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.