K. Nakano,*¹,*² Y. Goto,*² Y. Miyachi,*³ K. Nagai,* 4 S. Sawada,*²,* 5 and T. -A. Shibata*²,*6 for the E906/SeaQuest Collaboration The nuclear effects on the partonic structure of nucle- ons signify that the distribution of partons in a nucleon bound inside a heavy nucleus changes from that in a free nucleon. It was first discovered by the EMC col- laboration in 1983 1 ) and is being studied using various measurements and theories to understand its mecha- nism.  Most measurements made use of deep inelastic scattering (DIS), which does not differentiate between quarks and anti-quarks. The E772 experiment utilized the Drell-Yan process to extract the nuclear effects on anti-quarks. 2 ) It indicates that the effects are smaller than those observed in DIS, although the statistical pre- cision was limited.  If the difference between quarks and anti-quarks in the nuclear effects is confirmed, it will be a strong constraint in understanding the mechanism because it contradicts, for example, the conventional nuclear model.3) A number of theoretical models were proposed to reproduce the measured difference. 4) The E906/SeaQuest experiment at Fermilab aims to study the anti-quark distributions in the nucleon and nuclei. It utilizes the 120-GeV proton beam from the Fermilab Main Injector ( √ s = 15 GeV). It employs liq- uid hydrogen (LH 2 ), liquid deuterium (LD 2 ), carbon, iron, and tungsten as targets to measure the Drell- Yan process in p + p, p + d, and p + A reactions. In the Drell-Yan process, a quark in one hadron and an anti-quark in the other hadron are annihilated to pro- duce a virtual photon, which then decays into a lepton pair: q + q → γ* → l+ + l-. When it is measured at forward rapidity ( x F ≫ 0), an anti-quark nearly al- ways originates from the target-side hadron.  Therefore, the ratios of per-nucleon cross sections of two targets, R(A/D) = (σp+A/A)/(σp+d/2), are sensitive to the nuclear effects on anti-quarks. The SeaQuest spectrometer5) detects the final-state muon pair of the Drell-Yan process. SeaQuest ac- quired physics data from November 2013 to July 2017 with a summer accelerator shutdown each year. It has recorded 1.4 × 10¹ 8 beam protons on targets and an- alyzed approximately 40% of the recorded data.  We selected dimuons with the invariant mass larger than 4.2 GeV, for which the Drell-Yan events are domi- nant. The dimuon yields have been corrected for track- reconstruction efficiency and background dimuons by using the extrapolation method. 6 ) Fig. 1. Ratios of per-nucleon cross sections of C, Fe, and W to D versus the Bjorken x of target-side partons ( x T ). The SeaQuest preliminary result is drawn with blue cir- cles, and the E772 result is drawn with red squares. Figure 1 shows the preliminary result of R(A/D) measured using SeaQuest, together with the result ob- tained using E772 for comparison. The systematic un- certainty arises mostly from the fitting shape used in the extrapolation method. SeaQuest is consistent with E772 in the commonly measured region, xt ～ 0.2, with better accuracy at higher xt. It favors the observation by E772 that the nuclear effects on anti-quarks differ from those on quarks. Improvements in the measured precision and comparisons to theoretical models in or- der to finalize the result are ongoing.