  The EM field pattern created by spectators in relativistic heavy-ion
collisions plants a seed of positive (negative) magnetic helicity in the
hemisphere above (below) the reaction plane. Owing to the chiral anomaly, the
magnetic helicity interacts with the fermionic helicity of the collision
system, and causes photons emitted in upper- and lower-hemispheres to have
different preferences in the circular polarization. In this paper, we lay down
a procedure to measure the variation of the circular polarization w.r.t the
reaction plane in relativistic heavy-ion collisions for massless photons, as
well as similar polarization patterns for vector mesons decaying into two
daughters. We propose to study the yield differentially and compare the yield
between upper- and lower-hemispheres in order to identify and quantify such
effects.
