Origin of heavy-flavor azimuthal anisotropy in small collision systems

Determine the exact origin of the positive azimuthal anisotropies (elliptic flow coefficient v2) measured for heavy-flavor particles—including prompt D0 mesons, non-prompt D0 mesons, J/ψ mesons, and leptons from heavy-flavor hadron decays—in high-multiplicity proton–proton and proton–lead collisions at LHC energies, by disentangling and quantifying the relative contributions of initial-state effects modeled within the Color Glass Condensate framework and final-state interactions such as anisotropic parton escape and hydrodynamic-like collective flow.

Background

The review summarizes multiple measurements of elliptic flow (v2) signals for heavy-flavor particles in small systems at LHC energies, including pp and p–Pb collisions. These measurements show positive v2 values and mass-dependent trends similar to those in heavy-ion collisions, prompting questions about the underlying mechanisms in systems that may not form a large, long-lived quark–gluon plasma.

Comparisons with models based on initial-state correlations (Color Glass Condensate) and final-state dynamics (AMPT with anisotropic parton escape, hydrodynamic-like behavior) both qualitatively describe the data. However, because these distinct mechanisms can each reproduce the observed anisotropies under certain conditions, isolating their respective contributions remains unresolved.

References

Both models qualitatively provide compatible results and are able to describe the data, indicating that both initial- and final-state effects could explain the azimuthal anisotropies observed in small systems, while leaving the question of the exact origin of these effects still open.

— Recent Findings from Heavy-Flavor Angular Correlation Measurements in Hadronic Collisions  (2403.01035 - Thomas et al., 2024) in Section 6 (Summary)

Quantitative calculations, however, find that energy loss cannot generate a $v_2$ of the measured size in small systems including collisions, leaving the origin of the high-$p_T$ $v_2$ an open question.

— Centrality-dependent nuclear modification from hard-soft correlations in the glasma  (2609.29886 - Faraday et al., 24 Sep 2026) in Section 1, Introduction