Probing the chiral magnetic effect via transverse spherocity event classification in relativistic heavy-ion collisions

Abstract: We present the first study of the Chiral Magnetic Effect (CME) using transverse spherocity as an event-shape classifier in Pb+Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV, simulated with the A Multi-Phase Transport (AMPT) model with a realistic CME implementation. Transverse spherocity separates events into jetty and isotropic topologies based on the geometric distribution of transverse momentum. Unlike traditional event shape engineering methods, which use the flow vector as an event classifier that is itself contaminated by the very backgrounds it is intended to suppress, spherocity provides a cleaner, geometry-driven classification that avoids this circular limitation. CME inclusion shifts the spherocity distribution toward more isotropic events, confirming its sensitivity to CME-induced charge separation. The charge-dependent azimuthal correlator $Δγ$ and correlated background coupled with elliptic flow are consistently higher in jetty events. The scaled ratio $Δγ/v_2$ shows enhanced values for isotropic events, confirming effective background suppression after elliptic flow scaling. Our results demonstrate that isotropic event selection via transverse spherocity provides a cleaner and more reliable environment for CME searches by simultaneously suppressing flow-driven and resonance-decay backgrounds, making it a powerful complementary method to existing flow-vector-based methods.

• The paper introduces transverse spherocity as a novel event classifier to enhance the sensitivity of the chiral magnetic effect (CME) signal in heavy-ion collisions.
• It employs the AMPT model with a realistic CME implementation, using Δγ correlators to distinguish between jetty and isotropic events while suppressing v₂-driven backgrounds.
• The results indicate that isotropic events, selected via high spherocity, yield a clearer CME signal, paving the way for refined experimental strategies in probing QCD anomalous transport.

Probing the Chiral Magnetic Effect with Transverse Spherocity Event Classification in Relativistic Heavy-Ion Collisions

Introduction

The study addresses the persistent challenge of isolating the Chiral Magnetic Effect (CME) in relativistic heavy-ion collisions, specifically within the context of Pb+Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV modeled by the AMPT framework with a realistic CME implementation. The CME links topological fluctuations of the QCD vacuum and strong magnetic fields to observable charge separation perpendicular to the reaction plane, rendering it a key probe for local parity violation in QCD under extreme conditions. Despite extensive theoretical and experimental effort, the interpretation of experimental evidence for the CME is hampered by backgrounds—especially those correlated with elliptic flow $v_2$ and resonance decays. Traditional event-shape engineering methods, which classify events based on $v_2$, are inherently limited by their reliance on the very variable that characterizes the dominant backgrounds.

This work introduces transverse spherocity ($S_0$), a geometric and topology-driven event-shape variable, as an alternative event classifier. Unlike $v_2$-based classifiers, spherocity categorizes events into jetty and isotropic classes without direct coupling to the flow vector, thereby circumventing autocorrelation limitations. The paper specifically investigates the efficacy of spherocity-based selection in enhancing CME sensitivity while suppressing conventional backgrounds.

Methodology

CME Observable and Backgrounds

The analysis focuses on the charge-dependent three-particle azimuthal correlator $$\Delta\gamma = \gamma_{\mathrm{OS}} - \gamma_{\mathrm{SS}}$$ as the principal CME-sensitive observable. Here, $\gamma_{\alpha\beta}$ quantifies correlations of particle pairs relative to the reaction plane. Backgrounds—primarily resonance decays, local charge conservation, and jet-induced effects—contribute to $\Delta\gamma$ and scale approximately linearly with $v_2$, making them difficult to disentangle from the CME when using $v_2$ as the event classifier.

CME Implementation in AMPT

The AMPT model (string-melting mode) facilitates a controlled implementation of the CME by exchanging the $v_2$0 components for a subset of downward-moving quarks and upward-moving antiquarks, governed by an adjustable charge separation fraction $v_2$1. This prescription ensures strict momentum conservation while introducing a tunable CME signal. The study uses $v_2$2 minimum-bias Pb+Pb events, covering both mid-central and peripheral centrality classes.

