Hadronization

Hadronization is the process by which color-charged quarks and gluons produced in high-energy reactions become color-neutral hadrons. Because color confinement forbids free quarks and gluons from propagating over long distances, partons emerging from hard scatterings or parton showers must hadronize as the strong interaction becomes non-perturbative at low energy scales (~1 GeV).

Two widely used pictures describe hadronization in phenomenology: string fragmentation and cluster hadronization. In the string picture, a color flux tube connects separating partons; when it stretches, it breaks by creating quark-antiquark pairs, producing a cascade of hadrons that forms jets. In the cluster approach, colored partons recombine into color-singlet clusters that decay into hadrons.

Hadronization occurs in electron-positron annihilation, deep-inelastic scattering, hadron-hadron collisions, and heavy-ion collisions. In heavy-ion environments, hadronization competes with the expansion of a deconfined medium and can proceed via recombination or fragmentation, contributing to phenomena such as jet quenching and enhanced baryon production at intermediate transverse momentum.

Experimental observables include hadron spectra, multiplicities, baryon-to-meson ratios, and fragmentation functions, which encode how parton momentum is distributed among final-state hadrons. These are modeled with tunable parameters in Monte Carlo event generators and tested against jet and inclusive-hadron data.

Theoretically, hadronization reflects non-perturbative QCD; lattice QCD has limited applicability for real-time hadronization, while phenomenological models remain essential for translating parton-level predictions into observable hadrons.

Hadronization is central to interpreting collider experiments and testing QCD, providing a bridge between the quark-gluon level and the particle-level signals detected by detectors.