Leading-twist Single-transverse-spin asymmetries: Drell-Yan and Deep-Inelastic Scattering

TL;DR

The paper analyzes leading-twist single-spin asymmetries in SIDIS and Drell–Yan, showing they arise from transverse-momentum dependent PDFs with Wilson-line operators (the Sivers effect) and are consistent with factorization. It demonstrates that time-reversal arguments do not forbid these T-odd distributions because Wilson lines distinguish DIS and Drell–Yan, predicting a sign reversal between the two processes. It also identifies an additional T-odd distribution, $h_1^ot$, that can generate azimuthal asymmetries in unpolarized Drell–Yan and discusses the role of Sudakov evolution. The work provides testable predictions for RHIC and frames a broader approach to interpreting Sivers- and Boer–Mulders-type effects in hadronic structure.

Abstract

Recently, Brodsky, Hwang and Schmidt have proposed a new mechanism that gives a transverse spin symmetry at leading twist in semi-inclusive deep-inelastic scattering. I show that the new mechanism is compatible with factorization and is due to an transverse-spin asymmetry in the k_T distribution of quarks in a hadron (the "Sivers asymmetry"). An earlier proof that the Sivers asymmetry vanishes because of time-reversal invariance is invalidated by the path-ordered exponential of the gluon field in the operator definition of parton densities. Instead, the time-reversal argument shows that the Sivers asymmetry is reversed in sign in hadron-induced hard processes (e.g., Drell-Yan), thereby violating naive universality of parton densities. Previous phenomenology with time-reversal-odd parton densities is therefore validated.

Figures (4)

• Figure 1: Graph with SSA for SIDIS.
• Figure 2: Factorization at low transverse momentum for SIDIS.
• Figure 3: Light-like surface used to define the light-front wave functions and parton densities.
• Figure 4: Graph with SSA for Drell-Yan.