Dilepton emission as a novel probe of QCD critical point

Abstract

In this work, we propose dilepton emission rate (DER) as a sensitive probe of QCD critical point based on the extended Polyakov-quark-meson model. The model could successfully capture two main mechanisms for dilepton production, $π^\pm$ and quark-antiquark annihilations on one hand, and self-consistently account for chiral transition and (de-)confinement on the other hand. Along the chemical freezeout lines, all the moments of the DER peak show similar extremal features as those of light quark mass, thus the DER can well reflect the change of chiral symmetry and criticality. More importantly, the DER can be directly measured in heavy ion collisions: Compared to the baryon number fluctuations, the DER fluctuations are found to be more drastic in the critical region and more sensitive to the relative location of the critical point. However, recent statistics of DER in heavy ion collision is not large enough to perform event-by-event measurement, instead we propose to study the baseline subtracted DER: For a given dilepton center of mass, the largest deviation shows up around the point where the light quark mass fluctuation is the strongest.

Figures (5)

• Figure 1: The main dilepton production mechanisms in QCD matter: (a) direct quark-antiquark annihilations through QED, (b) indirect charged hadron annihilations in term of the imaginary part of the retarded propagator of $\rho^0$ meson.
• Figure 2: Along the freezeout line with $n=2$, the integrated dilepton emission rates ${\cal R}$ as functions of the dilepton invariant mass $M$ for several values of baryon chemical potential $\mu_{\rm B}$. The solid lines are full DERs receiving contributions from both Fig. \ref{['mechanisms']} (a) and (b), and the dashed ones are the baselines ${\cal R}_0$ where only pions with fixed mass contribute Gale:1990pn. The insertion are the curvatures $\kappa_{\rm p}$ of the DER peaks as functions of the baryon chemical potential $\mu_{\rm B}$ for the full calculation (solid line) and baseline (dashed line).
• Figure 3: A comparison between the moments of the reduced light quark mass, $\tilde{m}^{(n)}_{\rm l}$, and those of reduced dilepton emission rate, $\tilde{\cal R}^{(n)}_{\rm p}$, as functions of the baryon chemical potential $\mu_{\rm B}$ and colliding energy $\sqrt{S_{NN}}$. Here, the black solid and green dashed lines correspond to the freezeout lines $n=2$ and $n=3$, respectively.
• Figure 4: The relative deviations of the full DERs from the baselines, $\Delta {\cal R}/{\cal R}_0$, as functions of the baryon chemical potential $\mu_{\rm B}$ and colliding energy $\sqrt{S_{NN}}$ along the freezeout line $n=2$ for two center of masses, $M=0.5$ and $1\,{\rm GeV}$.
• Figure 5: The self-consistent Feynman diagrams for the dressed propagator of $\rho^0$ meson (thick dashed line) in terms of its bare propagator (thin dashed line), charged pion loop (dotted loop) and quark loops (arrowed solid loop).