[Paper Review] Status of the Chiral Magnetic Effect Search in Relativistic Heavy-Ion Collisions

J. Zhao, Zhoudunming Tu+1 more|arXiv (Cornell University)|Jul 12, 2018

High-Energy Particle Collisions Research1 references12 citations

TL;DR

This paper reviews the experimental status of the chiral magnetic effect (CME) search in relativistic heavy-ion collisions, focusing on background suppression techniques and novel analysis methods. Despite observing a 1–2σ signal in isobaric collisions (Ru+Ru vs. Zr+Zr), the CME signal remains at ~5% of inclusive Δγ, consistent with zero within large uncertainties, highlighting persistent challenges in isolating the CME from physics backgrounds.

ABSTRACT

Quark interactions with topological gluon fields in QCD can yield local $\mathcal{P}$ and $\mathcal{CP}$ violations which could explain the matter-antimatter asymmetry in our universe. Effects of $\mathcal{P}$ and $\mathcal{CP}$ violations can result in charge separation under a strong magnetic field, a phenomenon called the chiral magnetic effect (CME). Experimental measurements of the CME-induced charge separation in heavy-ion collisions are dominated by physics backgrounds. Major theoretical and experimental efforts have been devoted to eliminating or reducing those backgrounds. We review the current status of these efforts in the search for the CME in heavy-ion collisions.

Motivation & Objective

• To assess the current experimental status of the chiral magnetic effect (CME) search in relativistic heavy-ion collisions at RHIC and the LHC.
• To address the dominant physics backgrounds contaminating CME measurements, particularly nonflow correlations and momentum conservation effects.
• To evaluate the viability of isobaric collisions (e.g., 96Ru+96Ru vs. 96Zr+96Zr) as a tool for isolating the CME signal by comparing charge separation in systems with similar geometry but different initial magnetic fields.
• To review and evaluate novel analysis techniques such as event-by-event v₂, event-shape engineering, and pair invariant mass dependence to enhance CME sensitivity.
• To provide guidance for future experimental programs, including data collection and detector upgrades, to improve CME detection sensitivity.

Proposed method

• Utilizes the three-point correlator γ = ⟨cos(φα + φβ − 2ψRP)⟩ to probe charge separation correlated with the reaction plane (RP), with Δγ = γOS − γSS used to isolate CME signals from background.
• Applies the three-particle correlation method to estimate γ without explicit RP determination, using a third particle c as a proxy: ⟨cos(φα + φβ − 2φc)⟩ / v₂,c.
• Employs event-by-event elliptic flow (v₂) and event-shape engineering to suppress nonflow correlations and isolate genuine CME-related correlations.
• Uses isobaric collisions (96Ru+96Ru and 96Zr+96Zr) with similar geometry but different initial magnetic fields (B_sq) to test CME sensitivity, based on differences in proton and neutron density distributions.
• Performs AMPT simulations with density distributions from energy density functional theory (DFT, SLy4 + HFB) to model nuclear structure and predict relative differences in v₂ and B_sq between Ru+Ru and Zr+Zr collisions.
• Analyzes the dependence of charge separation on pair invariant mass (m_inv) to distinguish CME from background effects, as CME is expected to be more prominent in low-m_inv pairs.

Experimental results

Research questions

• RQ1 To what extent can the CME signal be isolated from dominant physics backgrounds such as nonflow correlations and momentum conservation effects in heavy-ion collisions?
• RQ2 How effective are isobaric collisions (e.g., 96Ru+96Ru vs. 96Zr+96Zr) in suppressing background contributions and enhancing CME sensitivity?
• RQ3 What are the predicted differences in v₂ and B_sq between Ru+Ru and Zr+Zr collisions based on realistic nuclear density distributions from DFT calculations?
• RQ4 How do novel analysis techniques like event-shape engineering and m_inv dependence improve the sensitivity to the CME signal?
• RQ5 What is the expected strength of the CME signal relative to the inclusive Δγ measurement, and what does this imply for its detectability?

Key findings

• The isobaric collision data from RHIC suggest a 1–2σ effect in the CME signal, assuming the CME signal is 5% of the inclusive Δγ, based on the latest STAR results.
• Simulations using DFT-based nuclear densities show that the relative difference in v₂ and B_sq between Ru+Ru and Zr+Zr collisions is ~3% with respect to the event plane (ψ_EP), and ~10% with respect to the reaction plane (ψ_RP).
• The relative difference in B_sq is estimated to be ~20% with respect to ψ_EP, indicating a significant initial magnetic field contrast between the two isobaric systems.
• The CME signal strength is estimated to be on the order of a few percent of the inclusive Δγ, consistent with zero within large uncertainties.
• The premise of isobaric collisions for CME searches may not be as robust as initially anticipated due to non-negligible differences in initial geometry and flow, suggesting caution in interpreting such data.
• Future CME searches require more statistics from Au+Au and Pb+Pb collisions, along with detector upgrades and development of novel analysis techniques to enhance sensitivity.