The quark transversity distribution is one of three leading-twist parton distribution functions describing the structure of the nucleon. The transversity distribution is particularly difficult to measure because it is a chiral-odd distribution, manifesting as an interference effect between multiple helicity amplitudes. It appears convolved with other chiral odd distributions/fragmentation functions in the cross section of several processes. In particular, in semi inclusive deep inelastic scattering (SIDIS), it appears convolved with the Collins function, resulting in an azimuthal asymmetry. The Collins function also appears independently (convolved with itself) in the cross section for semi-inclusive hadron pair production in lepton-antilepton collisions (SIHPP), resulting a Collins-effect asymmetry for this process as well. Using a phenomenological parametrization of the Collins functions and transversity distribution, one can perform a simultaneous fit of the Collins asymmetries in SIDIS and SIHPP, thereby extracting the transversity distribution and the Collins functions.

Similar extractions have been performed in the past with some success. UCLA’s own Dr. Zhongbo Kang (et al.) showed in a 2017 paper (https://arxiv.org/pdf/1707.00913.pdf) that similar asymmetries can be found for hadron-production inside jets (HIJ), and that the Collins functions appearing are likely the same universal Collins functions appearing in SIDIS and SIHPP, providing another source of data for a global extraction of the distributions. Data on these asymmetries have been obtained, as well as new data on the asymmetries in the SIDIS and SIHPP processes.

In this project we are performing multiple global fits of the distributions using the available data for these processes. To start out, we are only considering the SIDIS and SIHPP processes, but performing fits using all of the data that has accumulated since the last such analysis was performed. This gives a baseline for comparison with the results of the full fit, where the hadron-in-jet data will also be included, and where the Q^2 evolution of the distributions will be accounted for in more detail. After having extracted the distributions, we will then be able to make predictions for Collins asymmetries in future SIDIS, SIHPP, and HIJ experiments which can then be compared with the results of these future measurements.

Author: Alexander M. Czajka, UCLA Physics Department, Kang Research Group