TpPb

TpPb, commonly written p-Pb in many contexts, refers to collisions between a high-energy proton beam and a lead nucleus in accelerator-based nuclear and particle physics. The term TpPb appears in some papers and notes, but p-Pb remains the standard notation. TpPb collisions are used to study cold nuclear matter effects and to provide a baseline for heavy-ion Pb-Pb collisions.

These collisions are typically studied at the Large Hadron Collider. They are designed to probe how a proton interacts with a heavy nucleus without forming the hot, dense quark-gluon plasma associated with central Pb-Pb collisions. By comparing TpPb results to proton-proton (pp) data, physicists aim to isolate nuclear effects in the initial state and to test quantum chromodynamics in a nuclear environment. Observables include charged-particle production, identified hadrons, jets, heavy-flavor production, and quarkonia, as well as collective motion signals in high-multiplicity events.

A central quantity is the nuclear modification factor RpPb, defined as RpPb = (1/A) [d^2N_pPb/dpT dy] / [d^2σ_pp/dpT dy], where A is the lead mass number (A = 208). RpPb near unity at high transverse momentum suggests binary nucleon-nucleon scaling, while deviations at low pT indicate cold nuclear matter effects such as shadowing, the Cronin effect, or initial-state parton energy loss. Experiments at the LHC—ALICE, ATLAS, CMS, and LHCb—measure RpPb over broad ranges of pT and rapidity, often with a rapidity shift due to the asymmetric beam energies.

TpPb data serve as essential baselines for Pb-Pb studies, constrain nuclear parton distribution functions, and test perturbative QCD in nuclear environments. They also help tune event generators and improve understanding of particle production in nuclei. See also p-Pb, Pb-Pb, and heavy-ion physics.