Elucidating the Partonic Structure of the Photon with Drell-Yan at the Electron-Ion Collider

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 photon PDF uncertainties typically one or two orders of magnitude larger than their proton counterparts. The high luminosity and particle identification capabilities of the Electron-Ion Collider (EIC) make it an attractive system for studying photon parton distribution functions (PDFs). Similar to the $p+p$ or $\pi+p$ systems, photoproduction can be thought of as two a priori unknown parton distributions colliding. One of the most powerful processes in $h+h$ collisions is Drell-Yan production of lepton pairs, i.e. $h+p\rightarrow l^+l^-+X$. For $\gamma+p$ Drell-Yan at the EIC, the ability to tag the flavor of the single jet produced in the photon-going direction can provide novel constraints on the flavor-dependent photon PDFs. Furthermore, with more precise knowledge of the proton transverse-momentum-dependent (TMD) PDFs promised by the EIC, the photoproduction Drell-Yan process can provide the first experimental constraints on the TMDPDFs of the photon.