Charged elliptic flow at zero charge asymmetry

Abstract

The difference between the flow ellipticities of oppositely charged pions, \Delta v_2=v_2[\pi-]-v_2[\pi+], measured recently by STAR collaboration at RHIC shows a linear dependence on the event charge asymmetry A_+-={(N_+-N_-)/ (N_++N_-)}: \Delta v_2(A_+-)= \Delta v_2(0)+ r A_+- with a slope r>0 and a non-zero intercept \Delta v_2(0)>0 of order 10^{-4}. We point out two novel mechanisms which could explain the non-zero value of the charged elliptic flow \Delta v_2 at zero charge asymmetry A_+-=0, i.e., the non-zero positive intercept \Delta v_2(0). Both effects are due to the electric fields created by the colliding ions. These fields have quadrupole asymmetry of the magnitude and the sign needed to account for the nonzero intercept \Delta v_2(0) in the RHIC data. One of the mechanisms also involves chiral magnetic effect. This mechanism, although negligible at RHIC energies, may become important at LHC energies.