Probing Parton Distributions in Proton using Drell-Yan and Charmonium Production in $p+p$ and $p+d$ Interactions with 120 GeV Proton Beam at Fermilab

E906/SeaQuest is a fixed-target experiment at Fermilab with a 120 GeV proton beam. The primary goal of the experiment is to study the partonic structure of the nucleon. Muon pairs with mass between 2 to 9 GeV from the interaction of proton beam with various targets, including liquid hydrogen and deuterium, have been detected. These events contains muons originating from the Drell-Yan process as well as charmonium decays. The measurement of the Drell-Yan $\sigma_{pd}/2\sigma_{pp}$ cross section ratio is particularly sensitive to the light sea-quark asymmetry in the proton. The SeaQuest measurement provides information on the $\bar{d}/\bar{u}$ asymmetry for Bjorken $x$ ranging from 0.1 to 0.45. The improved statistics of the SeaQuest measurement allows for a check on the surprising behavior of $\bar{d}/\bar{u}$ at large $x$ observed in E866/NuSea. The Drell-Yan cross section ratio is extracted from the entire SeaQuest dataset using two independent methods. The measured $\sigma_{pd}/2\sigma_{pp}$ ratio is found to remain about unity, indicating the $\bar{d}>\bar{u}$ for $x$ up to 0.45. The new result is compared with various parton distribution functions and theoretical calculations. In contrast to the Drell-Yan process, the charmonium production can be used to probe both the quark content as well as the gluon content. While the charmonium production mechanism is not as well understood as the Drell-Yan process, the large $J/\psi$ production cross section allows for a higher statistics measurement. The SeaQuest charmonium production measurement covers the forward rapidity region of $0.5 < x_F <0.9$. The measured cross sections are found to be in good agreement with theoretical calculations based on nonrelativistic QCD (NRQCD). The $\sigma_{\psi^\prime} / \sigma_{J/\psi}$ cross section ratios are found to increase as $x_F$ increases, indicating that the $q\bar{q}$ annihilation process has larger contributions to the $\psi'$ production than the $J/\psi$ production. The $\sigma_{pd}/2\sigma_{pp}$ cross section ratios are observed to be significantly different between the Drell-Yan process and charmonium production, reflecting the different production mechanisms. The $P_T$ distributions for $J/\psi$ and $\psi^\prime$ production are also presented and compared with data collected at higher energies.

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