Search for CME in U+U and Au+Au collisions in STAR with different approaches of handling backgrounds

Abstract

The chiral magnetic effect (CME) refers to charge separation along a strong magnetic field between left- and right-handed quarks, caused by interactions with topological gluon fields from QCD vacuum fluctuations. We present two approaches to handle the dominant elliptic flow ($v_2$) background in the three-particle correlator ($\Delta {\gamma }_{112}$), sensitive to CME. In the first approach, we present the $\Delta {\gamma }_{112}$ and $\Delta {\gamma }_{123}$ measurements in U+U and Au+Au collisions. While hydrodynamic simulations including resonance decays and local charge conservation predict that $\Delta {\gamma }_{112}$ scaled by $N_{\mathrm{part}}/v_2$ will be similar in U+U and Au+Au collisions, the projected B-field exhibits a distinct difference between the two systems and with varying $N_{\mathrm{part}}$. Therefore, U+U and Au+Au collisions provide configurations with different expectations for both CME signal and background. Moreover, the three-particle observable $\Delta {\gamma }_{123}$ scaled by $N_{\mathrm{part}}/v_3$ provide baseline measurement for only the background. In the second approach, we handle the $v_2$ background by measuring $\Delta {\gamma }_{112}$ with respect to the planes of spectators measured by Zero Degree Calorimeters and participants measured by Time Projection Chamber. These measurements contain different amounts of contributions from CME signal (along B-field, due to spectators) and $v_2$ background (determined by the participant geometry). With the two $\Delta {\gamma }_{112}$ measurements, the possible CME signal and the background contribution can be determined. We report such a measurement in Au+Au collisions at $\sqrt{s_{NN}}=$ 27 GeV with the newly installed event plane detector, and report the new findings in U+U system where the spectator-participant plane correlations are expected to differ from those in Au+Au collisions.

4 pages, 4 figures, Quark Matter 2019

References in corpus (7)

• Experimental searches for the chiral magnetic effect in heavy-ion collisions
• Multi-particle and charge-dependent azimuthal correlations in heavy-ion collisions at the Relativistic Heavy-Ion Collider
• The STAR Event Plane Detector
• Charge Fluctuations from the Chiral Magnetic Effect in Nuclear Collisions
• Separation of flow from chiral magnetic effect in U+U collisions using spectator asymmetry
• Search for the Chiral Magnetic Effect in Relativistic Heavy-Ion Collisions
• Measurements of the chiral magnetic effect with background isolation in 200 GeV Au+Au collisions at STAR

Cited by in corpus (5)

• Chiral Magnetic Effect in Heavy Ion Collisions: The Present and Future
• Two- and three-particle nonflow contributions to the chiral magnetic effect measurement by spectator and participant planes in relativistic heavy ion collisions
• Revisit the Chiral Magnetic Effect Expectation in Isobaric Collisions at the Relativistic Heavy Ion Collider
• Decipher the $R_{{\Psi }_m}$ correlator in search for the chiral magnetic effect in relativistic heavy ion collisions
• Difference between signal and background of the chiral magnetic effect relative to spectator and participant planes in isobar collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}=200$ GeV