protonproton

Proton-proton interactions refer to collisions between two protons, a central topic in high-energy particle physics. Protons are composite particles made of valence quarks (two up quarks and one down quark) bound by gluons, with additional sea quarks and gluons. In these interactions, electromagnetic repulsion is overcome at short distances by the strong force, described by quantum chromodynamics (QCD).

At high energies, proton-proton collisions proceed through parton-level processes in which quarks and gluons from each proton scatter, radiate, and hadronize into observable final states. The outcomes are categorized as elastic scattering, where protons remain intact, and inelastic scattering, where the protons break up and produce multiple hadrons. Electroweak processes can also occur, producing W and Z bosons or, in rarer cases, Higgs bosons and heavy quarks.

Key observables include the total cross section, elastic and inelastic cross sections, and differential cross sections as a function of momentum transfer. The proton-proton cross section generally rises with collision energy, a behavior described by concepts from Regge theory and perturbative QCD at high momentum transfers. Parton distribution functions describe the momentum sharing among constituent quarks and gluons, and factorization theorems separate short- and long-range effects in calculations and simulations.

Proton-proton collisions are central to experiments at the Large Hadron Collider, where high-energy pp collisions enable precision tests of the Standard Model and searches for new physics. They also model cosmic ray interactions in the atmosphere and form the dominant background in many new-physics analyses. Theoretical work combines perturbative QCD, lattice methods for nonperturbative effects, and phenomenological modeling to interpret data and generate event simulations.