Charting the Luminosity Capabilities of the CERN Large Hadron Collider with Various Nuclear Species

The Large Hadron Collider (LHC) at CERN has been instrumental in recent advances in experimental high energy physics by colliding beams of protons and heavier nuclei at unprecedented energies. The present heavy-ion programme is based mainly on colliding lead nuclei. For future ion runs, there is strong interest to achieve a significantly higher integrated nucleon-nucleon luminosity, which might be achieved through collisions of species other than Pb. In this paper, we explore the nucleon-nucleon luminosity projections in the LHC for a selection of ion species ranging from He to Xe, and including Pb as reference. Alternative beam production schemes are investigated as a way to mitigate effects such as space charge that degrade the beam quality in the LHC injectors. In the most optimistic scenarios, we find up to about a factor~4 improvement in integrated nucleon-nucleon luminosity for a typical future one-month run, with respect to the present Pb programme. We also outline a future study programme and experiments to test the assumptions and refine the simulated projections put forward in this article.

💡 Research Summary

The paper addresses a pressing limitation of the CERN Large Hadron Collider’s (LHC) heavy‑ion program, which to date has been dominated by lead‑lead (Pb‑Pb) collisions. While the upcoming ALICE 3 detector and other upgrades aim to study the quark‑gluon plasma with unprecedented precision, the integrated nucleon‑nucleon (N‑N) luminosity achievable with the current Pb‑Pb schedule (≈13 nb⁻¹ over Runs 3 and 4) is insufficient to meet the ambitious physics goals. The authors therefore explore whether colliding other fully‑stripped ion species—ranging from helium (He) to xenon (Xe), with lead (Pb) retained as a reference—could provide a substantial increase in integrated N‑N luminosity.

To answer this question, the authors develop a detailed numerical “Injector Model” that propagates beam parameters through the entire LHC injector complex: the electron‑cyclotron‑resonance ion source (ECRIS) → Linac3 → Low‑Energy Ion Ring (LEIR) → Proton Synchrotron (PS) → Super Proton Synchrotron (SPS) → LHC. For each ion species the model takes as input the charge state, the current that can be extracted from Linac3 (based on past fixed‑target tests), and a set of empirically‑derived transmission efficiencies (e.g., 50 % injection efficiency into LEIR, 76 % LEIR‑PS transmission, 92 % PS‑SPS transmission). Crucially, the model incorporates the dominant physical limitations that have been identified in Pb‑Pb operation:

1. Space‑charge (SC) tune shift – The most restrictive effect in the low‑energy rings (LEIR and SPS). The authors compute the SC tune shift ΔQ using the standard integral over the lattice, scaling the limit observed for Pb‑Pb (≈2.3 × 10⁸ ions per bunch) to other species based on charge, mass, and transverse beam size.
2. Electron cooling – Applied in LEIR to reduce emittance and mitigate SC. The model scales cooling performance from measured Pb‑Pb data, acknowledging that cooling efficiency depends on charge state and beam current.
3. Intra‑beam scattering (IBS) – Becomes significant at higher energies (SPS, LHC) and drives emittance growth, thereby reducing luminosity. IBS rates are calculated from the beam intensity, energy, and lattice functions.
4. Stripping losses – At the transitions Linac3→LEIR and PS→SPS the remaining electrons are removed using stripper foils. A nominal 90 % stripping efficiency is assumed for all species unless otherwise noted.

The authors consider three operational scenarios:

• Baseline – Mirrors the present Pb‑Pb production chain, using the 50 ns slip‑stacking scheme in the SPS that has been commissioned for Run 3.
• Optimistic – Introduces several upgrades: up to eight Linac3 injections into LEIR, improved electron‑cooling parameters, higher stripping efficiencies (via new foil materials), and additional bunch splitting in the PS to reduce per‑bunch intensity while increasing total bunch count.
• 25 ns – Envisions a future hardware upgrade that halves the bunch spacing in the SPS and LHC, effectively doubling the number of bunches that can be stored (from ≈1240 to ≈2240 per ring).

For each ion species and scenario the model predicts the maximum achievable bunch intensity at LHC injection, the resulting per‑bunch luminosity, and finally the integrated N‑N luminosity for a typical one‑month physics fill. The key findings are:

• Light to medium‑mass ions (O⁸⁺, Ar¹¹⁺, Ca¹⁷⁺) can deliver 2–3 times the integrated N‑N luminosity of Pb‑Pb under the Optimistic scenario, and up to a factor of four in the 25 ns scenario.
• Very light ions (He²⁺, He‑like species) suffer from low Linac3 currents (≈3–4 µA) despite negligible SC limits, resulting in modest luminosity gains.
• Heavy ions (Xe⁵⁴⁺, Pb⁸²⁺) remain limited by the same SC and IBS constraints that bound Pb‑Pb, so their gains are modest (≈1.2×).
• The overall uncertainty on the predicted luminosities is estimated at 20–30 %, stemming from variations in Linac3 current, transmission efficiencies, and stripping losses.

The authors stress that while the model is grounded in extensive operational data from Pb‑Pb runs, many of the proposed improvements (especially electron‑cooling upgrades and 25 ns slip‑stacking) require dedicated experimental validation. They propose a staged test program: (i) benchmark electron‑cooling performance for O and Ar in LEIR, (ii) evaluate new high‑efficiency stripper foils in the PS‑SPS transfer line, and (iii) perform pilot 25 ns slip‑stacking tests with a low‑intensity Xe beam to assess beam stability and loss patterns.

In conclusion, the paper demonstrates that the LHC’s heavy‑ion luminosity ceiling is not fundamentally fixed by the machine’s design but is largely dictated by injector‑chain limitations that can be mitigated. By judiciously selecting alternative ion species and implementing realistic upgrades to the injector complex, the integrated nucleon‑nucleon luminosity could be increased by up to a factor of four relative to the present Pb‑Pb program. Such an increase would substantially enhance the physics reach of future experiments like ALICE 3, enabling more precise measurements of the quark‑gluon plasma and other strongly interacting phenomena. The study provides a quantitative roadmap for the LHC heavy‑ion community and outlines concrete next steps to turn these projections into operational reality.