In summer 2025, the LHC entered new territory. For the first time, the machine delivered p–O, O–O and Ne–Ne collisions to its experiments, as part of a short but technically demanding special ion run. For LHCb, the run provided a rare opportunity to explore heavy-ion physics in the forward region with new light-ion systems, extending the experiment’s programme beyond the familiar pp, p–Pb and Pb–Pb configurations.

Figure 1: Overview of the 2025 LHC special light-ion run. Timeline of the p–O, O–O and Ne–Ne running periods, including commissioning, luminosity scans, physics fills and machine development activities. The figure illustrates the rapid transitions between three non-standard LHC configurations and the successful delivery of physics data despite the compressed schedule.

The outcome was highly successful. LHCb had requested 2 nb⁻¹ of integrated luminosity in p–O and 0.5 nb⁻¹ in O–O. By the end of the run, the experiment had collected approximately 33 nb⁻¹ in p–O and around 5.5 nb⁻¹ in O–O, exceeding its original targets by more than an order of magnitude. A shorter Ne–Ne period also surpassed expectations, delivering about 0.6 nb⁻¹ to LHCb against a target of 0.1 nb⁻¹. These preliminary luminosity numbers, pending final calibration, underscore both the LHC’s performance and the efficiency of the LHCb data-taking strategy.

These datasets are already being turned into physics results. The first measurements of charm production in light-ion collisions, based on D⁰ mesons reconstructed in O–O and Ne–Ne collisions at √s_NN = 5.36 TeV, show LHCb’s potential for precision studies of heavy flavour in these new systems. In particular, the comparison of D⁰ production in Ne–Ne and O–O collisions shows a non-constant dependence on transverse momentum, providing a new way to probe the possible onset of medium effects in small nuclear systems (arXiv:2605.27273).

Light-ion collisions occupy an important intermediate regime between proton–nucleus and large heavy-ion systems. They make it possible to test how collective behaviour, energy loss and heavy-flavour transport evolve with system size. The comparison between O–O and Ne–Ne is especially valuable because the two systems are close in size but differ in nuclear structure, offering a sensitive test of models that aim to describe the emergence of QGP-like phenomena in smaller collision systems.

An important feature of the 2025 datasets is that LHCb can reconstruct the full centrality range in these light-ion systems, including the most head-on collisions. This gives the experiment access to the region where collective effects and possible QGP-like behaviour are expected to be most pronounced.

Figure 2: LHCb luminosity accumulation across Run 3 light-ion and fixed-target samples. Recorded and integrated luminosities are shown for several LHCb collision systems, including the 2025 special light-ion run. LHCb far exceeded its initial targets for the special run, recording approximately 33 nb⁻¹ in p–O, 5.5 nb⁻¹ in O–O and 0.56 nb⁻¹ in Ne–Ne collisions. These datasets provide the basis for new studies of heavy-flavour production, nuclear effects and collective behaviour in light-ion systems.

Heavy-flavour particles offer a particularly powerful probe of these systems. Charm quarks are produced in the earliest stages of the collision, before any medium is formed, and can therefore carry information about the full evolution of the system. In light-ion collisions, D⁰ mesons provide a sensitive test of whether QGP-like effects begin to appear between small and large nuclear collision systems. Reconstructed beauty and charm mass peaks measured in O–O collisions already demonstrate LHCb’s ability to identify heavy-flavour particles in this new collision environment.

Figure 3: Ratio of D⁰ meson production in Ne–Ne and O–O collisions as a function of transverse momentum. The measurement shows a non-constant dependence on p_T, providing new insight into how charm production varies between two light-ion systems. The data are compared with calculations including cold nuclear matter effects and possible energy loss in the QGP.

Figure 4: Reconstructed mass peaks of beauty and charm hadrons measured by LHCb in O–O collisions. These first signals demonstrate the potential of the 2025 O–O dataset for heavy-flavour studies in light-ion collisions.

First preliminary data on Upsilon suppression have also been made public, further highlighting the physics reach of the LHCb light-ion dataset. By measuring Upsilon states in the pp reference run, p–O and O–O collisions, LHCb observes a suppression pattern that points to the possible emergence of QGP-like effects in O–O collisions. Since quarkonium suppression is one of the classic signatures of deconfinement in heavy-ion collisions, these early studies underline the potential of O–O and Ne–Ne collisions for probing strongly interacting matter in smaller systems (LHCb-PAPER-2026-027, in preparation).

