Probing gluonic saturated matter

To advance our understanding of gluonic saturated matter at the LHC, the ALICE collaboration has presented a new study using photon-induced interactions in ultra-peripheral collisions (UPCs). In this type of collision, one beam emits a very high energetic photon that strikes the other beam, giving rise to photon–proton, photon–nucleus and even photon–photon collisions. 

While we know that the proton – and most of the visible matter of the universe – is made of quarks bound together by gluons, quantum chromodynamics (QCD)  has not yet provided a complete understanding of the rich physics phenomena that occur in high-energy interactions involving hadrons. For example, it is not known how the distribution of gluons evolve at low values of Bjorken-x. The rapid increase in gluon density observed with decreasing x cannot continue forever as it would eventually violate unitarity. At some point “gluon saturation” must set in to curb this growth.

So far, it has been challenging to experimentally establish when saturation sets in. One can expect, however, that it should occur at lower energies for heavy nuclei than for protons. Thus, the ALICE Collaboration has studied the energy dependence of UPC processes for both protons and heavy nuclei. At the same time, other physics phenomena, such as gluon shadowing originating from multi-scattering processes, can exist with similar experimental signatures. The interplay between these phenomena is still an open problem in QCD.

ALICE has presented new results on J/ψ meson-production UPC, where the photon probes the whole nucleus. The new ALICE results, analysed using LHC Run 1 and Run 2 data, probe a wide range of photon-nucleus collision energies from around 10 GeV to 1000 GeV. These results confirm previous measurements by ALICE, obtained at lower energies, that indicated a strong nuclear suppression when such photon–nucleus data are compared to expectations from photon–proton interactions. The present analysis employs novel methods for extracting the energy dependence, providing new information to test theo­retical models. The present data at high energies can be described by both saturation-based and gluon shadowing models. The coherent J/ψ meson production at low energy, in the anti-shadowing region, is not described by these models, nor can available models fully describe the energy dependence of this process over the explored energy range.

ALICE will continue to investigate these phenomena in LHC Runs 3 and 4, where high-precision measurements with larger data samples and upgraded detectors will provide more powerful tools to better understand gluonic saturated matter.