Saturation measurements at the Relativistic Heavy Ion Collider

Over half a century of research has shown that nucleons themselves consist of quarks bound together by gluons, leading to the development of Quantum Chromodynamics (QCD) as the fundamental theory. Recent colliders, including HERA (Hadron-Electron Ring Accelerator), the only lepton-proton collider, have provided precise measurements of quark distributions within nucleons. However, the study of gluon dynamics has been more limited, particularly regarding nonlinear gluon dynamics - such as gluon saturation - which QCD predicts will emerge in high-energy collisions and remains largely unexplored.

Finding clear evidence of gluon saturation is a primary goal of the Cold QCD program at the Relativistic Heavy Ion Collider (RHIC) and the future Electron-Ion Collider (EIC) in the U.S. At RHIC, protons and deuterons serve as probes to study gluon saturation inside heavy nuclei in p(d)+A collisions. For the future EIC, electrons are expected to offer a cleaner probe in e+A collisions. RHIC, with its collision energy similar to that of the EIC, provides access to gluon dynamics in an overlapping phase space, making it an ideal hadron collider for testing the universality of gluon saturation across different collision systems. The complementary nature of the probes further enhances RHIC’s role in this research.

In this presentation, we will review both the inclusive and back-to-back correlation measurements conducted at RHIC in the search for gluon saturation, as well as future measurements planned with upgraded detectors. These efforts aim to test the universality of gluon saturation across different colliders and probes, preparing us well for the data-taking phase of the EIC.