Measurement of the charge separation along the magnetic field with Signed Balance Function in 200 GeV Au + Au collisions at STAR

Abstract

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 rrest, the out-of-plane to in-plane ratio of ΔB measured in pair's rest frame, where ΔB is the difference between signed balance functions; The other is a double ratio, RB = rrest/rlab, where rlab is a measurement similar to rrest 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 rrest and RB being larger than unity can be regarded as a case in favor of the existence of CME. It is found experimentally that rrest, rlab and RB 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.