Unambiguous Observation of Initial-State Energy Loss in Cold Nuclear Matter at E906 SeaQuest

Partonic energy loss is an essential probe of the nuclear medium properties in fixed target and heavy ion collisions, yet remains imprecisely determined up to now. The E906 SeaQuest spectrometer at Fermilab measures charmonium and Drell-Yan dimuon production in p+A collisions with fixed targets H, D, C, Fe, and W, at $\sqrt{s_{NN}} = 15$ GeV ($E_{\text{beam}} = 120$ GeV). The experiment provides an ideal phase space with minimal shadowing for measuring unambiguous signals of initial-state energy loss in cold nuclear matter. We observe a clear suppression in the nuclear modification in good agreement with theoretical predictions. These measurements will put new constraints on the partonic stopping power, radiation length, and transport coefficient.