Measurement of $(p+d)/(p+p)$ $J/\psi$ cross section ratio with 120 GeV proton beam in the SeaQuest experiment

The SeaQuest experiment at Fermilab was designed to measure dimuons produced in the interaction of 120 GeV proton beam with liquid hydrogen and liquid deuterium targets. First result on the measurement of sea-quark flavor asymmetry from the $(p+d)/(p+p)$ Drell-Yan cross section ratio is expected be reported soon. The SeaQuest experiment also provides a measurement of the $(p+d)/(p+p)$ $J/\psi$ cross section ratio, which offer an independent means to examine the sea-quark asymmetry in proton. The interest for measuring the $(p+d)/(p+p)$ $J/\psi$ cross section ratio will be discussed. Current status of the analysis will also be presented in this talk.