Finite V 2 Δ puzzle in low-multiplicity p p collisions from ultralong-range azimuthal correlations in the string-shoving model

Ultralong range angular correlations have been recently reported by the ALICE collaboration in pp collisions at s = 13 TeV below d N ch / d η = 7 . The measurements have been performed as a function of the charged-particle multiplicity at midrapidity ( N ch in | η | < 0.8 ), which is known to be strongly sensitive to local multiplicity fluctuations. The present work investigates the impact of the event-activity estimator on ultralong range angular correlations. The study is conducted in the framework of 8 with the string shoving mechanism since it gives a nonzero elliptic flow coefficient, V 2 Δ . Such collectivelike effects arise from repulsive interactions between overlapping color strings, without invoking hydrodynamic collectivity. The analysis is conducted as a function of N ch , the number of parton-parton scatterings ( N mpi ) and flattenicity. Surprisingly, for ultralong range correlations, pp collisions with N mpi = 1 (dijets) seem to be the most sensitive to string shoving. The effect diminishes with increasing N mpi . While in data, within uncertainties, V 2 Δ exhibits a weak multiplicity dependence; the string shoving mechanism gives a V 2 Δ that decreases with the increase in N ch . The present work therefore supports the picture stating that mechanisms such as string shoving might explain the low multiplicity limit, whereas, hydro becomes relevant in high-multiplicity pp collisions. This work also suggests that flattenicity might be more effective than N ch to better handle collectivelike effects.