Charge dependent correlations relative to the 4th-harmonic event plane
in Au+Au collisions at 27 and 39 GeV at RHIC/STAR

In the chiral magnetic effect (CME) [1], an electric current is induced
in the presence of a strong magnetic field and a chirality imbalance in
the medium created in high-energy nuclear collisions. One corresponding 
observable for the charge separation across the reaction plane ($\psi$) 
is the charge dependent two-particle azimuthal correlator, 
$\gamma= <cos(\phi_1 + \phi_2 - 2\psi)>$.
However, the $\gamma$ contains both the CME signal and the flow background,
complicating the interpretation of the data. In this poster, we
investigate the background mechanism with a modified correlator,
$\gamma^{II} = <cos(2\phi_1 + 2\phi_2 - 4\psi)>$. The $\gamma^{II}$ only 
contains the background, and reflects the role played by the collective flow 
in the original $\gamma$ correlator. We will present the STAR data of
$\gamma^{II}$ as a function of centrality measured in Au+Au collisions 
at 27 and 39 GeV. The results will be compared with those obtained by the
ALICE experiment at a much higher collision energy, and will also be compared 
with model calculations. The physics implications will be discussed.

[1]D. Kharzeev, Phys. Lett. B 633 (2006) 260.