Ultra-long range angular correlations have been recently reported by the ALICE collaboration in pp collisions at $\sqrt{ s}=13$ TeV below ${ \rm d}N_{ \rm ch}/{ \rm d}\eta=7$. The measurements have been performed as a function of the charged-particle multiplicity at midrapidity ($N_{ \rm ch}$ in $|\eta|<0.8$), which is known to be strongly sensitive to local multiplicity fluctuations. The present work investigates the impact of the event-activity estimator on ultra-long range angular correlations. The study is conducted in the framework of PYTHIA8 with the string shoving mechanism since it gives a non-zero elliptic flow coefficient, $V_{ 2\Delta}$. The analysis is conducted as a function of $N_{ \rm ch}$, the number of parton-parton scatterings ($N_{ \rm mpi}$) and flattenicity. Surprisingly, for ultra-long range correlations, pp collisions with $N_{ \rm mpi}=1$ (dijets) seems to be the most sensitive to string shoving. The effect diminishes with increasing $N_{ \rm mpi}$. While in data, within uncertainties, $V_{ 2\Delta}$ exhibits a weak multiplicity dependence; the string shoving mechanism gives a $V_{ 2\Delta}$ that decreases with the increase in $N_{ \rm ch}$. 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 $N_{ \rm ch}$ to better handle non-flow effects.
Categories: hep-ph