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

Understand

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.