RpPb

RpPb, or R_pPb, is a nuclear modification factor used in high-energy nuclear physics to quantify how particle production in proton–lead (p+Pb) collisions differs from proton–proton (pp) collisions when scaled by the expected number of binary nucleon-nucleon interactions. It provides a way to study the influence of the nuclear environment on particle yields and to distinguish initial-state effects from final-state interactions observed in heavy-ion collisions.

A common definition is RpPb(pT, η) = [1/N_events^pPb d^2N_pPb/dpT dη] / [⟨N_coll⟩ × (1/N_events^pp d^2N_pp/dpT dη)], where N_events^pPb and N_events^pp are the numbers of analyzed events in p+Pb and pp data, respectively, and ⟨N_coll⟩ is the average number of binary nucleon–nucleon collisions in p+Pb. An equivalent formulation uses the nuclear overlap function, RpPb = [d^2N_pPb/dpT dη] / [⟨T_pPb⟩ d^2σ_pp/dpT dη], with ⟨T_pPb⟩ related to ⟨N_coll⟩. In either form, RpPb ≈ 1 indicates yields scale with the number of binary collisions, while deviations point to cold nuclear matter effects such as nuclear parton distribution function modifications, multiple scattering (Cronin effect), or energy loss in the nucleus.

Measurements by LHC experiments (ALICE, ATLAS, CMS) have generally found RpPb values near unity over a broad range of transverse momentum, with species- and rapidity-dependent deviations in some regions. RpPb serves as a baseline for interpreting results in nucleus–nucleus collisions and for constraining nuclear parton distribution functions and other cold nuclear matter phenomena.