Examinations of the early degrees of freedom in ultrarelativistic nucleus-nucleus collisions

The parton-hadron string dynamics (PHSD) transport model is used to study the impact of the choice of initial degrees of freedom on the final hadronic and electromagnetic observables in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV. We find that a nonperturbative system of massive gluons (scenario I) and a system dominated by quarks and antiquarks (scenario II) lead to different hadronic observables when imposing the same initial energy-momentum tensor $T_{\mu \nu }\left(x\right)$ just after the passage of the impinging nuclei. In case of the gluonic initial condition the formation of $s,\overline{s}$ pairs in the quark-gluon plasma (QGP) proceeds rather slowly, such that the antistrange quarks and accordingly the $K^{+}$ mesons do not achieve chemical equilibrium even in central Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV. Accordingly, the $K^{+}$ rapidity distribution is suppressed in the gluonic scenario, and is in conflict with the data from the BRAHMS Collaboration. The proton and antiproton rapidity distributions also disfavor scenario I. Furthermore, a clear suppression of direct photon and dilepton production is found for the pure gluonic initial conditions, which is not so clearly seen in the present photon and dilepton spectra from Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV due to a large contribution from other channels. It is argued that dilepton spectra in the invariant mass range $1.2 <M<$ 3 GeV will provide a definitive answer once the background from correlated $D$-meson decays is subtracted experimentally.

Figure 1

(a) Time evolution of the gluon number (red solid line) as well as the total quark+antiquark number (divided by 2) (dashed blue line) and strange+antistrange quark number (divided by 2) (dot-dashed green line) as a function of time $t$ for a central ($b=2$ fm) Au+Au collision at $\sqrt{s_{NN}}=200$ GeV in logarithmic representation for scenario I (gluonic initial conditions). (b) Same quantities as in (a) but for scenario II (fermionic initial conditions).

Figure 2

(a) The quark interaction rate $d{N}_q/dt$ from PHSD as a function of time $t$ for a central ($b=2$ fm) Au+Au collision at $\sqrt{s_{NN}}=200$ GeV in scenario I (lower dark blue dashed line) and scenario II (light blue upper line). (b) Same quantities as in (a) but for the gluon interaction rate $d{N}_g/dt$ (see legend).

Figure 3

Rapidity distributions for ${\pi }^{+}$ (a) and $K^{+}$ (b) mesons from PHSD for scenarios I and II in comparison to the results of the BRAHMS Collaboration [62] for 5% central Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.

Figure 4

Transverse momentum spectra for ${\pi }^{+}$ (a) and $K^{+}$ (b) mesons from PHSD for scenarios I and II in comparison to the results of the PHENIX Collaboration [63] for 5% central Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.

Figure 5

The rapidity distribution of protons (a) and antiprotons (b) for 5% central Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV from scenarios I and II in comparison to the experimental data from the BRAHMS [65] and PHENIX collaborations [63] (the data as well as calculations are without including the feed–down from strange baryons).

Figure 6

The elliptic flow $v_2\left(p_T\right)$ of charged hadrons for minimum bias Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV from scenarios I and II in comparison to the experimental data from the STAR Collaboration [66].

Figure 7

The thermal photon yield from partonic channels versus transverse momentum $p_T$ from PHSD (at midrapidity) in scenarios I (lower dash-dotted purple lines) and II (dashed green lines) for Au-Au reactions at $\sqrt{s_{NN}}=200$ GeV for 0–20% centrality (a) and 20—40% centrality (b). The solid red lines reflect the thermal photon spectrum when adding the hadronic channels—dominated by $mm$ and $mB$ bremsstrahlung—for scenario I while the dash-dotted blue lines represent the same quantity for the scenario II. The data for the thermal photons are from the PHENIX Collaboration [71]. The panel (c) shows a comparison of the elliptic flow $v_2\left(p_T\right)$ for scenario I (solid red line) and scenario II (dashed blue line) with the PHENIX data. The hatched area displays the statistical uncertainty of the PHSD calculations.

Figure 8

Comparison of the QGP di-electron contribution as a function of the invariant mass $M$ for scenario I (solid red line) and scenario II (dash-dotted blue line) for central Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV from the PHSD.

Figure 9

The transverse mass spectra of $e^{+}{e}^{-}$ pairs from Au+Au reactions at $\sqrt{s_{NN}}=200$ GeV for 0–80% centrality (a) and 0–92% (b). The thermal dilepton yield from partonic channels is displayed for scenarios I (lower dash-dotted purple lines) and II (dashed green lines). The solid red lines reflect the total dilepton spectrum when adding the residual channels—dominated by correlated $D$-meson decays—for scenario I while the dash-dotted blue line represents the same quantity for scenario II. The dilepton data are from the STAR Collaboration [72] (a) and from the PHENIX Collaboration [73] (b).