Electromagnetic Radiation from Baryon-Rich Matter in Heavy-Ion Collisions

Abstract

We perform a study of electromagnetic radiation in heavy-ion collisions at Relativistic Heavy Ion Collider (RHIC) Beam Energy Scan (BES) and SPS energies using the iEBE-MUSIC framework, which includes 3D dynamical Monte Carlo Glauber initial conditions, MUSIC (3+1)D viscous relativistic hydrodynamics, and the UrQMD hadronic afterburner. The multistage modeling has been calibrated to hadronic data at RHIC-BES energies using a Bayesian analysis. Integrating the thermal photon emission rates with the medium evolution, we study the direct photon yield and elliptic flow and how they vary with collision energy and emission source. We compare with results obtained by the STAR and PHENIX Collaborations. We employ next-to-leading order thermal QCD dilepton emission rates to compute dilepton invariant mass spectra and extract the effective temperature of the quark-gluon plasma at different collision energies.

Figures (14)

• Figure 1: Identified particle yields $dN/dy$, their average transverse momentum $\langle p_T \rangle$, and charged hadron anisotropic flow coefficients $v_n\{2\}$ with $n = 2, 3$ as functions of collision centrality in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 62.4, 54.4, 39, 27 and 14.6 GeV. Experimental data are taken from the STAR Collaboration STAR:2019vcpSTAR:2017salSTAR:2017idkSTAR:2016vqt.
• Figure 2: Rapidity distributions of $\pi^-$ (first row), $K^+$ (second row), and protons (third row) in Pb+Pb collisions at $\sqrt{s_{\rm NN}} = 17.3$ GeV, compared with experimental measurements from the NA49 Collaboration NA49:2012rsi.
• Figure 3: The top panel shows the transverse momentum ($p_T$) distributions of direct photon production and the individual contributions from prompt and thermal photons in 0–20% central Au+Au collisions at $\sqrt{s_{\rm NN}} = 200$ GeV. Experimental data from the PHENIX PHENIX:2014nkk and STAR STAR:2016use Collaborations are included for comparison. The bottom panel shows the ratio of experimental data to the direct photon yield calculated by the model (STAR: red circles with open red boxes; PHENIX: black squares with open black boxes). The green box indicates the averaged relative model uncertainty, including statistical and posterior uncertainties in the direct photon calculations.
• Figure 4: Transverse momentum ($p_T$) distributions of direct photon production and the individual contributions from prompt and thermal photons in minimum-bias Au+Au collisions at RHIC-BES energies. Experimental data from the STAR and PHENIX Collaborations xianwenQMPHENIX:2022qfp are included for comparison. The direct photon yield (prompt + thermal) is shown as a solid black curve. The prompt and thermal contributions are shown as blue dashed lines and red dotted lines, respectively. Bands indicate the model uncertainties for the direct components. STAR (PHENIX) data points are shown as black circles (red squares). The bottom panel in each subfigure shows the ratio of experimental data to the calculated direct photon yield, with the green box indicating the $p_T$-averaged relative direct photon uncertainty.
• Figure 5: The top panel shows the transverse momentum ($p_T$) distributions of direct photon production and the individual contributions from prompt and thermal photons in 0–10% central Pb+Pb collisions at $\sqrt{s_{\rm NN}} = 17.3$ GeV. Experimental data from the WA98 Collaboration WA98:2000vxl are included for comparison. The bottom panel shows the ratio of experimental data to the direct photon yield calculated by the model. The green box represents the averaged relative model uncertainty in the direct photon calculations.