2 Experiments at Particle Colliders

Measure what is measurable and make measurable what is not so.

– Galileo Galilei (attributed)

In Chapter 1, we reviewed the most successful testable theory to date describing the properties and dynamics of our universe at the most fundamental scales. Clear limitations of the Standard Model as it is currently formulated are known, such as the complete omission of gravity forces or the absence of viable dark matter candidates, motivating the quest for alternative unified descriptions of the physical world. A direct path to verify the predictions of the Standard Model up to high accuracy and test alternative theoretical models is to collide high energy particles in a controlled setting and quantitatively study the properties of the particles produced as an outcome of the scattering. That is the aim of the Large Hadron Collider (LHC) and the experiments set up around its collision points. In this chapter, the main design characteristics of a general purpose high-energy physics experiment, namely the Compact Muon Solenoid (CMS) detector at the LHC, will be explored. Given the data-centric nature of the next chapters, particular significance will be given to the acquisition, processing and simulation of individual experimental observations, commonly referred to as events.