Simplified recursive P30 model for the fragmentation of polarized quarks

Abstract

We revisit our recursive model for the fragmentation of polarized quarks based on the string+ 3 P0 mechanism of qq pair creation. We make a different choice for one input function of the model that simplifies the implementation in a Monte Carlo program. No new parameters are introduced, and the relevant results are the same apart from the suppression of the spin-independent correlations between successive quarks. In addition, the present version is more suitable for an interface with external event generators. The theoretical aspects and the simulation results obtained with a stand alone program are discussed in detail and compared with those of the previous version of the model.

Simulation of the Polarized Quark Fragmentation with Vector Meson Production

“…The 3 P 0 mechanism is parametrized by a complex mass parameter µ taken as in Ref. [5], the imaginary part of which is responsible for the transverse spin effects. The coupling of quarks to VMs with linear and transverse polarization with respect to the string axis (defined by the momentum of q A in the q A qB rest frame), is parametrized respectively by the complex parameters G L and G T .…”

“…A solid model of the fragmentation process of polarized quarks is the string+ 3 P 0 model which extends the Lund string fragmentation model [3] by the addition of the quark spin degree of freedom. Two slightly different versions of the model restricted to pseudoscalar mesons, M18 in [4] and M19 in [5], have been proposed and have been implemented in stand alone MC programs which give similar results. The simpler version M19 has been interfaced to the Pythia 8 hadronization [6] via the StringSpinner [7] package which allows the simulation of the Collins and dihadron asymmetries in SIDIS.…”

StringSpinner -- adding spin to the PYTHIA string fragmentation

“…The former rejection forces the fragmentation chain to evolve from the q A side towards the remnant side to emulate the recursive string+ 3 P 0 model of Ref. [12], and this restriction is set as default in the plugin. Alternatively, if h is not a pseudoscalar, it is rejected with a probability 1/2 in order to not change the composition of hadrons in the event, but only the k ′ T distributon of the leading pseudoscalars.…”

“…Otherwise the hadron is rejected and a new one is tried by PYTHIA until all selections are passed. The probability p is inspired to the splitting function of the recursive string+ 3 P 0 model [12], k ′ T is the transverse momentum of q ′ in the string rest frame and µ the complex mass parameter introduced by the 3 P 0 mechanism to parametrize the spin effects and assumed to be equal for all quark flavors. The imaginary part of µ is responsible for the transverse spin effects in the fragmentation process.…”

“…In this article we present StringSpinner, a program module that can be added to PYTHIA 8 [11] as a plugin for the simulation of the polarized SIDIS process, with the aim to making the code available to the comunity. StringSpinner allows to emulate the recursive string+ 3 P 0 model [12] of polarized quark fragmentation in the hadronization part of PYTHIA. The string+ 3 P 0 model is based on the Lund Model [13] of string hadronization already implemented in PYTHIA, with an additional assumption that the q q pairs at string breakups are produced with the vacuum quantum numbers L = 0, S = 1 and J = 0.…”