The $\cos 2\varphi$ azimuthal asymmetry of unpolarized dilepton production at the $Z$-pole

Abstract

We calculate the Boer-Mulders effect contribution to the $\cos 2\varphi$ 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.

comments and references added, journal version

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• Drell-Yan lepton angular distributions in perturbative QCD
• Azimuthal asymmetries in lepton-pair production at a fixed-target experiment using the LHC beams (AFTER)
• Boer-Mulders effect in the unpolarized pion induced Drell-Yan process at COMPASS within TMD factorization
• Boer-Mulders function of pion meson and $q_T$-weighted $\cos 2\varphi$ asymmetry in the unpolarized ${\pi }^{-}p$ Drell-Yan at COMPASS
• Perturbative $T$-odd asymmetries in the Drell-Yan process revisited