Protonlead

Proton-lead, or p-Pb, denotes collisions between protons and lead nuclei in high-energy physics experiments. In collider programs at the Large Hadron Collider (LHC), p-Pb runs complement proton-proton and lead-lead data by isolating effects associated with cold nuclear matter and by providing baselines for interpreting heavy-ion collisions. The system enables studies of how parton distributions are modified inside nuclei and how particle production is affected by the nuclear environment without the formation of a hot quark-gluon plasma.

At the LHC, p-Pb collisions are conducted at a center-of-mass energy per nucleon pair of sqrt(s_NN) = 5.02 TeV. The beams are asymmetric; the proton beam carries about 4 TeV while the lead beam provides 1.58 TeV per nucleon, which produces a rapidity shift of roughly 0.465 in the center-of-mass frame. These conditions influence detector acceptance and the interpretation of rapidity-dependent measurements.

Key measurements include the nuclear modification factor RpPb, particle spectra, jets, heavy-flavor hadrons, and quarkonia across a range of transverse momenta and rapidities. By comparing p-Pb to p-p and Pb-Pb results, researchers assess cold nuclear matter effects such as nuclear shadowing, anti-shadowing, and initial-state parton energy loss, as well as possible gluon saturation at small x.

Experiments at the LHC, notably ALICE, ATLAS, CMS, and LHCb, have accumulated p-Pb data to map these effects across different kinematic regions. ALICE emphasizes low-pT and identified hadrons; LHCb focuses on forward rapidities; CMS and ATLAS cover mid- to high-pT ranges and jets.

Overall, RpPb measurements generally show consistency with unity at high pT, indicating limited final-state energy loss in cold nuclear matter, while deviations at low pT and forward rapidities reflect initial-state nuclear effects. Proton-lead data thus provide a crucial baseline for Pb-Pb studies and help constrain nuclear parton distribution functions.