CERN Accelerating science

Thanks to the large Pb-Pb data sample and to the enhanced spatial resolution of the upgraded ALICE Inner Tracking system, the first-ever measurement of the prompt Λ_c⁺ baryon elliptic flow v₂ was obtained at a center-of-mass energy per nucleon pair of √s_NN = 5.36 TeV. The results appeared recently on arXiv (arXiv:2603.18966). A characteristic baryon-meson splitting of v₂ at intermediate transverse momentum, commonly interpreted as a consequence of quark recombination of partons propagating in a common velocity field and already observed for light-flavour hadrons, is now also experimentally observed in the charm sector.

This observation highlights that the splitting is driven by the number of constituent quarks rather than by the particle mass. The measurement therefore provides further support for the presence of partonic collectivity and for a recombination-driven hadronisation mechanism also in OO collisions, as reported in Pb-Pb and in high-multiplicity pp and p-Pb collisions.

Further systematic measurements of the elliptic flow for different identified particles are being carried out to characterise the conditions required for the appearance of collectivity and medium effects. To that end, the v₂ of charm D mesons has been measured to be significant in OO collisions, despite the significantly smaller system size. The results on the elliptic flow v₂ of prompt D⁰, D⁺, and D_s⁺  are shown in the right figure as a function of the transverse momentum for OO collisions at √s_NN = 5.36 TeV. The measured v₂ is qualitatively described by transport models that include the presence of a QGP phase.

Probing the onset of medium effects with quarkonium in OO collisions

Preliminary measurements of the J/ψ nuclear modification factor ROO collisions at √s_NN = 5.36 TeV reveal a remarkably different trend vs p_T with respect to Pb-Pb collisions. At low p_T​, the nuclear modification factor is stronger in OO than in Pb-Pb, indicating that a larger regeneration could be at play in Pb-Pb with respect to OO collisions.  A clear difference is observed also at intermediate and high p_T, where the suppression is smaller for OO collisions.  This suggests a different interplay between suppression and regeneration contributions. The first measurement of the J/ψ v₂ has also been reported and points to an elliptic flow consistent with zero within uncertainties, a result that is consistent with transport models that include an expanding medium and which predict a very small J/ψ v₂. 

Resonances surviving the hadronic phase in OO collisions

Strangeness production has long been considered a key probe of the hot and dense QCD medium created in heavy-ion collisions, with an enhanced production of strange hadrons historically proposed as a signature of quark–gluon plasma formation. Measurements across collision systems have shown that strange-hadron production scales primarily with the final-state multiplicity rather than with the system geometry or beam energy, making OO collisions an ideal system to bridge small and large collision systems and to test the origin of this scaling behaviour.

ALICE presents the first measurement of strange hadron and resonance production in OO collisions at 5.36 TeV. The ratio of K* to K_s⁰ shows a decreasing trend with increasing multiplicity, consistent with the suppression of short-lived resonances due to rescattering in the hadronic phase. On the contrary, the Φ/K0s ratio stays approximately constant, reflecting the much longer lifetime of the Φ meson, which makes it largely insensitive to hadronic phase effects. The yields of strange and multi-strange hadrons (Λ, K0s, Ξ, and Ω) exhibit the familiar increase with event multiplicity already observed in other collision systems. When compared across systems, the OO results follow the same smooth strangeness-content-dependent scaling from pp to Pb–Pb collisions. The enhancement becomes stronger for particles carrying higher strangeness content, supporting the picture of a universal strangeness production mechanism driven by the final-state multiplicity.

New insights into hadronisation of strange-charm hadrons with multiplicity studies of D_s/D 

ALICE released new, unprecedentedly precise measurements of the strange-to-non-strange meson ratio D_s⁺​/D⁰ in fine centrality intervals, aiming at constraining hadronization across different collision systems. In pp collisions, from very low to high multiplicity, the D_s^+​/D⁰ yield ratio shows no significant dependence on multiplicity and p_T, as expected by models that assume independent fragmentation. In contrast, significantly larger yield ratios are observed in Pb–Pb collisions, increasing towards more central collisions. Describing this observation presents a challenge to models incorporating different hadronization mechanisms, which currently fail to simultaneously reproduce the data across the full multiplicity range and across different collision systems. 

Accessing the final state interaction with charm and strange hadrons

Accessing the residual strong force between hadrons with charm and strange content is extremely challenging due to their very short lifetimes, which makes it practically unfeasible to have stable beams and targets. With the data sample collected during Run 2, ALICE has been delivering high-precision data on many interactions involving strange and charm particles thanks to the femtoscopy technique. The basics of this approach rely on the measurements of two-particle correlation functions in the relative momentum space of the pair of interest. 

With the enhanced Run 3 statistics in pp collisions at 13.6 TeV and thanks to the development of dedicated offline triggers, the first measurement of a proton interacting with a D⁺ meson was performed, providing the first evidence of residual strong interaction in the
charm sector (left figure). 

In addition, the upgrade of the ITS, with the inclusion of an additional layer, allowed for the possibility of reconstructing Σ^- baryons via their hadronic decay into neutrons and pions. ALICE reports the first measurement of the pΣ^- correlation function, delivering important input for understanding the behaviour of strange baryons in dense nuclear matter. The measured correlation indicates a slightly repulsive NΣ interaction, which is in tension with available models. 

In addition, the femtoscopy technique has also been extended to the triplet case, providing unique access to three-body dynamics and allowing ALICE to present the first femtoscopic measurement of a three-baryon system in the strangeness sector, studying the ppΞ correlation. The data show evidence of a final-state interaction, with a significant deviation from the Coulomb-only scenario at low Q3.

First measurement of fully reconstructed B⁰ meson at very low p_T at midrapidity (arXiv:2603.18904)

Another important result enabled by the large sample of pp collisions collected in 2023 and 2024 (L_int ≃ 50 pb^-1) and exploiting offline triggers for rare-channels is the measurement of the B0 production cross section via full hadronic reconstruction performed for the first time at midrapidity (|y| < 0.5) down to p_T = 1 GeV/c, at √s = 13.6 TeV (arXiv:2603.18904). The measurement significantly extends the kinematic region of previous results of beauty hadrons performed by the CMS and the LHCb Collaborations. Further, a preliminary measurement of the B+ meson cross section is also being made available at SQM2026 and complements the existing publication. These precise measurements provide constraints to pQCD calculations and serve as a crucial reference for future measurements in heavy-ion collisions, where the production of beauty hadrons is expected to be modified at low p_T due to the interaction of beauty quarks in the colour-deconfined medium.