ppb

pPb denotes proton-lead collisions, a high-energy nuclear collision in which a single proton beam collides with a beam of lead nuclei. Studied in collider experiments at CERN and elsewhere, pPb provides insight into quantum chromodynamics in cold nuclear matter and serves as a baseline for heavier systems. The center-of-mass energy per nucleon pair, sqrt(s_NN), for pPb at the LHC has been 5.02 TeV, with explorations at other energies. The system is inherently asymmetric because the proton and the lead ion carry different numbers of nucleons and beam energies.

Experiments such as ALICE, ATLAS, CMS, and LHCb measure particle production, jets, heavy-flavor hadrons, and quarkonia in pPb collisions. The aim is to understand initial-state effects in the nucleus, such as nuclear parton distribution functions and saturation, and to separate them from final-state effects seen in PbPb collisions. Observables include the nuclear modification factor RpPb, rapidity-dependent yields, and collective-like azimuthal correlations observed in high-multiplicity events, which have sparked discussion about possible hot-medium behavior in small systems.

pPb data help calibrate theoretical models of QCD in nuclei and test ideas about gluon density, energy loss in cold nuclear matter, and parton energy transport. While overall signatures of a quark-gluon plasma are weaker than in PbPb, certain measurements show flow-like patterns and particle production asymmetries that challenge simple interpretations, prompting ongoing theoretical and experimental work. Ongoing and planned pPb programs continue to refine measurements across energy, rapidity, and event activity, contributing to a consistent picture of nuclear effects in high-energy collisions.