Exploring the Quark Transversity and the Collins Fragmentation Functions Using Polarized pp Collisions at STAR

Abstract

Understanding the internal spin structure of the nucleon still remains a challenge in strong interac-tion physics. Transversity, which describes the transverse spin structure of quarks in a transversely polarized proton, is poorly constrained by experimental data. Since it is chiral-odd, it can only be ac-cessed through channels that couple to other chiral-odd distributions, like the Collins fragmentation functions (so-called Collins e ff ect) or the interference fragmentation functions. Recently, a detailed calculation using the soft-collinear e ff ective theory found that the Collins e ff ect in pp collisions involves a mixture of collinear and transverse momentum dependent (TMD) factorization. The Collins e ff ect provides a direct probe to the Collins fragmentation function and enables testing of its evolution, universality and factorization breaking in the transverse momentum dependent formalism. In 2018, STAR published the first measurements of Collins asymmetries for charged pions in jets in polarized pp collisions at √ s = 500 GeV based on data taken during 2011. These measurements probe Q 2 scales one to two orders of magnitude larger than similar measurements in semi-inclusive deep-inelastic scattering (SIDIS) and the results are consistent with predictions based on global analyses of e + e − and SIDIS data. In 2012 and 2015, STAR collected ∼ 14 pb − 1 and ∼ 52 pb − 1 of transversely polarized pp data at √ s = 200 GeV, respectively. These datasets provide the most precise measurement of the Collins e ff ect in 200 GeV pp collisions to date, especially at the quark momentum fractions 0 . 1 ≤ x ≤ 0 . 4. These proceedings present the preliminary results for Collins asymmetries of identified pions in jets in pp collisions at √ s = 200 GeV and comparisons to theory predictions.