The photon is arguably the most universally important particle across all fields of physics. Despite its status as a fundamental particle, at high energies the photon can be seen as a hadronic source of partons. The partonic content of the photon is very poorly constrained compared to that of the proton, with uncertainties in the photon parton distribution functions typically 1 or 2 orders of magnitude larger than their proton counterparts, despite the fact that its source, the $\gamma \to q\overline{q}$ splitting, is perturbatively calculable. The high luminosity, excellent particle identification, and far-backward electron tagging capabilities of the Electron-Ion Collider (EIC) make it an ideal environment for studying photon parton distribution functions. Similar to the $p+p$ or $\pi +p$ systems, photoproduction at the EIC can be thought of as two parton distributions colliding. One of the most powerful processes in such collisions is production of lepton pairs, i.e. $h+p\to l^{+}{l}^{-}+X$, known as the Drell-Yan process. This process has the ability to access for the first time the transverse-momentum-dependent parton distributions of the photon. The transversely polarized proton beam of the EIC additionally provides a possible means of accessing the transversity distribution of the proton without relying on fragmentation functions.

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• Lepton production
• Parton distribution functions
• Quantum chromodynamics
• Transverse momentum dependent distribution
• Photons
• Pions
• Protons