Three-Pion Hanbury Brown–Twiss Correlations in Relativistic Heavy-Ion Collisions from the STAR Experiment

Data from the first physics run at the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory, $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=130\mathrm{G}\mathrm{e}\mathrm{V}$, have been analyzed by the STAR Collaboration using three-pion correlations with charged pions to study whether pions are emitted independently at freeze-out. We have made a high-statistics measurement of the three-pion correlation function and calculated the normalized three-particle correlator to obtain a quantitative measurement of the degree of chaoticity of the pion source. It is found that the degree of chaoticity seems to increase with increasing particle multiplicity.

Figure 1

Three-pion correlation function for central Au-Au events using ${\pi }^{-}$ triplets. Statistical and statistical + systematic errors are shown.

Figure 2

Calculation of $r_3$ for (a) central and (b) midcentral ${\pi }^{-}$ events, and (c) central and (d) midcentral ${\pi }^{+}$ events. The fits shown use Eq. (3). Statistical and statistical + systematic uncertainties are shown.

Figure 3

$r_3(Q_3=0)/2$ from STAR and two other experiments [4, 5]. For STAR, (a) central and (b) midcentral results are shown for ${\pi }^{-}$ (circles) and ${\pi }^{+}$ (squares) data. The other experiments use ${\pi }^{-}$ data only. STAR results for fitting with both a quadratic and quartic functions are shown.

Figure 4

Chaotic fraction, calculated from Eq. (2), and plotted versus charged particle multiplicity per unit pseudorapidity for the same experiments as in Fig. 3. The meanings of the symbols used in this figure are the same as in Fig. 3.