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Open Access Publications from the University of California

Measurement of Transverse Single Spin Asymmetries for $\pi^{0}$ and Jets at Large $x_{F}$ in $\sqrt{s}$ = 500 GeV Polarized Proton Collisions at STAR

  • Author(s): Pan, Yuxi
  • Advisor(s): Huang, Huanzhong
  • et al.
Abstract

Asymmetries in the production of final state particles from collider or fixed-target experiments with polarized beams or target could provide valuable information on the spin structure of the nucleon. Transverse and longitudinal spin asymmetry measurements are complementary approaches for probing the nucleon spin structure functions and understanding the QCD dynamics for such phenomena, due to the non-commutative nature of boosts and rotations in relativistic regime.

The research presented in this thesis focuses on characterizing the Transverse Single Spin Asymmetries (TSSA) of inclusive $\pi^{0}$ and jet productions in polarized proton collisions at large Feynman-x ($x_{F}$) region. Theoretically these observables are explained by means of collinear twist-3 multi-parton correlations. In this picture the asymmetries originate from initial-state twist-3 parton distributions in the polarized proton and/or through the coupling between proton's initial-state transversity distribution and the final-state twist-3 fragmentation functions. The measurement of SSA for forward inclusive $\pi^{0}$ produced in $pp$ collisions up to high transverse momentum helps examine the validity and interplay of these initial- and final-state models. These models can be further explored by investigating the dependence of the TSSA on event topologies. This thesis presents the STAR measurement of TSSA for forward inclusive $\pi^{0}$ at $2.8 < \eta < 4.0$ in $\sqrt{s}$ = 500 GeV $pp$ collisions taken in 2011 with integrated luminosity of 22 $pb^{-1}$ as well as the dependence of $\pi^{0}$ TSSA on event topologies. In addition we show results of the analyses of Sivers and Collins asymmetries for forward jet-like events. The physics implications of these measurements will also be discussed.

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