Speaker
Description
Metastable domains of topological charges in QCD can cause chirality
imbalance and, under the strong magnetic field present in heavy-ion
collisions, result in charge separation along the magnetic field, a
phenomenon called the Chiral Magnetic Effect (CME) [1]. Charge
separation can also be caused by intrinsic particle correlations coupled
with elliptic flow anisotropy, a major background for the CME search. In
this talk, we report results by two analysis methods designed to reduce
or eliminate, model-independently, background contributions.
In the first method [2], a correlator Cp is constructed as the ratio of
real-event to shuffled-event differences between positive and negative
charge distributions of a correlation variable with respect to the event
plane. A second correlator Cp_perp is similarly constructed using the
particles perpendicular to the event plane. The shape of the
double-ratio $C_p/C_{p_{perp}}$ is concave for a CME-associated charge
asymmetry and flat or convex for all non-CME associated effects. The
observation of a concave shape for $C_p/C_{p_{perp}}$ in the data confirms
the presence of a CME associated charge asymmetry. Quantification of
this signal is obtained via comparisons with data driven simulations.
In the second method [3], correlators are constructed from multiplicity
asymmetries of positive and negative particles across the event plane.
The same- and opposite-sign difference is studied as a function of the
event-by-event anisotropy of the measured particles. A linear dependence
is observed in both real and mixed-events. The intercept or the
difference between real- and mixed-events measures the charge separation
with reduced flow background.
We present the results obtained via these two methods for Au+Au
collisions at several collision centralities and beam energies at RHIC.
We discuss our results in terms of the CME search.
[1] D. Kharzeev, Phys. Lett. B633, 260 (2006).
[2] N. N. Ajitanand, R. A. Lacey, A. Taranenko, and J. M.Alexander,
Phys. Rev. C 83, 011901(R) (2011).
[3] L. Adamczyk, et al. (STAR Collaboration), Phys. Rev. C 89, 044908
(2014).
| Preferred Track | Collective Dynamics |
|---|---|
| Collaboration | STAR |
