hadronisation

Hadronisation, or hadronization, is the process by which colored quarks and gluons produced in high-energy interactions form color-neutral hadrons. It is a non-perturbative aspect of quantum chromodynamics (QCD) that follows parton showering and precedes the observation of final-state hadrons, yielding the jets seen in detectors.

The dominant modelling approaches are fragmentation (string/cluster hadronization) and quark recombination. In fragmentation, color fields between separating partons form a flux tube that breaks by producing quark–antiquark pairs, creating hadrons along the string. The Lund string model, implemented in event generators such as PYTHIA, and cluster models used in HERWIG are widely employed frameworks. Fragmentation functions describe the probability that a parton will form a given hadron with a particular momentum fraction.

Recombination or coalescence is an alternative mechanism that gains relevance in hot, dense media such as a quark–gluon plasma, where nearby quarks can combine into hadrons as the system cools. This process can enhance baryon production and modify flow observables, especially at intermediate transverse momentum. In heavy-ion collisions, both fragmentation and recombination can contribute to the final hadron spectrum.

Hadronisation reflects confinement and non-perturbative QCD physics. It is constrained by experimental data from electron–positron, lepton–hadron, and hadron–hadron collisions, and by lattice QCD studies of the QCD transition. The transition from quark–gluon plasma to hadrons is believed to be a crossover occurring around 150–160 MeV. Understanding hadronisation is essential for interpreting jet structure and particle production in high-energy experiments.