Light ion collisions at the LHC
In the week of 11-15 November 2024, more than 150 theorists, experimentalists and accelerator physicists met at CERN to discuss newly emerging opportunities with light ion beams, cern.ch/lightions. This workshop responded to the growing realisation that light ion collisions open qualitatively novel possibilities for the study of QCD at extreme temperature or parton densities. In particular, it has been understood recently that even short special runs can address central open questions resulting from recent discoveries of the LHC heavy ion programme, and that such short runs are also of interest for neighbouring physics communities including nuclear structure and cosmic rays. With this consensus report, the workshop participants summarise the main motivations for future light ion collisions at the LHC.
The existing CERN accelerator complex allows for the injection and acceleration of a broad range of light and heavy ion species across the entire accelerator chain up to the LHC. For light ions, the resulting scientific opportunities have as yet not been explored. However, in recent years, dedicated machine development periods set aside for CERN’s injectors were used to inject and accelerate ions for short (~ one to few days) pilot runs into the LHC. There is ample evidence by now that even such small data taking periods yield fundamentally new insights since they inform questions about the nuclear size dependence of the phenomena observed in heavy ion collisions at the LHC. The addition of a second ion source would significantly increase the versatility with which different ion beams could be delivered for short runs to the SPS and to the LHC.
Light ion beams provide a unique tool to follow up with precision measurements on the LHC discovery of collectivity in small systems. So far, smaller systems have been engineered mainly by colliding heavy ion collisions at increasing impact parameter or by colliding protons on heavy ions. While this has led to significant insights, the determination of system size in non-central collisions relies on the correlation between event activity and impact parameter that, for small systems, becomes overshadowed by fluctuations. Proton-ion collisions similarly have exciting signs of collectivity, but its precise understanding is hindered by our poor knowledge of the geometry of the system that is formed when a single proton hits a nucleus. Multiple studies have revealed by now that a much-enhanced precision on initial conditions and system geometry can be reached by varying instead the size of the colliding ions towards small mass numbers. In particular, light ion beams allow varying the initial conditions from which the QGP is formed and consequently the spatio-temporal extent over which collective phenomena develop. They provide a novel precision tool for understanding how fluid-like responses seen in heavy ion collisions emerge with increasing system size. This informs on central questions of QCD thermodynamics and QCD non-equilibrium dynamics, namely: how does hydrodynamic evolution of a locally equilibrated system emerge and how do far-out-of-equilibrium excitations relax to such a locally equilibrated system?
In heavy ion collisions, our understanding of how spatial correlations and fluctuations in the initial conditions are mapped by the QGP dynamics onto the observed multi-particle momentum distributions is by now so precise that it may be used as a tool for the diagnostics of nuclear structure. In particular, the density of matter in the colliding nuclei shows characteristic inhomogeneities and asymmetries that arise, e.g., from alpha-clusters, from the thickness of the neutron skin, or from nuclear deformations. The experimentally accessible, fluid-like response of light ion collisions is known to be sensitive to these imprints of nuclear structure in the initial conditions. The large Lorentz contraction in ultra-relativistic collisions implies that the heavy ion experiments have access to instantaneous two-dimensional projections of the three-dimensional nuclear shape rather than seeing only directional averages. Beyond contributing to central open questions in the field of hot QCD, light ion beams thus fertilise the exchange with neighbouring scientific communities and offer novel synergies with nuclear structure physics.
Enhanced experimental control over initial conditions in light ion collisions is also regarded as crucial for making progress on the central question of how jet quenching emerges as a function of system size. Jet quenching is a generic phenomenon in collisions of large nuclei. It is expected to manifest itself also in small collision systems, albeit with smaller signal sizes, but has not been observed yet. In addition, light ion beams at LHC would provide a unique opportunity for improving our knowledge of the dependence of the nuclear parton distribution functions (nPDFs) on the nuclear mass number. Proton-nucleus collisions of light nuclei are one particularly clean way of accessing this information. Light ion beams and light ion-proton collisions have thus the potential to deepen our understanding of how the characteristic modifications of PDFs (i.e. shadowing, anti-shadowing, EMC effect and Fermi-motion) emerge as a function of nuclear size. This is an important question in its own right, but it is also a prerequisite for a precise understanding of high-momentum transfer processes in nuclear collisions and it is thus a prerequisite for a precise understanding of jet quenching.
In this short report, we mention only in passing other physics opportunities with light ion beams that were discussed at the light ion workshop. For instance, in cosmic ray physics, there is interest in cross section measurements (at both collider energies and SPS fixed target energies) to improve air shower simulations. Other interesting opportunities arise via the use of light ion beams in fixed-target mode, for instance via the SMOG2 system at LHCb, or the the NA61/SHINE detector at SPS.
We conclude by repeating our main message: Even very short “endoscopic” experimental programmes with light ions at the LHC (that seems compatible with the overarching priorities of the HL-LHC running schedule) could produce over one billion collisions. This would enable us to boost our understanding of emergent collective phenomena in QCD matter from the analysis of the low-momentum sector, while advancing our understanding of its high-momentum counterpart that involves nPDFs and jet quenching. As such it would address important problems that have been opened by the discoveries of the LHC heavy ion programme. Therefore, such light ion runs would contribute significantly to the richness of results produced at the LHC. We strongly argue that short light ion runs should become part of the full exploitation of the scientific opportunities arising from HL-LHC.
Drafted by the workshop organisers [names] as the consensus view developed and discussed by the participants of the CERN TH Institute workshop “Light ion collisions at the LHC”.
