Contribution invited talk
Experimental Signals of Low-x Saturation
The power-law growth of gluon and sea quark PDFs that has been experimentally observed in the small-x regime is fundamentally inconsistent with basic tenets of quantum field theory. This phenomenon is driven by the explosive rate of soft gluon bremsstrahlung which is a fundamental feature of QCD (or any non-Abelian field theory). In order to be consistent with essential features such as unitarity, this power-law growth of color-charge density at small x must eventually be softened by the onset of a high-density phase of QCD. This behavior, termed “gluon saturation,” is a non-negotiable consequence of QCD at high energies or small x. Observing the onset of gluon saturation and characterizing its properties would elucidate not only the nature of the exotic high-density regime of QCD, but also the fundamental mechanisms by which any quantum field theory can achieve UV completeness.
At present, there has been no unambiguous detection of gluon saturation. There are, however, a range of tantalizing signals consistent with saturation physics seen in electron-proton, proton-nucleus, and nucleus-nucleus collisions. The high-density phase of QCD can be described by the “color-glass condensate” effective field theory, which makes simultaneous consistent predictions for diverse phenomena from diffractive cross sections to jet quenching to multiparticle correlations. In this talk, I will discuss the common theoretical framework that underpins these predictions and compare it to the hints seen in experiment. I will also discuss the prospects for peering into the saturation regime with a future high-energy Electron-Ion Collider.