Exploring the limit of partonic collectivity in small collision systems at the LHC

The existence of a partonic phase in small collision systems has been investigated previously through measurements of the elliptic flow (v₂) of mesons (π±, K±, and KS⁰) and baryons (p(p̄), Λ(Λ)) in high-multiplicity p-Pb collisions at √sNN = 5.02 TeV and pp collisions at √s = 13 TeV recorded by ALICE. The results have revealed a grouping and splitting behavior of v₂ for baryons and mesons at intermediate transverse momentum (pT), establishing that partonic collectivity, as observed in heavy-ion collisions, is also present in these systems. In this Letter, the pT-differential v₂ of various hadronic species is investigated in p-Pb collisions for the multiplicity intervals Nch ≥ 50, 25 ≤ Nch < 50, and 15 ≤ Nch < 25, as well as in pp collisions for 25 ≤ Nch < 50 and 15 ≤ Nch < 25. Here, Nch is the efficiency-corrected number of charged particles produced in the kinematic region of $|η|< 0.8$ and 0.2 < pT < 3.0 GeV/c. The characteristic mass dependence of v₂, as well as the grouping and splitting patterns among identified particles, are examined. The results in both p-Pb and pp collisions with Nch ≥ 25 are consistent with the development of partonic collectivity. In the low-multiplicity region, 15 ≤ Nch < 25, the baryon-to-meson v₂ splitting, as quantified by the baryon-to-meson v₂ ratios, remains broadly consistent with that at higher multiplicities. However, its significance decreases to ~2σ in p-Pb collisions and to less than $1σ$ in pp collisions owing to the sizable experimental uncertainties. These measurements constitute the first steps towards identifying the limits of partonic flow in small collision systems at the LHC.