Origin of common source-size scaling for non-identical hadron pairs in pp collisions

Determine the physical origin and underlying mechanism that produce the observed common scaling of the hadron-pair emission source size with pair transverse mass in high-multiplicity proton–proton collisions at sqrt(s)=13 TeV, particularly for non-identical particle pairs such as proton–pion, proton–kaon, and proton–Lambda.

Background

ALICE measurements in high-multiplicity pp collisions at sqrt(s)=13 TeV have shown that the extracted emission source size of hadron pairs decreases monotonically with the pair transverse mass. This scaling appears consistently across multiple pair types, including p–p, p–Λ, p–K+, and π±–π±, suggesting a common emission source behavior.

While such a scaling is anticipated for identical particles from collective-like effects, its emergence for non-identical hadron pairs is not straightforward. The paper emphasizes that the mechanism leading to this common scaling—especially for non-identical pairs—remains unidentified and requires further investigation.

References

However, the origin and underlying mechanism of this behavior remains, especially for pairs of non-identical particles, an open question.

— Investigating the p-$π^{\pm}$ and p-p-$π^{\pm}$ dynamics with femtoscopy in pp collisions at $\sqrt{s} = 13$ TeV  (2502.20200 - Collaboration, 27 Feb 2025) in Section 1, Introduction

So far, no definitive explanation has been found, that could account for the non-Gaussian sources and scaling simultaneously.

— Levy-shaped femtoscopic sources from an exact solution of relativistic hydrodynamics  (2610.13352 - Szentkuti et al., 9 Oct 2026) in Section 1.6, “Source size scaling in femtoscopy and the Lévy-hydro puzzle”