Spin asymmetries for vector boson production in polarized p+p collisions

TL;DR

This paper analyzes vector boson production in polarized proton collisions, presenting cross sections and spin asymmetries using TMD factorization, and assesses their experimental feasibility at RHIC to probe nucleon spin structure.

Contribution

It provides the first detailed calculation of vector boson spin asymmetries in polarized collisions using TMD formalism and estimates their size for experimental testing at RHIC.

Findings

Some spin asymmetries can be sizable without strong TMD evolution suppression.

W boson production can probe the Sivers function and transverse helicity distributions.

Results suggest potential for constraining TMD universality and nucleon structure.

Abstract

We study the cross section for vector boson ($W^{\pm}/Z^0/\gamma^*$) production in polarized nucleon-nucleon collisions for low transverse momentum of the observed vector boson. For the case where one measures the transverse momentum and azimuthal angle of the vector bosons, we present the cross sections and the associated spin asymmetries in terms of transverse momentum dependent parton distribution functions (TMDs) at tree level within the TMD factorization formalism. To assess the feasibility of experimental measurements, we estimate the spin asymmetries for $W^{\pm}/Z^0$ boson production in polarized proton-proton collisions at the Relativistic Heavy Ion Collider (RHIC) by using current knowledge of the relevant TMDs. We find that some of these asymmetries can be sizable if the suppression effect from TMD evolution is not too strong. The $W$ program at RHIC can, thus, test and constrain spin theory by providing unique information on the universality properties of TMDs, TMD evolution, and the nucleon structure. For example, the single transverse spin asymmetries could be used to probe the well-known Sivers function $f_{1T}^{\perp q}$, as well as the transversal helicity distribution $g_{1T}^{q}$ via the parity-violating nature of $W$ production.