Identifying the Charge Carriers of the Quark-Gluon Plasma

Charge correlations in lattice gauge calculations suggest that up, down, and strange charges move independently in the quark-gluon plasma, and that the density of such charges is similar to what is expected from simple thermal arguments. Here, we show how specific elements of the charge-charge correlation matrix in the quark-gluon plasma survive hadronization and become manifest in final-state charge-charge correlation measurements.

Figure 1

The correlation for ${\pi }^{+}{\pi }^{-}$ scaled by multiplicity and separated by the QGP and hadronization contributions (upper panel). The tail of the correlation function is dominated by the QGP portion and is more pronounced for larger ${\sigma }_{\mathrm{QGP}}$, as is seen in the lower panel.

Figure 2

Correlations involving other hadronic species are more effective at separating the QGP and hadronization components because the two components might contribute with opposite signs. Such is the case for the $p\overline{p}$ and $p{K}^{-}$ correlations shown in the upper panels of Figs. 2, 3, respectively. The dip in the $p\overline{p}$ correlation and the switch of signs in the $p{K}^{-}$ correlation only exist if there are competing correlations of an opposite sign. Proton-antiproton correlations test the idea that charge production comes in two separate waves because the hadronization and QGP contributions have opposite signs, as seen in the upper panel. The middle panel illustrates the sensitivity to reducing the diagonal elements of ${\chi }_{ab}^{\mathrm{QGP}}$ by a factor of 2, and the effect of adding large off-diagonal components as in Eq. (10) is displayed in the lower panel.

Figure 3

The hadronization and QGP components of the $p{K}^{-}$ correlations, shown in the upper panel, have opposite signs, which results in a correlation that changes sign. This would represent dramatic evidence of the two-wave nature of charge production. The $p{K}^{-}$ correlation is sensitive to the off-diagonal elements of ${\chi }^{\mathrm{QGP}}$, as can be seen by comparing the upper panel, where there are no such elements, to the lower panel, which uses the elements described by Eq. (10).

Figure 4

Since few strange quarks are produced during hadronization, the hadronization component for $K^{+}{K}^{-}$ correlations is small, as shown in the upper panel. If the QGP had half the strangeness content, the hadronization component would be larger and one would have a much narrower correlation, as seen in the lower panel.