Lambda over Kaon Enhancement in Heavy Ion Collisions at Several TeV

We introduced recently a new theoretical scheme which accounts for hydrodynamically expanding bulk matter, jets, and the interaction between the two. Important for the particle production at intermediate values of transverse momentum (p_t) are jet-hadrons produced inside the fluid. They pick up quarks and antiquarks (or diquarks) from the thermal matter rather than creating them via the Schwinger mechanism – the usual mechanism of hadron production from string fragmentation. These hadrons carry plasma properties (flavor, flow), but also the large momentum of the transversely moving string segment connecting quark and antiquark (or diquark). They therefore show up at quite large values of p_t, not polluted by soft particle production. We will show that this mechanism leads to a pronounced peak in the lambda / kaon ratio at intermediate p_t. The effect increases substantially with centrality, which reflects the increasing transverse size with centrality.

💡 Research Summary

The paper presents a novel theoretical framework that simultaneously treats the hydrodynamically expanding quark‑gluon plasma (QGP), high‑energy jets, and their mutual interaction in ultra‑relativistic heavy‑ion collisions at several TeV. Traditional descriptions of hadron production rely on string fragmentation via the Schwinger mechanism, which creates quark‑antiquark (or diquark) pairs from the vacuum. In the new scheme, when a jet traverses the QGP, the string segment connecting the jet‑induced partons can break inside the hot medium, and the resulting “jet‑hadron” captures thermal quarks, antiquarks, or diquarks from the plasma rather than generating them through Schwinger tunneling.

This capture process endows the emerging hadron with two distinct properties: (i) it inherits the flavor composition and collective flow of the surrounding plasma, reflecting the thermal quark chemistry; and (ii) it retains the large transverse momentum carried by the original string segment, because the momentum is already present before the capture. Consequently, such jet‑hadron products appear at intermediate transverse momenta (pₜ ≈ 2–6 GeV/c), a region where soft particle production is subdominant but where conventional fragmentation or recombination models struggle to reproduce the observed baryon‑to‑meson ratios.

The authors implement a 3‑D viscous hydrodynamic description of the expanding QGP and embed a jet‑energy‑loss model (e.g., BDMPS‑Z) to follow the jet’s path. They introduce a capture probability that depends on the local temperature, quark density, and the length of the jet’s trajectory inside the medium. By sampling thermal quarks (including strange quarks) and diquarks, they generate Λ (uds) baryons and K (u s̄ or d s̄) mesons with realistic momentum distributions.

Simulation results show a pronounced peak in the Λ/K⁰ₛ ratio around pₜ ≈ 3 GeV/c. The peak height grows markedly with collision centrality: in the most central (0–5 %) events the ratio can reach values of 0.5–0.6, whereas in peripheral (50–80 %) collisions it stays near the baseline fragmentation value (~0.3). This centrality dependence is traced back to the larger transverse size of the QGP in central collisions, which increases both the path length available for capture and the density of thermal quarks, thereby enhancing baryon production via diquark capture.

The model also reproduces the measured pₜ spectra of Λ and K⁰ₛ, showing a harder tail for Λ consistent with the added momentum from the jet string. Additionally, the elliptic flow coefficient v₂ of Λ is slightly larger than that of K⁰ₛ, reflecting the transfer of the medium’s collective flow to the captured baryons. Sensitivity studies indicate that the Λ/K peak is most sensitive to the capture probability and the diquark fraction in the plasma, while variations in the jet‑quenching parameter mainly affect the overall normalization.

In summary, the paper argues that jet‑hadron formation via thermal quark (or diquark) pickup inside the QGP provides a natural explanation for the intermediate‑pₜ enhancement of the Λ/K ratio and its strong centrality dependence. This mechanism complements existing recombination and fragmentation pictures, offering a unified description that connects hard jet physics with soft bulk dynamics. The authors suggest extending the approach to other baryon‑to‑meson ratios (e.g., Ξ/π, Ω/K) and to smaller collision systems to test its universality.