The $\cos2\phi$ azimuthal asymmetry of unpolarized dilepton production at the $Z$-pole

TL;DR

This paper predicts a negative $ ext{cos}2 ext{phi}$ azimuthal asymmetry in unpolarized dilepton production near the Z-pole due to the Boer-Mulders effect, contrasting with the positive asymmetry at low $Q^2$, and suggests it is measurable at RHIC.

Contribution

It provides the first calculation of the Boer-Mulders effect contribution to the azimuthal asymmetry near the Z-pole, highlighting a sign change and its experimental implications.

Findings

Asymmetry near Z-pole is negative, opposite to low $Q^2$ region.

Magnitude of asymmetry is several percent, making it measurable.

Experimental confirmation would reveal the chiral odd structure of quarks.

Abstract

We calculate the Boer-Mulders effect contribution to the $\cos2\phi$ azimuthal asymmetry of unpolarized dilepton production near the $Z$-pole. Based on the tree-level expression in the transverse momentum dependent factorization framework, we show that the corresponding asymmetry near the $Z$-pole is negative, which is opposite to the asymmetry in the low $Q^2$ region, dominated by the production via a virtual photon. We calculate the asymmetry generated by the Boer-Mulders effect near the $Z$-pole at RHIC, with $\sqrt{s}=500$ GeV. We find that the magnitude of the asymmetry is several percent, and therefore it is measurable. The experimental confirmation of this sign change of the asymmetry from the low $Q^2$ region to the $Z$-pole provides direct evidence of the chiral odd structure of quarks inside an unpolarized nucleon.