  The search for the chiral magnetic effect (CME) in heavy-ion collisions has
been impeded by the significant background arising from the anisotropic
particle emission pattern, particularly elliptic flow. To alleviate this
background, the event shape selection (ESS) technique categorizes collision
events according to their shapes and projects the CME observables to a class of
events with minimal flow. In this study, we explore two event shape variables
to classify events and two elliptic flow variables to regulate the background.
Each type of variable can be calculated from either single particles or
particle pairs, resulting in four combinations of event shape and elliptic flow
variables. By employing a toy model and the realistic event generator,
event-by-event anomalous-viscous fluid dynamics (EBE-AVFD), we discover that
the elliptic flow of resonances exhibits correlations with both the background
and the potential CME signal, making the resonance flow unsuitable for
background control. Through the EBE-AVFD simulations of Au+Au collisions at
$\sqrt{s_{NN}} = 200$ GeV with various input scenarios, we ascertain that the
optimal ESS strategy for background control entails utilizing the
single-particle elliptic flow in conjunction with the event shape variable
based on particle pairs.
