This lesson is still being designed and assembled (Pre-Alpha version)

Introduction to ATLAS Offline Software


Teaching: 20 min
Exercises: 0 min
  • What is atlas/athena and what release should I be using?

  • Introduce the athena repository.

  • Define the concept of a package.

  • Define the contect of a project.

  • Describe what a release means.

In this episodes, we hope to introduce the organization of the ATLAS software (athena).

What is Athena?

All of the central ATLAS software is stored in a public repository called Athena. It contains all of the code used for simulation, data reconstruction, triggering, calibration and object identification.

This lesson will not touch any of this source code in detail. How to develop new Athena software is covered in the official ATLAS documentation. Instead we will focus on using the pre-compiled versions of a few athena packages.

Athena Packages

The Athena repository contains several packages. A single package is a collection of related libraries, programs and other supporting files that are maintained by a dedicated group of developers. For example, the Reconstruction/Jet/JetCalibTools/JetCalibTools package contains all of the code necessary to calibrate jets.

Each package, for example Package, is a directory with the following structure:

The structure itself is very flexible. Technically, only the CMakeLists.txt file is required to be there. However maintaining consistency across the experiment is very important for code readability.

You can also have packages outside of the athena repository. For example, you can think of your analysis code as an external package.

Projects and Releases

The entire athena repository is huge and has many dependencies between its packages. New code is being added all the time. To keep everything stable, there is a complex release procedure for “public” code.

First of all, there is the concept of an athena project. An athena project is a collection of packages required to accomplish a certain task. For example, if you only care to analyze events, then you don’t need all of the infrastrucure used during detector operation. Reducing the package list to only what is required has several advantages:

The following list of projects has been tabled from the ATLAS Software Documentation.

Project Purpose
Athena Functional for event generation, simulation, digitisation reconstruction and DAOD derivations (but rarely validated for anything except digi+reco)
AthenaP1 Used to run the High Level Trigger and online monitoring at P1
AthGeneration For event generation
AthSimulation For full Geant4 simulation
AthDerivation For generating derived AODs from AOD and Athena based analysis
AthAnalysisBase Athena based analysis
AnalysisBase Non-athena ROOT based analysis

Each project is released independently and versioned using the following scheme: A.B.X[.Y], where

The official releases are compiled and copied to CVMFS. For certail projects, Docker images are also prepared (more on this tomorrow!).

Our Project,Release

Our goal is to analyze data using a very lightweight framework. What project and release will we be using? (Hint: It is in the form of Project,A.B.75).


We will be using AnalysisBase,21.2.75. We need the AnalysisBase project to get the basic tools without the full environment and release 21.2 is designed for analyzing Run 2 data. The minor release, 75, was the latest one available at the time of writing.

Contribution to Athena

You might be asked to improve or fix a package inside the athena repository during your career in ATLAS. The procedure for doing so is very straightforward, if you are paying attention this week. It uses a lot of the git workflow (yesterday) and CI validation (tomorrow) that we are covering. However we will not go into details of submitting a merge request to atlas/athena. It is very nicely documented in the ATLAS Software Documentation.

Key Points

  • Athena is a collection of common packages that are compiled and released regulariy.

  • Your analysis project is just an external package.