CERN Accelerating science

Ultrarelativistic heavy-ion collisions performed at the Large Hadron Collider (LHC) are thought to reach such high temperatures that a new state of matter known as a quark– gluon plasma (QGP) is created. This state of strongly interacting matter is predicted to have dominated the early Universe a few microseconds after the Big Bang and understanding its properties is crucial for understanding its subsequent evolution. However, in order to understand heavy-ion collisions reference measurements in so-called small systems, at the LHC typically proton-proton (pp) and proton-lead (p–Pb) collisions, are required. Measurements in pp collisions represent the so-called baseline or vacuum measurements where the particle production rates in a vacuum can be determined. Whereas measurements in p–Pb collisions act as an intermediary which are used to determine any modifications to the particle production rates that arise from cold nuclear matter (CNM) effects, in contrast to hot matter effects originating from the QGP. In recent years, signatures that were originally attributed to the QGP have been observed in small system collisions, further complicating the current picture of heavy-ion collisions. Deeper studies into these small systems are now required to clear up the situation.