Intrinsic coupling between transverse spherocity and elliptic flow in heavy-ion collisions

Abstract

Transverse spherocity ($S_{0}$) is an event-shape observable widely used to classify collision events according to their topology, particularly to distinguish jet-like from isotropic events. Low-spherocity events are generally interpreted as being associated with enhanced jet activity. This event classification has also been applied to heavy-ion collisions to investigate the influence of event topology on several observables, including elliptic flow and constituent-quark-number scaling. In this work, we demonstrate that such an interpretation requires careful reconsideration. Using toy Monte Carlo simulations, A Multiphase Transport (AMPT) model calculations, and an analytical formulation of the spherocity observable, we show that transverse spherocity is intrinsically related to the elliptic flow coefficient, $v_{2}$. This connection arises because the axis that minimizes the spherocity aligns with the event symmetry plane, causing events with larger elliptic anisotropy to naturally exhibit smaller spherocity values even in the absence of genuine jet-like topologies. We further show that this intrinsic relation gives rise to an inherent anti-correlation between transverse spherocity and $v_{2}$, implying that several characteristics previously attributed to the enhanced jet-like nature of low-spherocity events in heavy-ion collisions can instead be understood as consequences of collective anisotropic flow. Our results indicate that, in heavy-ion collisions, transverse spherocity should be interpreted primarily as a probe of the collective momentum-space anisotropy rather than as a direct measure of jetty event topology. Consequently, physics conclusions drawn from spherocity-selected events should explicitly account for its intrinsic correlation with elliptic flow.