[8] and numerous bugs are fixed with every release.
The Toolkit for Multivariate Data Analysis with ROOT (TMVA) is a ROOT-integrated project providing a machine learning environment for the processing and evaluation of multivariate classification, both binary and multi class, and regression techniques targeting applications in high-energy physics (hereorhere).
