Introduction

Last updated on 2024-07-02 | Edit this page

Estimated time: 5 minutes

Overview

Questions

  • What is C++?
  • What is ROOT?
  • What is the point of these exercises?

Objectives

  • Learn a bit about C++ and how to compile C++ code.
  • Learn how to use ROOT to write and read from files, using the C++ libraries.
  • Learn how to use ROOT to investigate data and create simple histograms.
  • Explore the ROOT python libraries.

Let’s talk about ROOT!

From the ROOT website:

Testimonial

ROOT is a framework for data processing, born at CERN, at the heart of the research on high-energy physics. Every day, thousands of physicists use ROOT applications to analyze their data or to perform simulations.

In short, ROOT is an overarching toolkit that defines a file structure, methods to analyze particle physics data, visualization tools, and is the backbone of many widely used statistical analysis tool kits, such as RooFit and RooStats. You don’t need to use ROOT for your own analysis, but you will have to have some familiarity with it when you are first accessing the open data.

OK, that sounds cool. So what’s the deal with C++?

ROOT and C++

In the mid-80’s, C++ extended the very popular C programming language, primarily with the introduction of object-oriented programming (OOP). This programming paradigm was adopted by many particle physics experiments in the 1990’s and when ROOT was written, it was written in C++. While there are now python hooks to call the ROOT functions, the underlying code is all in C++.

Because C++ is a compiled code, it is usually much faster than a scripted language like python, though that is changing with modern python tools. Still, since much of the original analysis and access code was written in C++ and calling the C++ ROOT libraries, it’s good to know some of the basics of ROOT, in a C++ context.

Most CMS analysts interface with ROOT using python scripts and you may find yourself using a similar workflow. Later on in this lesson, we’ll walk you through some very basic python scripts and point you toward more in-depth tutorials, for those who want to go further.

You still have choices!

Just to emphasize, you really only need to use ROOT and C++ at the early stages of analyzing CMS Open Data in the AOD (Run 1) or MiniAOD (Run 2) formats. These datasets require using CMS-provided tools that perform much better in C++ than python. However, downstream in your analysis or to analyze Run 2 NanoAOD files, you are welcome to use whatever tools and file formats you choose.

Key Points

  • C++ has a reputation for being intimidating, but there are only a few things you need to learn to edit the open data code for your own uses.
  • You can use the ROOT toolkit using both C++ and python.
  • Some ROOT code is written in C++ to access the datafiles.
  • People will often use simpler C++ scripts or python scripts to analyze reduced datasets.