Measurement of non-flow influence on the CMW-sensitive slope parameter from STAR

4 Nov 2019, 17:40
Wanda Han Show Theatre & Wanda Reign Wuhan Hotel

Wanda Han Show Theatre & Wanda Reign Wuhan Hotel

Poster Presentation Chirality, vorticity and spin polarization Poster Session


Fuqiang Wang (Purdue University (US))


The charge asymmetry ($A_{\rm ch}$) dependence of the $\pi^{+}$ and $\pi^{-}$ elliptic flow difference, $\Delta v_{2}(A_{\rm ch})\equiv v_{2}^{\pi^{-}}(A_{\rm ch}) - v_{2}^{\pi^{+}}(A_{\rm ch})$, is sensitive to the Chiral Magnetic Wave (CMW). Previous measurements in 200 GeV Au+Au collisions by STAR indicated a positive $\Delta v_{2}(A_{\rm ch})$ slope and, in central and peripheral collisions, a negative triangular flow $\Delta v_{3}(A_{\rm ch})$ slope. Since only backgrounds contribute to the latter, the results disfavor a pure background scenario for the $\Delta v_{2}(A_{\rm ch})$ slope.

We show in this poster, however, that including all charged particles as reference in the Q-cumulant flow method automatically introduces a trivial linear term in $v_{n}(A_{\rm ch})$ if non-flow correlations differ between same-sign and opposite-sign particle pairs. This contributed artificial slopes to the previous $\Delta v_{n} (A_{\rm ch})$ measurements. After eliminating this non-flow artifact, the $\Delta v_{2}(A_{\rm ch})$ and $\Delta v_{3}(A_{\rm ch})$ slopes, normalized by the respective $v_{2}$ and $v_{3}$ magnitudes, are consistent with each other within errors. The present error on the $\Delta v_{3}(A_{\rm ch})$ slope is relatively large: the average normalized $\Delta v_{3}(A_{\rm ch})$ slope in $0-80\%$ centrality is about 2.2$\sigma$ above zero, and that in $20-60\%$ is about 1.5$\sigma$ above zero. The implications of our results in terms of the possible CMW signal and local charge conservation backgrounds are discussed.

Primary author

Fuqiang Wang (Purdue University (US))

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