Recent Developments of Small-$x$ Evolution for Quark and Gluon Helicity

Abstract

Helicity of quarks and gluons inside the proton at small Bjorken $x$ is one of the missing pieces of the proton spin puzzle, with limited experimental results due to various challenges. To address the problem, we derive under the framework of color glass condensate (CGC) effective theory a renormalization group equation in rapidity for the parton helicity distributions, which resums double-logarithmic factor, $\alpha_s\ln^2(1/x)$, with $\alpha_s$ being the strong coupling constant. With running coupling, the equation produces results at Bjorken $x\leq 0.1$ that are consistent with the world polarized scattering data. There is an evidence for a significant parton helicity contribution from the small-$x$ region. However, the helicity estimate still contains a large uncertainty that will dramatically reduce with the upcoming measurements from the Electron-Ion Collider (EIC). Ongoing attempts to reduce the uncertainty of our helicity prediction in the medium term will also be discussed.