Observation of Suppressed Charged-Particle Production in Ultrarelativistic Oxygen-Oxygen Collisions

A hot and dense state of nuclear matter, known as the quark-gluon plasma, is created in collisions of ultrarelativistic heavy nuclei. Highly energetic quarks and gluons, collectively referred to as partons, lose energy as they travel through this matter, leading to suppressed production of particles with large transverse momenta ((Formula presented)). Conversely, high-(Formula presented) particle suppression has not been seen in proton-lead collisions, raising questions regarding the minimum system size required to observe parton energy loss. Oxygen-oxygen (OO) collisions examine a region of effective system size that lies between these two extreme cases. The CMS detector at the CERN LHC has been used to quantify charged-particle production in inclusive OO collisions for the first time via measurements of the nuclear modification factor ((Formula presented)). The (Formula presented) is derived by comparing particle production to expectations based on proton-proton ((Formula presented)) data and has a value of unity in the absence of nuclear effects. The data for OO and (Formula presented) collisions at a nucleon-nucleon center-of-mass energy (Formula presented) correspond to integrated luminosities of (Formula presented) and (Formula presented), respectively. The (Formula presented) is below unity with a minimum of (Formula presented) around (Formula presented). The data exhibit better agreement with theoretical models incorporating parton energy loss as compared to baseline models without energy loss.