Measurements of the chiral magnetic effect with background isolation in 200 GeV Au+Au collisions at STAR

Abstract

Using two novel methods, pair invariant mass ($m_{inv}$) and comparative measurements with respect to reaction plane ($Ψ_{\rm RP}$) and participant plane ($Ψ_{\rm PP}$), we isolate the possible chiral magnetic effect (CME) from backgrounds in 200 GeV Au+Au collisions at STAR. The invariant mass method identifies the resonance background contributions, coupled with the elliptic flow ($v_{2}$), to the charge correlator CME observable ($Δγ$). At high mass ($m_{inv}>1.5$ GeV/$c^{2}$) where resonance contribution is small, we obtain the average $Δγ$ magnitude. In the low mass region ($m_{inv}<1.5$ GeV/$c^{2}$), resonance peaks are observed in $Δγ$($m_{inv}$). An event shape engineering (ESE) method is used to model the background shape in $m_{inv}$ to extract the potential CME signal at low $m_{inv}$. In the comparative method, the $Ψ_{\rm RP}$ is assessed by spectator neutrons measured by ZDC, and the $Ψ_{\rm PP}$ by the 2$^{nd}$-harmonic event plane measured by the TPC. The $v_{2}$ is stronger along $Ψ_{\rm PP}$ and weaker along $Ψ_{\rm RP}$; in contrast, the magnetic field, mainly from spectator protons, is weaker along $Ψ_{\rm PP}$ and stronger along $Ψ_{\rm RP}$. As a result, the $Δγ$ measured with respect to $Ψ_{\rm RP}$ and $Ψ_{\rm PP}$ contain different amounts of CME and background, and can thus determine these two contributions. It is found that the possible CME signals with background isolation by these two novel methods are small, on the order of a few percent of the inclusive $Δγ$ measurements.