CERN's ALICE Zoomed In on Lead Nuclei to a Fifth of a Femtometer and Found Fewer J/ψ Particles Than the Old Model Allows. Gluons Are Piling Up on Each Other.

Almost all of the mass of ordinary matter comes not from quarks but from the gluons that bind them, and physicists have long suspected that inside a heavy nucleus those gluons get so crowded that they start interfering with one another. The ALICE collaboration at CERN's Large Hadron Collider has now published the sharpest evidence yet that this is happening. Looking inside lead nuclei at a resolution of 0.2 femtometers, about a quarter the size of a proton, the experiment found significantly fewer J/ψ particles than the conventional description of nuclear structure predicts.

The measurement, reported in Physical Review Letters under the title "Evidence for J/ψ Suppression in Incoherent Photonuclear Production," uses a trick that turns the LHC into something closer to a microscope than a demolition site. In so-called ultra-peripheral collisions, two lead nuclei pass close enough to feel each other's electromagnetic fields but do not physically collide. A photon from one nucleus strikes the other and can produce a J/ψ, a bound state of a charm quark and its antiquark. How often that happens, and with what momentum, depends on the density of gluons at the point of impact.

Previous studies of this process measured the "coherent" case, where the photon interacts with the nucleus as a whole and the result is an average over the entire nucleus. ALICE instead measured "incoherent" production, where the photon interacts with a small region and the nucleus breaks up. "Our experiments using incoherent production is like switching from a blurry image to a high-resolution microscope," said Daniel Tapia Takaki, a physicist at the University of Kansas who played a leading role in the analysis. The data, collected during the LHC's Run 2, span photon-nucleus energies from 20 to 633 billion electron volts and three spatial scales: 0.6, 0.3 and 0.2 femtometers. It is the first such measurement to track energy and momentum transfer at the same time.

Two ideas compete to explain what gluons do inside a nucleus. Nuclear shadowing treats the gluons in overlapping nucleons as partially obscuring one another, like layers of cloud blocking sunlight, and predicts a mild, smooth reduction in J/ψ yield. Gluon saturation, a prediction of quantum chromodynamics, says that at high enough density the gluons begin to interact strongly with each other, recombining and capping how many can occupy a given volume. Saturation predicts a stronger suppression that grows as the probe resolves finer and finer structure.

That is what ALICE saw. At the largest scale, the yields are consistent with shadowing. At the smallest, the J/ψ rate falls well below the shadowing prediction, with a statistical significance of about three standard deviations. "At these extraordinary scales, we observe evidence that gluons begin to behave collectively, a phenomenon known as gluon saturation," Tapia Takaki said. Three sigma is the threshold particle physicists call "evidence" rather than "discovery," which requires five, but the pattern across energies and scales is exactly the one saturation models produce.

The result matters beyond bookkeeping. Saturation is the expected state of gluons in the instant before two heavy nuclei collide, and it sets the initial conditions for the quark-gluon plasma that the LHC and the Relativistic Heavy Ion Collider study. It is also the primary physics goal of the Electron-Ion Collider under construction at Brookhaven National Laboratory, which is designed to map gluon density directly. ALICE's measurement suggests the phenomenon is already within reach at the LHC, and the collaboration expects the far larger Run 3 data set to push the significance higher.