Proton–Lead Collisions at the CERN LHC

The single proton–lead collision run performed at the LHC has had profound consequences for the entire field of hot and dense QCD. The prior, 20-year-long experimental exploration of high-energy nucleus–nucleus collisions indicated that a hot QCD medium is created in a very short time, a fast equilibration. The striking similarities found in these nucleus–nucleus data and the a priori simpler proton–lead collisions, where no hot medium was expected, are refining our understanding of the whole process of thermalization at the most fundamental level. We review the most relevant sets of data taken during the proton–lead collisions of Run 1 at the LHC and the implications they have for our understanding of the underlying physical mechanisms and characterization of hot and dense QCD systems.