Nuclear Physics Seminar

The Collins Fragmentation Functions allow access to parton transversity distribution function in nucleon in SIDIS experiments, and have been widely explored recently both in theoretical and experimental studies. Most notably, the experimental measurements hint that, unlike the unpolarized fragmentation functions, the favored and unfavored Collins functions have a similar size but opposite sign. This is in distinct contrast with most theoretical models, which lack a mechanism for calculating the unfavored fragmentation functions altogether. In this talk I will present our recent work on extending the NJL-jet model to accommodate hadronization of a transversely polarized quark and explore the Collins effect. Our model calculation of transverse momentum dependent pion and kaon Collins functions will be presented. The remarkable feature of this model is that, for very clear physical reasons which we explain, the one-half moment of the favored Collins fragmentation functions, are positive and peak at large values of $z$, but decrease and oscillate at small values of $z$. The one-half moment of the unfavored Collins functions have comparable magnitude but opposite sign to the favored functions, vanish at large $z$ and peak at small values of $z$. Moreover, the transverse momentum dependence of the model Collins functions differs significantly from the Gaussian form widely used in empirical parametrizations.