Ultra-long range angular correlations have been recently reported by the ALICE collaboration in pp collisions at √ s = 13 TeV below dNch/dη = 7. The measurements have been performed as a function of the charged-particle multiplicity at midrapidity (Nch 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 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, V2∆. The analysis is conducted as a function of Nch, the number of parton-parton scatterings (Nmpi) and flattenicity. Surprisingly, for ultra-long range correlations, pp collisions with Nmpi = 1 (dijets) seems to be the most sensitive to string shoving. The effect diminishes with increasing Nmpi. While in data, within uncertainties, V2∆ exhibits a weak multiplicity dependence; the string shoving mechanism gives a V2∆ that decreases with the increase in Nch. 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 Nch to better handle non-flow effects.

In this study, we report the first identification of the Einstein–de Haas (EdH) effect in the QCD matter. The EdH effect is a fundamental magnetomechanical coupling wherein magnetic-field-induced spin alignment generates a compensating collective rotation to conserve the total angular momentum. Using an equilibrium hadron gas under an external magnetic field, we show that even remnant magnetic fields at the freeze-out produce induced rotations (omega_EdH) comparable to typical estimates of fluid vorticity in heavy-ion collisions as inferred from final-state hyperon polarization. This rotation emerges from the magnetic field alone, without any initial vorticity as input. The Einstein–de Haas effect thus establishes hot QCD matter as a self-vortical magnetofluid, where collective rotation can be generated purely from spin alignment, and identifies spin-rotation coupling as a potentially important, previously overlooked component of angular momentum dynamics in relativistic nuclear collisions.

Dushmanta.Sahu@cern.ch

imdushmanta@gmail.com