Indications for freeze-out of charge fluctuations in the quark-gluon plasma at the LHC

Abstract

The D-measure of net-charge fluctuations quantifies the variance of net charge in strongly interacting matter. It was introduced over 20 years ago as a potential signal of quark-gluon plasma (QGP) in heavy-ion collisions, where it is expected to be suppressed due to the fractional electric charges of quarks. Measurements have been performed at RHIC and LHC, but the conclusion has been elusive in the absence of quantitative calculations for both scenarios. We address this issue by employing a recently developed formalism of density correlations and incorporate resonance decays, local charge conservation, and experimental kinematic cuts. We find that the hadron gas scenario is in fair agreement with the ALICE data for $\sqrt{s_{\rm NN}} = 2.76$ TeV Pb-Pb collisions only when a very short rapidity range of local charge conservation is enforced, while the QGP scenario is in excellent agreement with experimental data and largely insensitive to the range of local charge conservation. A Bayesian analysis of the data utilizing different priors yields moderate evidence for the freeze-out of charge fluctuations in the QGP phase relative to hadron gas. The upcoming high-fidelity measurements from LHC Run 2 will serve as a precision test of the two scenarios.

Figures (6)

• Figure 1: Temperature dependence of the normalized charge variance $\omega = \kappa_2[Q]/\langle N_{\rm ch}^{\rm prim} \rangle$ computed through Eq. \ref{['eq:omega']} using lattice QCD data for $\chi_2^Q$ and $s/T^3$ (gray band), as well as the hadron resonance gas (dashed blue line) and free massless QGP (dashed red line) limits. The error bands reflect the uncertainties in lattice QCD data Borsanyi:2011swBorsanyi:2013bia and $S/\langle N_{\rm ch} \rangle$Hanus:2019fnc.
• Figure 2: The corrected D-measure as a function of pseudorapidity cut $\tilde{\eta}_{\rm cut}$ in central Pb–Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV calculated via Eq. (\ref{['eq:Dmeasure']}) for hadron gas (blue lines) and QGP (red lines) scenarios with global (solid lines) and local (styled lines) charge conservation. The symbols depict the experimental data of the ALICE Collaboration ALICE:2012xnj.
• Figure 3: Posterior distribution for $\omega$ and $V_c/V_{\rm tot}$ obtained from our Bayesian analysis of $\sqrt{s_{\rm NN}} = 2.76$ TeV data employing uniform (red) and local conservation (blue) priors. The labels indicate the 68% confidence interval around the median. The contours in the lower left plot represent 10%, 30%, 50%, 68%, and 95% confidence regions for the joint $(\omega,V_c/V_{\rm tot})$ distribution.
• Figure S.1: Predictions for the (uncorrected) D-measure in 0–5% central Pb–Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV, plotted as a function of the pseudorapidity acceptance $\tilde{\eta}_{\mathrm{cut}}$ for four different transverse momentum ($p_T$) ranges analyzed by the ALICE Collaboration. The curves compare the hadron-gas (blue) and QGP (red) freeze-out scenarios under three assumptions about charge conservation: global (solid) and local with two different rapidity ranges, $\sigma_y = 1.20$ (dashed) and $\sigma_y = 0.78$ (dash-dotted).
• Figure S.2: Same as Fig. \ref{['fig:ALICERun2']} but for corrected $D$-measure ($D^{\rm corr}$).