StringSpinner - adding spin to the PYTHIA string fragmentation

StringSpinner is a plugin allowing for the introduction of spin effects in the hadronization part of the PYTHIA 8 event generator. In this first version the implementation is limited to handling pesudoscalar mesons in events without parton showers. The spin effects are generated by propagating the quark polarization along the fragmentation chain according to the rules of the quantum mechanical string+P03 model. By using parametrizations for the transversity parton distribution functions, StringSpinner allows to simulate the polarized semi inclusive deep inelastic scattering process on protons and neutrons. The instructions to use StringSpinner and to modify the parametrizations of the transversity distribution functions are presented, as well as an example of the calculation of the Collins asymmetries as obtained from simulations of the transversely polarized semi inclusive deep inelastic scattering process. Program summary: Program Title: StringSpinner CPC Library link to program files: https://doi.org/10.17632/37jrpmyz3t.1 Developer's repository link: https://gitlab.com/albikerbizi/stringspinner.git Licensing provisions: GNU GPL v2 or later Programming language: C++, Fortran Nature of problem: To analyse and interpret the experimental data on the study of the nucleon spin structure, as well as to make predictions for future experiments simulations are needed. Solution method: Introduce spin effects in the hadronization part of a complete Monte Carlo event generator, largely used in the community of high energy particle physicists.