Collective effects in PYTHIA 8 simulations of pp and p-Pb collisions

Measurements of two- and multi-particle azimuthal correlations (cumulants) provide valuable information on the properties of the system created in collisions of hadrons and nuclei at high energy. In particular, they revealed an unexpected collective behavior in small collision systems similar to the one exhibited by the quark--gluon plasma in heavy-ion collisions. The origin of collectivity in small collision systems is still not understood. Measurements of the second harmonic two- and four-particle cumulants of unidentified charged particles are reported from PYTHIA 8 simulations of pp collisions at s=13. 6 TeV and p--Pb collisions at s₍₍=5. 02 TeV. The second harmonic two-particle cumulants decrease with charged-particle multiplicity and || gap introduced to suppress the contribution from few-particle correlations (``nonflow"). A weak dependence on the || gap is still observed at high multiplicity. The second harmonic four-particle cumulants exhibit a strong dependence on multiplicity for N₂₇ 2 gap is employed. This is qualitatively similar to the elliptic-flow pattern observed in heavy-ion collisions.