Azimuthal Charged-Particle Correlations and Possible Local Strong Parity Violation

TL;DR

A three-particle azimuthal correlator is investigated which is a P even observable, but directly sensitive to the charge separation effect, and measurements of charged hadrons near center-of-mass rapidity with this observable are reported.

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 system's orbital momentum axis. We investigate a three particle azimuthal correlator which is a ¶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 Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$=200 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.

Figures (4)

• 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. \ref{['eq:expansion']} and \ref{['e3p']} are depicted here.
• Figure 2: $\left\langle \cos(\phi_a +\phi_\beta -2\Psi_{RP}) \right\rangle$ in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}=200$ GeV calculated using Eq. \ref{['e3p']}. The thick solid (Au+Au) and dashed (Cu+Cu) lines represent HIJING calculations of the contributions from 3-particle correlations. Shaded bands represent uncertainty from the measurement of $v_2$. Collision centrality increases from left to right.
• Figure 3: Dependence of $\left\langle \cos(\phi_{\alpha}+\phi_{\beta}-2\Psi_{RP}) \right\rangle$ 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: $\left\langle \cos(\phi_{\alpha}+\phi_{\beta}-2\Psi_{RP}) \right\rangle$ results from 200 GeV Au+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.