The p–O sample adds another important element to the programme. Proton–oxygen collisions provide an asymmetric system involving a light nucleus, offering a clean way to study how ordinary nuclear matter affects particle production. In the forward region covered by LHCb, such measurements are complementary to those of the central LHC experiments and are relevant for studies of heavy-flavour production, quarkonia and other probes of nuclear effects. They also provide valuable input for understanding the development of air showers induced by cosmic protons in the Earth’s atmosphere.

The p–O part of the run also posed operational challenges, as protons and oxygen ions have different charge-to-mass ratios and therefore different revolution frequencies in the LHC. Once collisions were established, however, LHCb was able to profit from favourable data-taking conditions and accumulated a dataset far beyond its original request.

The O–O period offered a different perspective. Unlike p–O, O–O collisions are symmetric, but much lighter than Pb–Pb collisions. They therefore provide an important testing ground for understanding how collective behaviour emerges as the size of the colliding system changes. The O–O run was performed at 5.36 Z TeV, matching the energy requested for comparison with pp reference data, and the optics were squeezed to increase the available luminosity.

In practice, the O–O run was highly efficient. The LHC delivered seven back-to-back physics fills between 5 and 7 July. A single six-hour fill would already have been sufficient to reach LHCb’s nominal luminosity target, but excellent machine availability allowed data taking to continue for around 2.6 days. This brought the LHCb O–O sample to more than eleven times the original request.

Although the 2025 Ne–Ne collider period was short, it adds an important element to this broader programme. The switch from oxygen to neon was completed rapidly, with the first neon beam injected into the LHC about eight hours after the source switch, although a cryogenics fault delayed physics production. Only one Ne–Ne physics fill was delivered, but it still exceeded LHCb’s luminosity target. Together with the O–O sample, this first Ne–Ne collider dataset opens the way to direct comparisons between two light-ion systems at the LHC.

LHCb’s fixed-target programme with SMOG2 provides a complementary handle on these questions. By injecting gases into the LHC beam pipe, SMOG2 allows the experiment to record fixed-target collisions alongside standard collider-mode data, a capability unique among the LHC experiments. During the 2025 special run, this capability was also used to collect additional fixed-target samples, including configurations with hydrogen during O–O running and neon during Ne–Ne running.

The new collider-mode datasets also complement LHCb’s fixed-target heavy-ion programme with SMOG2. Earlier PbNe and PbAr measurements at √s_NN = 70.9 GeV, already discussed in a previous EP News article on SMOG2, showed how collective-flow observables can probe the deformation of the ²⁰Ne nucleus. The 2025 O–O and Ne–Ne samples now extend this light-ion programme to collider collisions at the LHC.

Figure 5: The images above show the distribution of the v₂ and v₃ flow coefficients as a function of centrality, alongside the ratio between PbNe and PbAr collisions. The comparison points to a clear signature of the peculiar bowling-pin shape of the ²⁰Ne nucleus and supports the use of hydrodynamic descriptions for the hot medium formed in these collisions.

LHCb brings a distinctive contribution to these studies. Its forward geometry gives access to regions of phase space that complement the central LHC detectors, while SMOG2 allows the experiment to combine collider and fixed-target data in a way that is unique at the LHC. The 2025 special light-ion run adds important new collider-mode datasets to this expanding programme.

Taken together, the run was a remarkable demonstration of what can be achieved in a tightly constrained machine schedule. The LHC operated three special configurations in rapid succession, including collision systems never before delivered at the collider. For LHCb, the outcome is especially promising: sizeable p–O and O–O datasets in the forward region, a first Ne–Ne collider sample, and complementary fixed-target measurements with SMOG2.

The first analyses are already turning these special collision samples into physics results. Measurements of charm production and preliminary studies of quarkonium suppression point to a rich programme ahead, from heavy-flavour transport to the emergence of collective behaviour in small collision systems. With these datasets, LHCb is now exploring how particle production, nuclear structure and collective behaviour evolve across some of the lightest ion systems ever studied at the LHC.