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SpinQuest will measure the correlation between the angular distribution of the di-muons and the proton spin. If this is non-zero, then the antiquarks must have some orbital angular momentum.
The first measurement of the sea quark Sivers function is not SpinQuest but has been performed by the Star collaboration at RHIC using W/Z boson production rather than Drell-Yan. https://arxiv.org/pdf/1511.06003.pdf
This may be replaced by:
Initial attempts to measure the Sivers asymmetry for sea quark Sivers have been reported by the STAR collaboration at RHIC using W/Z boson production. Their data is statistically limited and favor a sign-change only, if TMD evolutions effects are significantly smaller than expected. https://arxiv.org/pdf/1511.06003.pdf
SpinQuest will perform the first measurement of the Sivers asymmetry in Drell-Yan pp scattering from the sea quarks.
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