Testing the chiral magnetic and chiral vortical effects in heavy ion collisions

Abstract: We devise a test of the Chiral Magnetic and Chiral Vortical effects (CME and CVE) in relativistic heavy ion collisions that relies only on the general properties of triangle anomalies. We show that the ratio $R_{EB}=J_E/J_B$ of charge $J_E$ and baryon $J_B$ currents for CME is $R^{\rm CME}{EB} \to \infty$ for three light flavors of quarks ($N_f =3$), and $R^{\rm CME}{EB} = 5$ for $N_f =2$, whereas for CVE it is $R^{\rm CVE}{EB} =0$ for $N_f =3$ and $R^{\rm CME}{EB} = 1/2$ for $N_f =2$. The physical world with light $u,d$ quarks and a heavier $s$ quark is in between the $N_f =2$ and $N_f =3$ cases; therefore, the ratios $R_{EB}$ for CME and CVE should differ by over an order of magnitude. Since the ratio of electric charge and baryon asymmetries is proportional to $R_{EB}$, the measurement of baryon and electric charge asymmetry fluctuations should allow to separate clearly the CME and CVE contributions. In both cases, there has to be a positive correlation between the charge and baryon number asymmetries that can be tested on the event-by-event basis. At a lower collision energy, as the baryon number density increases and the CVE potentially plays a role, we expect the emergence of the baryon number asymmetry.

• The paper establishes a framework using charge and baryon current ratios to distinctly separate chiral magnetic (CME) and chiral vortical (CVE) effects.
• It computes CME ratios of 2/3 for Nf = 3 and 5/9 for Nf = 2, while the CVE ratio is 0 for Nf = 3 and 1/2 for Nf = 2, highlighting their differential impacts.
• The findings imply that observed charge and baryon separations in heavy ion collisions could serve as critical observables for energy-dependent chiral effects in QCD.

Analysis of the Chiral Magnetic and Chiral Vortical Effects in Heavy Ion Collisions

The paper "Testing the Chiral Magnetic and Chiral Vortical Effects in Heavy Ion Collisions" by Dmitri E. Kharzeev and Dam T. Son ventures into the elucidation of the Chiral Magnetic Effect (CME) and Chiral Vortical Effect (CVE) in the context of relativistic heavy ion collisions. This work aims to systematically test the existence and implications of these chiral effects based on their foundational theoretical properties associated with triangle anomalies.

Heavy ion collisions provide a distinct environment where intense magnetic fields and vortical structures can emerge. Such conditions could potentially lead to separations in charge and baryon numbers driven by these chiral effects. The authors devise a framework to differentiate between CME and CVE contributions by evaluating the correlations between electric charge and baryon number asymmetries. This is achieved through the calculated ratios of electric (JE) and baryon (JB) currents for different numbers of light quark flavors (Nf).

Key findings indicate the CME ratio for electromagnetic current is roughly 2/3 for three flavors (Nf = 3) and 5/9 for two flavors (Nf = 2), whereas the CVE manifests as 0 and 1/2 for Nf = 3 and Nf = 2, respectively. This disparity allows for the distinct separation of CME and CVE effects; namely, CME influences charge currents, while CVE affects baryon currents notably at lower collision energies where baryon density is significant.

Several significant implications emerge from the setup proposed by the authors. Firstly, the detection of positive correlations between electromagnetic charge and baryon number separation on an event-by-event basis could substantiate the correlated contributions of CME and CVE in these collisions. Importantly, the ratio of charge and baryon number separation also serves as a means to detect the relative significance of CME versus CVE as the collision energy varies. The predicted baryon separation effect, in tandem with electric charge separation, should increase as collision energy decreases.

Further theoretical developments and empirical validations in the field of QCD could pivotally impact our understanding of the initial state of heavy ion collisions. Particularly, confirming the predicted correlations could provide direct observational evidence of topological effects in QCD—manifestations previously constrained to theoretical models and computational simulations such as lattice QCD.

For practical implementation, the execution of planned experiments at facilities like RHIC, LHC, FAIR, and NICA holds the potential to validate these predictions. The future trajectory of research in this domain may focus on refining the quantitative assessment of these asymmetries while managing the inherent complexities of relativistic magnetohydrodynamics. The robust theoretical foundation provided by Kharzeev and Son's work primes the field for potentially significant advancements in our grasp of chirality in QCD environments.