  Experimental searches for Chiral Magnetic Effect (CME) in heavy-ion
collisions have been going on for a decade, and so far there is no conclusive
evidence for its existence. Recently, the Signed Balance Function (SBF), based
on the idea of examining the momentum ordering of charged pairs along the in-
and out-of-plane directions, has been proposed as a probe of CME. In this
approach, a pair of observables is invoked: one is $r_{\mathrm{rest}}$, the
out-of-plane to in-plane ratio of $\Delta B$ measured in pair's rest frame,
where $\Delta B$ is the difference between signed balance functions; The other
is a double ratio, $R_{\mathrm{B}} = r_{\mathrm{rest}}/r_{\mathrm{lab}}$, where
$r_{\mathrm{lab}}$ is a measurement similar to $r_{\mathrm{rest}}$ but measured
in the laboratory frame. These two observables give opposite responses to the
CME-driven charge separation compared to the background correlations arising
from resonance flow and global spin alignment. Both $r_{\mathrm{rest}}$ and
$R_{\mathrm{B}}$ being larger than unity can be regarded as a case in favor of
the existence of CME. It is found experimentally that $r_{\mathrm{rest}}$,
$r_{\mathrm{lab}}$ and $R_{\mathrm{B}}$ are larger than unity in Au+Au
collisions at 200 GeV, and larger than realistic model calculations with no CME
implemented. These findings are difficult to be explained by a background-only
scenario.
