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

Abstract

The existence of a partonic phase in small collision systems has been investigated previously through measurements of the elliptic flow ($v_{2}$) of mesons ($π^{\pm}$, K$^{\pm}$, and K$_{S}^{0}$) and baryons ($\rm{p}(\overline{\rm{p}})$, $Λ(\overlineΛ)$) in high-multiplicity p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV and pp collisions at $\sqrt{s} = 13$ TeV recorded by ALICE. The results have revealed a grouping and splitting behavior of $v_{2}$ for baryons and mesons at intermediate transverse momentum ($p_{\rm{T}}$), establishing that partonic collectivity, as observed in heavy-ion collisions, is also present in these systems. In this Letter, the $p_{\rm{T}}$-differential $v_{2}$ of various hadronic species is investigated in p-Pb collisions for the multiplicity intervals $N_{\rm ch} \geq 50$, $25 \leq N_{\rm ch} < 50$, and $15 \leq N_{\rm ch} < 25$, as well as in pp collisions for $25 \leq N_{\rm ch} < 50$ and $15 \leq N_{\rm ch} < 25$. Here, $N_{\rm ch}$ is the efficiency-corrected number of charged particles produced in the kinematic region of $|η|< 0.8$ and $0.2 < p_{\rm T} < 3.0$ GeV/$c$. The characteristic mass dependence of $v_{2}$, as well as the grouping and splitting patterns among identified particles, are examined. The results in both p-Pb and pp collisions with $N_{\rm ch} \geq 25$ are consistent with the development of partonic collectivity. In the low-multiplicity region, $15 \leq N_{\rm ch} < 25$, the baryon-to-meson $v_{2}$ splitting, as quantified by the baryon-to-meson $v_2$ ratios, remains broadly consistent with that at higher multiplicities. However, its significance decreases to $\sim2σ$ 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.