Observation of charge-dependent azimuthal correlations and possible local strong parity violation in heavy ion collisions

The collaboration
Phys.Rev.C 81 (2010) 054908, 2010.

Abstract (data abstract)
Parity-odd domains, corresponding to non-trivial topological solutions of the QCD vacuum, might be created during relativistic heavy-ion collisions. These domains are predicted to lead to charge separation of quarks along the orbital momentum of the system created in non-central collisions. To study this effect, we investigate a three particle mixed harmonics azimuthal correlator which is a $\mathcal{P}$-even observable, but directly sensitive to the charge separation effect. We report measurements of this observable using the STAR detector in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$ = 200 and 62 GeV. The results are presented as a function of collision centrality, particle separation in rapidity, and particle transverse momentum. A signal consistent with several of the theoretical expectations is detected in all four data sets. We compare our results to the predictions of existing event generators, and discuss in detail possible contributions from other effects that are not related to parity violation.

• #### Figure 03a

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$\langle cos(\phi_{\alpha}+\phi_{\beta}−2\phi_{c})\rangle$ as a function of reference multiplicity for different charge combinations, before corrections for acceptance effects. In the legend...

• #### Figure 03b

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$\langle cos(\phi_{\alpha}+\phi_{\beta}−2\phi_{c})\rangle$ as a function of reference multiplicity for different charge combinations, before corrections for acceptance effects. In the legend...

• #### Figure 04a

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$\langle cos(\phi_{\alpha}+\phi_{\beta}−2\phi_{c})\rangle$ as a function of reference multiplicity for different charge combinations, after corrections for acceptance effects. In the legend...

• #### Figure 04b

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$\langle cos(\phi_{\alpha}+\phi_{\beta}−2\phi_{c})\rangle$ as a function of reference multiplicity for different charge combinations, after corrections for acceptance effects. In the legend...

• #### Figure 05

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$\langle cos(\phi_{\alpha}-\phi_{\beta})\rangle$ as a function of centrality for different charge combinations and FF and RFF configurations. The data points corresponding...

• #### Figure 06a

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A comparison of the correlations obtained by selecting the third particle from the main TPC or from the Forward TPCs....

• #### Figure 06b

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A comparison of the correlations obtained by selecting the third particle from the main TPC or from the Forward TPCs...

• #### Figure 07

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$\langle cos(\phi_{\alpha}+\phi_{\beta}−2\Psi_{RP})\rangle$ in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$ = 200 GeV calculated using Eq. 7. The error-bars show the...

• #### Figure 08

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$\langle cos(\phi_{\alpha}+\phi_{\beta}−2\Psi_{RP})\rangle$ in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$ = 62 GeV calculated using Eq. 7. The error-bars show the...

• #### Figure 09a

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Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$ = 200 GeV. The correlations are scaled with the number of participants and are...

• #### Figure 09b

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Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$ = 200 GeV. The correlations are scaled with the number of participants and are...

• #### Figure 10a

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Au+Au at 200 GeV. The correlations dependence on pseudorapidity separation $\Delta\eta$ = |$\eta_{\alpha}$ − $\eta_{\beta}$| for centrality 30-50%. The shaded...

• #### Figure 10b

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Au+Au at 200 GeV. The correlations dependence on pseudorapidity separation $\Delta\eta$ = |$\eta_{\alpha}$ − $\eta_{\beta}$| for centrality 10-30%. The shaded...

• #### Figure 11a

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Au+Au at 200 GeV. The correlations dependence on (p$_{t,\alpha}$ + p$_{t,\beta}$)/2 for centrality 30-50%. The shaded band indicates uncertainty associated...

• #### Figure 11b

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Au+Au at 200 GeV. The correlations dependence on (p$_{t,\alpha}$ + p$_{t,\beta}$)/2 for centrality 10-30%. The shaded band indicates uncertainty associated...

• #### Figure 12a

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Au+Au at 200 GeV. The correlations dependence on |p$_{t,\alpha}$ - p$_{t,\beta}$| for centrality 30-50%. The shaded band indicates uncertainty associated...

• #### Figure 12b

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Au+Au at 200 GeV. The correlations dependence on |p$_{t,\alpha}$ - p$_{t,\beta}$| for centrality 10-30%. The shaded band indicates uncertainty associated...

• #### Figure 13

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Three-particle correlator in Au+Au and Cu+Cu collisions compared to HIJING calculations shown as thick lines. All three particles are taken...

• #### Figure 14

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$\langle cos(\phi_{\alpha} + \phi_{\beta} − 2\Psi_{RP})\rangle$ calculated for 200 GeV Au+Au events with event generators HIJING (with and without an...