Mechanism behind strangeness enhancement in small collision systems

Determine the exact microscopic production mechanism responsible for the observed increase in strangeness in small collision systems, specifically high-multiplicity proton–proton and proton–lead collisions at LHC energies, as quantified by strange-to-non-strange hadron yield ratios.

Background

Strangeness enhancement—an increased production of strange and multi-strange hadrons—has long been a signature associated with quark–gluon plasma formation in heavy-ion collisions. Surprisingly, recent measurements at the LHC show that strangeness enhancement also appears in high-multiplicity proton–proton and proton–lead collisions, scaling smoothly with event multiplicity across systems.

While several theoretical approaches (statistical thermal models, color reconnection and rope hadronization, and core–corona models) qualitatively reproduce aspects of the enhancement in small systems, the underlying microscopic mechanism driving the increase remains unresolved. This paper uses h–φ(1020) two-particle angular correlations in p–Pb collisions at √sNN = 5.02 TeV to separate jet-like from underlying-event-like production, aiming to clarify the origin of strangeness enhancement by comparing φ-to-hadron ratios in distinct angular regions.

References

While certain theoretical models — such as statistical thermal models, color reconnection and rope hadronization, and the dynamical core-corona model — can qualitatively explain this behavior in small collision systems, the exact production mechanism responsible for this increase in strangeness in these systems remains unknown.

— Investigating strangeness enhancement in jet and medium via $φ$(1020) production in p$-$Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV  (2405.14491 - Collaboration, 2024) in Section 1, Introduction (pages 2–3)

However, reconciling a threshold in $A$ at SPS energies with continuous multiplicity scaling at the LHC remains an open problem.

— System-size dependence of strangeness production from p+p to Pb+Pb: quantitative tests of the $K^+/π^+$ horn  (2608.28189 - Neeraj et al., 28 Aug 2026) in Section 1, Introduction

Our PHSD runs use IGLUE$\,=1$ (partonic mode) with ICSR$\,=0$ (CSR off). The conclusion that ``even a transport model with explicit QGP degrees of freedom cannot generate the non-monotonic structure'' (Sec.~\ref{sec:results_transport}) therefore applies to PHSD without CSR; whether the CSR mechanism, combined with the system-size scan, can reproduce the step-like onset remains an open question.

— System-size dependence of strangeness production from p+p to Pb+Pb: quantitative tests of the $K^+/π^+$ horn  (2608.28189 - Neeraj et al., 28 Aug 2026) in Section 7, Discussion, paragraph “PHSD and chiral symmetry restoration”