Experimental overview of electromagnetic radiation in heavy-ion collisions

TL;DR

Electromagnetic probes—photons and dileptons—provide a penetrating window into the space-time evolution of hot QCD matter in heavy-ion collisions. The paper reviews recent RHIC and LHC results on real direct photons and dielectrons, emphasizing the direct-photon puzzle, a proposed universal scaling with charged-particle density, and temperature extraction from dilepton spectra across a broad energy range. It discusses the experimental tension in photon yields and elliptic flow, the interpretation of scaling, and the potential roles of pre-equilibrium radiation and late-stage sources. It concludes with the prospects of future facilities and programs (ALICE Run 3/ALICE 3, NA60+, CBM) to map the QCD phase diagram with precision across a range of baryon chemical potentials.

Abstract

Electromagnetic (EM) probes such as photons and dileptons provide direct insight into the space-time evolution of the hot and dense matter formed in heavy-ion collisions. Being unaffected by strong interactions, they serve as penetrating messengers from all collision stages, from pre-equilibrium dynamics to the quark-gluon plasma (QGP) and hadronic phases. This contribution summarises recent experimental results on direct-photon and dilepton production from RHIC and LHC experiments, as well as at lower energies. Particular emphasis is given to the ongoing 'direct-photon puzzle', the study of universal scaling of direct-photon production over a large range of collision systems and energies. Recent dielectron measurements from ALICE, STAR, and HADES, as well as new experimental developments at the LHC, are presented, along with perspectives for future facilities.

Figures (5)

• Figure 1: Measurement of the direct-photon elliptic flow $v_{2}$ as a function of $p_{\rm T}$ by the PHENIX collaboration PHENIX:2025ejr in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 $\mathrm{GeV}$. The measurement is presented in the centrality intervals of 0-20% and 20-40% and is compared to results from previous analyses in both cases the $p_{\rm T}$ reach is extended significantly. The data are compared to different model calculations.
• Figure 2: Direct-photon yield measured in different collision systems, energies and centralities as a function of the charged particle density $\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta$. The PHENIX PHENIX:2022rsx and ALICE ALICE:2024vwy yields are extracted in $1 < p_{\rm T}\xspace < 5$$\mathrm{GeV}/c$, STAR Bao2025 used $1 < p_{\rm T}\xspace < 3$$\mathrm{GeV}/c$. The dashed lines indicate the universal scaling parameters extracted based on the latest PHENIX data ($\alpha \approx 1.1$) and the preliminary STAR data ($\alpha \approx 1.43$).
• Figure 3: The fraction of direct photons with respect to all photons ($r$) is presented as a function of $p_{\rm T}$ for minimum bias (left) and high-multiplicity (right) pp collisions at $\sqrt{s} = 13$ TeV measured with ALICE ALICE:2024vwy.
• Figure 4: Excess spectrum (after subtraction of hadronic contributions) measured by the HADES collaboration HADES:2019auvSchild2025 (left) and the STAR STAR:2024bpcBao2025 and NA60 NA60:2008dcb collaborations (right) in different collision systems and at varying centre-of-mass energies. The HADES data are compared to a in-medium spectral function based on coarse-grained simulations. Temperatures are extracted by parameterising the spectra as described in the text.
• Figure 5: Temperatures extracted from the dilepton excess spectrum in the low-mass region (LMR, $0.4 < m_{\rm ll} < 1.2~\mathrm{GeV}/c^{2}\xspace$) and the intermediate-mass region (IMR, $1 < m_{\rm ll} < 2.9~\mathrm{GeV}/c^{2}\xspace$) based on measurements by the STAR STAR:2024bpcBao2025 and NA60 NA60:2008dcb collaborations as we all by HADES HADES:2019auvSchild2025 are presented in the $T$ vs $\mu_{\mathrm{B}}\xspace$ plane. The data are shown together with freeze-out temperatures extracted in thermal model fits as well as predictions of the freeze-out temperature as a function of $\mu_{\mathrm{B}}$. Trajectories based on hydrodynamic calculations of Au+Au collisions at different energies indicate the path of a cooling system in the $T$ vs $\mu_{\mathrm{B}}$ plane. For more details see text.