Gluon saturation at small momentum fraction

5 problems

• What suppresses coherent $J/\psi$ photoproduction on lead at small $x$?

  Open

  In near-miss collisions of lead nuclei, a photon from one nucleus can knock a $J/\psi$ particle out of the other while leaving it intact, and the rate measures the gluon density. The measured rate is strongly reduced at very small momentum fractions, and whether this is saturation or a milder linear effect is unclear.

• Is forward dihadron suppression in proton-nucleus collisions a saturation signal?

  Contested

  When a proton hits a gold nucleus, pairs of particles flying forward are less often back-to-back than in proton-proton collisions, which saturation predicts. Other effects, such as extra kicks from ordinary scattering in the nucleus, might also cause it.

• Which Electron-Ion Collider measurement can decide on gluon saturation?

  Open

  The Electron-Ion Collider, being built at Brookhaven, is designed to settle whether gluons saturate. Which of its many possible measurements gives the cleanest yes or no, given its limited reach to small momentum fractions, still has to be determined.

• Does gluon density saturate at small momentum fraction?

  Contested

  The gluon density inside protons and nuclei rises steeply as the momentum fraction x shrinks, and theory says gluon recombination must eventually slow it. No measurement has yet shown this slowing in a way that ordinary, linear evolution of the gluon distribution cannot also explain.

• Size of the saturation scale in heavy nuclei

  Open

  The saturation scale is the momentum below which gluons in a nucleus are expected to be packed densely, and heavier nuclei should reach it sooner. Its actual value for gold or lead is uncertain by a large factor.