Azimuthal Charged-Particle Correlations and Possible Local Strong Parity Violation

Parity-odd domains, corresponding to nontrivial 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 system’s orbital momentum axis. We investigate a three-particle azimuthal correlator which is a $\mathcal{P}$ even observable, but directly sensitive to the charge separation effect. We report measurements of charged hadrons near center-of-mass rapidity with this observable in $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ using the STAR detector. A signal consistent with several expectations from the theory is detected. We discuss possible contributions from other effects that are not related to parity violation.

Figure 1

Schematic depiction of the transverse plane in a collision of two heavy ions (shown as dotted outlines—one emerging from and one going into the page). The azimuthal angles of the reaction plane and produced particles with charges $\alpha$ and $\beta$ as used in Eqs. (1) and (2) are depicted here.

Figure 2

$⟨\cos ({\varphi }_a+{\varphi }_{\beta }-2{\Psi }_{\mathrm{RP}})⟩$ in $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ collisions at $\sqrt{s_{\mathrm{NN}}}=200\mathrm{GeV}$ calculated using Eq. (2). The thick solid ($\mathrm{Au}+\mathrm{Au}$) and dashed ($\mathrm{Cu}+\mathrm{Cu}$) lines represent HIJING calculations of the contributions from three-particle correlations. Shaded bands represent uncertainty from the measurement of $v_2$. Collision centrality increases from left to right.

Figure 3

Dependence of $⟨\cos ({\varphi }_{\alpha }+{\varphi }_{\beta }-2{\Psi }_{RP})⟩$ on $\frac{1}{2}(p_{t,\alpha }+p_{t,\beta })$ calculated using no upper cut on particles’ $p_t$. Shaded bands represent $v_2$ uncertainty.

Figure 4

$⟨\cos ({\varphi }_{\alpha }+{\varphi }_{\beta }-2{\Psi }_{\mathrm{RP}})⟩$ results from 200 GeV $\mathrm{Au}+\mathrm{Au}$ collisions are compared to calculations with event generators HIJING (with and without an “elliptic flow afterburner”), UrQMD (connected by dashed lines), and MEVSIM. Thick lines represent HIJING reaction-plane-independent background.