Physics Opportunities with a Fixed-Target Program at the Electron-Ion Collider
A fixed-target program at the Electron-Ion Collider (EIC) would broaden the facility's scientific reach by providing key measurements for studies of cold nuclear matter (CNM), the QCD phase diagram, and nuclear reactions relevant for space radiation. Constraining CNM effects is essential for interpreting observables in proton-nucleus ($p+A$) and nucleus-nucleus ($A+A$) collisions, yet these effects are poorly understood at low center-of-mass energies. In particular, the range $\sqrt{s}\approx10$--20 GeV, where several CNM effects may compete at similar scales, has not been explored with high statistical precision. Mapping the QCD phase diagram similarly requires high-statistics $A+A$ data with broad rapidity and transverse-momentum coverage to probe the onset of deconfinement and the possible location of the QCD critical point (CP). Currently, such data are limited at 4.5 GeV $< \sqrt{s_{NN}} < 7.7$ GeV A fixed-target program at the EIC would fill these gaps, providing CNM baselines and complementary data for QCD CP studies. By delivering high luminosity and systematic measurements across a broad range of nuclear targets, the program would link $p+A$ and $A+A$ systems at the same center-of-mass energies, enabling a unified, quantitative description of cold QCD matter and clarifying the interpretation of QGP signatures in $A+A$ collisions at low energies where comparable $p+A$ data are lacking. Finally, the program would offer a unique opportunity to measure nuclear cross sections critical for improving cosmic-ray models, including studies of space-radiation protection for both autonomous spacecraft and long-duration human spaceflight.