Offline Polarization Measurement and Transverse Single Spin Asymmetry Analysis in Fermilab SpinQuest Experiment

Fermilab’s SpinQuest (E1039) experiment will deliver the measurement of transverse single spin asymmetry, A_N, in J/ψ production and of the sea–quark Sivers asymmetry in Drell–Yan proton-proton scattering. Observing a nonzero asymmetry would provide compelling evidence that orbital angular momentum of sea quarks contributes to the proton’s spin. The experiment consists of a transversely polarized solid NH₃/ND₃ target and a 120 GeV unpolarized proton beam. Target polarization is produced by Dynamic Nuclear Polarization (DNP) in a 5 T field with a 140 GHz Extended Interaction Oscillator (EIO); the target system features a ⁴He evaporation refrigerator operating near 1 K, a 17,000 m³/hr Roots pumping system, and a Q–meter based NMR system for continuous polarization monitoring. Polarizations above 90% have been achieved, and both commissioning and the start of physics runs were taken during Summer 2024. The J/ψ TSSA was analyzed using the initial physics data where the signal dimuon selection was obtained by employing a neural–network classifier. We analyze A_N in both low- and high-mass dimuon windows, and present dependencies versus x_F, q_T , x₁, and x₂. Current efforts focus on improved tuning of combinatorial backgrounds and a comprehensive Monte Carlo and systematic studies are underway. Continued SpinQuest data and refined analyses will sharpen extractions of A_N across the J/ψ, ψ′, and Drell–Yan regions. In this talk I will cover the DNP system, the offline polarization determination, and the present status of the TSSA analysis.