[Paper Review] Alternative Contributions to the Angular Correlations Observed at RHIC Associated with Parity Fluctuations

Scott Pratt|arXiv (Cornell University)|Feb 9, 2010

Quantum Chromodynamics and Particle Interactions19 citations

TL;DR

This paper proposes that angular correlations observed at RHIC—previously interpreted as evidence for parity-odd fluctuations in QCD—can instead be explained by charge and momentum conservation combined with elliptic flow. The author demonstrates that these conventional effects quantitatively reproduce the magnitude and sign of the measured correlations, particularly in same-sign and opposite-sign pairs, suggesting that parity-fluctuation signals may be overestimated without accounting for these background effects.

ABSTRACT

Recent measurements at RHIC of angular correlations of same-sign vs. opposite sign pairs have been interpreted as evidence for large-scale fluctuations of parity-odd fields. In this paper, we provide alternative explanations of the same phenomena based on correlations from charge and momentum conservation overlaid with elliptic flow. These effects are shown to produce correlations with similar magnitudes as those measured. Other correlations are also considered, but estimates of their size suggest they are inconsequential.

Motivation & Objective

• To challenge the interpretation of RHIC angular correlations as evidence for parity-odd field fluctuations in QCD.
• To investigate whether conventional conservation laws and collective flow can reproduce the observed correlation magnitudes.
• To quantify the contribution of charge and momentum conservation to same-sign and opposite-sign angular correlations.
• To assess the relative importance of other potential sources, such as fluctuating initial conditions or jet quenching.
• To provide a model-independent framework for disentangling genuine parity-odd signals from background effects.

Proposed method

• Modeling angular correlations using the observable $ \gamma_{\alpha\beta} = \left| \left\langle \cos(\phi_\alpha + \phi_\beta) \right\rangle \right| $, where $ \phi $ is the azimuthal angle relative to the reaction plane.
• Applying conservation laws: for every positive particle emitted at $ \phi_+ $, a negative particle is created nearby, inducing $ \phi_+ \approx \phi_- $ correlations.
• Incorporating elliptic flow ($ v_2 $) to model momentum-space anisotropy, which enhances correlations in the same rapidity and azimuthal range.
• Using $ p_t $-weighted correlations to reduce model dependence and better estimate momentum conservation contributions.
• Estimating the contribution of fluctuating initial conditions and spectator electric fields, comparing them to observed signal sizes.
• Analyzing the difference $ \gamma_{+-} - \gamma_{ss} $ to isolate parity-odd signal contributions, while accounting for background effects.

Experimental results

Research questions

• RQ1 Can charge and momentum conservation combined with elliptic flow quantitatively explain the magnitude and sign of the same-sign and opposite-sign angular correlations observed at RHIC?
• RQ2 How significant are the contributions from momentum conservation and elliptic flow compared to proposed parity-odd field fluctuations?
• RQ3 To what extent do initial condition fluctuations or spectator electric fields contribute to the observed correlations?
• RQ4 Can the $ p_t $-weighted correlation $ \gamma_{ss}' $ provide a more robust probe of transport properties than unweighted correlations?
• RQ5 Is the observed signal consistent with background effects, or does it still allow for a genuine parity-odd signal?

Key findings

• Charge conservation combined with elliptic flow explains approximately half or more of the observed $ \gamma_{+-} - \gamma_{ss} $ correlation signal.
• The momentum conservation contribution to same-sign correlations, when $ p_t $-weighted, is estimated to be significant, though model dependence remains a limitation.
• The contribution from fluctuating initial conditions and spectator electric fields is estimated to be too small to account for the observed signal, even with electric fields at 10% of the magnetic field strength.
• The model suggests that the observed correlations could be largely explained by conventional effects, implying that the signal may not require large parity-odd couplings in QCD.
• The $ p_t $-weighted same-sign correlation $ \gamma_{ss}' $ is proposed as a more robust observable for probing transport coefficients like viscosity.
• The study concludes that the bulk of the observed correlation can be accounted for by charge and momentum conservation with elliptic flow, casting doubt on the need to invoke parity-odd field fluctuations.