Transverse Spherocity as an Event Classifier

Transverse spherocity, defined as

$v_2$3

quantifies how collimated (jetty, $v_2$4) or isotropic ($v_2$5) each event’s transverse momentum flow is. Spherocity-based classification allows for systematic separation of jet-dominated backgrounds from bulk QGP emission and isolates event classes with minimal $v_2$6 and reduced non-flow contributions.

Results

Spherocity Distribution and CME Response

The spherocity distribution systematically shifts to higher $v_2$7 values upon inclusion of a CME-induced charge separation, indicating an overall increase in isotropic topologies. This shift is robust across centrality classes and not correlated with the value of the separation fraction $v_2$8, reflecting the collective and geometry-driven nature of the CME implementation. The spherocity classifier thus displays direct sensitivity to CME-induced dynamics in a manner decoupled from elliptic flow, forming a strong basis for its application in differential CME analyses.

Elliptic Flow and Background Suppression

Elliptic flow coefficients $v_2$9 were evaluated for different spherocity-selected event classes. Isotropic events ($v_2$0) exhibit significantly suppressed $v_2$1 over all $v_2$2 bins, while jetty events ($v_2$3) display enhanced $v_2$4. The magnitude of this separation increases with stricter spherocity selection (e.g., for (90%–10%) event-class cuts), indicating that spherocity is a robust geometric discriminator for aligning events with systematically different background contributions.

Resonance Decays Across Spherocity Classes

The yields of prominent resonance decay backgrounds, notably $v_2$5 and $v_2$6, were reconstructed and exhibit a monotonic decrease from jetty to isotropic event classes. The ratio of resonance yields in jetty versus isotropic classes becomes larger with stricter spherocity cuts, with jetty-enriched events showing up to 30% higher yields than the event-integrated average. This confirms that resonance backgrounds—key confounders in CME measurements—are suppressed in the isotropic regime.

Charge-Dependent Correlator and Scaling Analysis

Analysis of the $v_2$7 correlator reveals that jetty events retain higher $v_2$8, reflecting persistent background contamination, while isotropic events show a stable, lower value consistent with suppression of $v_2$9-coupled backgrounds. Importantly, the scaled ratio $S_0$0 is maximized in isotropic classes within the CME scenario, and this enhancement increases with stricter spherocity selection. This constitutes quantitative evidence that spherocity-based event selection enables simultaneous suppression of flow-driven and hadronic decay backgrounds and amplification of CME sensitivity.

In jetty events, even with the CME implemented, the signal-to-background ratio is consistently lower due to compounded backgrounds. Conversely, isotropic events provide an environment where any nonzero $S_0$1 excess is most plausibly attributable to the CME, given the marked suppression of $S_0$2-scaled backgrounds.

Implications and Future Directions

This work demonstrates that spherocity-based event-shape engineering is a powerful and complementary tool to traditional $S_0$3-based approaches, providing a cleaner separation of the CME signal from dominant backgrounds in heavy-ion collisions. The strong numerical enhancements seen in $S_0$4 for isotropic events—backed by robust model control via AMPT—inform a practical experimental strategy. Specifically, the application of stringent isotropic spherocity cuts can reveal CME-driven charge separation that would remain hidden within background-dominated ensembles.

The implications extend both practically and theoretically. Experimentally, application of this method to LHC or RHIC data can significantly tighten existing limits on CME observability. Theoretically, the findings facilitate a shift toward topology-differential analyses and may motivate further studies in small systems (p+Pb, Xe+Xe) and across different collision energies. Potential future directions include refining spherocity-based event selection in conjunction with other geometric observables, cross-examination with alternative background mitigation techniques, and systematic study of the scaling properties of higher-order correlators.

Conclusion

Transverse spherocity provides a geometry-driven, model-independent event classification that enhances CME sensitivity and suppresses backgrounds in relativistic heavy-ion collisions. By shifting the focus from final-state flow-dominated selection to a topology-informed regime, this approach bridges a central gap in the search for QCD anomalous transport. The strategy is immediately applicable to modern collider experiments and offers a concrete path toward resolving outstanding questions regarding the existence of CME-induced charge separation in the quark-gluon plasma.