In particle physics, hadronization (or hadronisation) is the process of the formation of hadrons out of quarks and gluons. This occurs after high-energy collisions in a particle collider in which quarks or gluons are created. Due to colour confinement, these cannot exist individually. In the Standard Model they combine with quarks and antiquarks spontaneously created from the vacuum to form hadrons. The QCD (Quantum Chromodynamics) of the hadronization process are not yet fully understood, but are modeled and parameterized in a number of phenomenological studies, including the Lund string model and in various long-range QCD approximation schemes.
The tight cone of particles created by the hadronization of a single quark is called a jet. In particle detectors, jets are observed rather than quarks, whose existence must be inferred. The models and approximation schemes and their predicted jet hadronization, or fragmentation, have been extensively compared with measurement in a number of high energy particle physics experiments, e.g. TASSO, OPAL and H1.
Hadronization also occurred shortly after the Big Bang when the quark–gluon plasma cooled to the temperature below which free quarks and gluons cannot exist (about 170 MeV). The quarks and gluons then combined into hadrons.
A top quark, however, has a mean lifetime of 5×10−25 seconds, which is shorter than the time scale at which the strong force of QCD acts, so a top quark decays before it can hadronize, allowing physicists to observe a "bare quark." Thus, they have not been observed as components of any observed hadron, while all other quarks have been observed only as components of hadrons.
Hadronization simulation and models
Hadronization can be explored using Monte Carlo simulation. After the particle shower has terminated, partons with virtualities (how far off shell the virtual particles are) on the order of the cut-off scale remain. From this point on, the parton is in the low momentum transfer, long-distance regime in which non-perturbative effects become important. The most dominant of these effects is hadronization, which converts partons into observable hadrons. No exact theory for hadronization is known but there are two successful models for parameterization.
These models are used within event generators which simulate particle physics events. The scale at which partons are given to the hadronization is fixed by the Shower Monte Carlo component of the event generator. Hadronization models typically start at some predefined scale of their own. This can cause significant issue if not set up properly within the Shower Monte Carlo. Common choices of Shower Monte Carlo are PYTHIA and HERWIG. Each of these correspond to one of the two parameterization models.
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