Modeling quark spin effects in hadronization

Hadronization is the process of formation of hadrons out of quarks and gluons, and it is one of the most intriguing and still poorly understood aspects of Quantum Chromodynamics (QCD). It belongs to the non- perturbative domain of QCD and is often described by means of fragmentation functions (FFs). Particularly relevant examples of FFs are the spin-dependent Collins FF and the interference FF, which describe the production of hadrons and hadron pairs in transversely polarized quark jets. They are mainly used to access the transverse spin and momentum distribution of quarks in hadrons, but at the same time they are also a precious source of information on the quark spin dependence of QCD. The seminar reviews the last developments on the modeling of quark spin effects in hadronization in the context of the “string+3P0” model, a quantum mechanical extension of the Lund string fragmentation model. The string+3P0 model combines the description of hadronization as the breaking of a relativistic string stretched between color charges with the assumption that the string breaking occurs via tunnelling of quark-antiquark pairs in the relative 3P0 state. It has been used for the simulation of the polarized semi-inclusive DIS (SIDIS) process, either via standalone Monte Carlo generators or by an interface to the known Pythia 8 event generator. More recently, the model has been extended to describe the quark spin effects in e+e- annihilation to hadrons, and simulations of this process have been enabled by an interface with the Pythia 8 generator. The main results obtained so far for SIDIS and e+e- annihilation, as well as the possible future applications of the model are presented and discussed.