Probing dense baryon-rich matter with virtual photons


About 10 μs after the Big Bang, the universe was filled—in addition to photons and leptons—with strong-interaction matter consisting of quarks and gluons, which transitioned to hadrons at temperatures close to kT= 150 MeV and densities several times higher than those found in nuclei. This quantum chromodynamics (QCD) matter can be created in the laboratory as a transient state by colliding heavy ions at relativistic energies. The different phases in which QCD matter may exist depend for example on temperature, pressure or baryochemical potential, and can be probed by studying the emission of electromagnetic radiation. Electron–positron pairs emerge from the decay of virtual photons, which immediately decouple from the strong interaction, and thus provide information about the properties of QCD matter at various stages. Here, we report the observation of virtual photon emission from baryon-rich QCD matter. The spectral distribution of the electron–positron pairs is nearly exponential, providing evidence for a source of temperature in excess of 70 MeV with constituents whose properties have been modified, thus reflecting peculiarities of strong-interaction QCD matter. Its bulk properties are similar to the dense matter formed in the final state of a neutron star merger, as apparent from recent multimessenger observation.

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Fig. 1: The conjectured QCD phase diagram of strong-interaction matter.
Fig. 2: Reconstructed e+e mass distribution from Au + Au collisions.
Fig. 3: Acceptance-corrected dilepton excess yield.
Fig. 4: Systematics of the e+e pair yield in A + A collisions attributed to excess radiation.
Fig. 5: Simulations of nuclear matter in collisions yielding extreme conditions of density and temperature.

Data availability

All data shown in figures are publicly available from HEPdata (

Code availability

The HADES raw data have been analysed with the ROOT-based customized reconstruction software HYDRA and stored in data summary files. Detailed information on the analysis procedures employed can be provided by the authors on request.


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The collaboration gratefully acknowledges the support by PTDC/FIS/113339/2009 LIP Coimbra (Portugal), 2013/10/M/ST2/00042, 2016/23/P/ST2/040 POLONEZ, 2017/25/N/ST2/00580, 2017/26/M/ST2/00600 SIP JUC Cracow, National Science Centre (Poland), VH-NG-823, DFG GRK 2128, DFG CRC-TR 211, ExtreMe Matter Institute EMMI at the GSI, BMBF:05P18RDFC1 TU Darmstadt (Germany), EU FP7:283286, BMBF:05P12CRGHE HZDR Dresden (Germany), HIC for FAIR (LOEWE), GSI F&E, EMMI, BMBF:05P15RFFCA Goethe-Universität, Frankfurt (Germany), DFG EClust 153, GSI F&E:TMLRG1316F, BMBF:05P15WOFCA, SFB 1258, DFG FAB898/2-2 TU München, Garching (Germany), BMBF:05P12RGGHM JLU Giessen, Giessen (Germany), INFN-LNS Catania, Catania (Italy), Russian Academic Excellence 02.a03.21.0005, Minobrnauka 3.3380.2017/4.6 NRNU MEPhI Moscow (Russia), UCY/3411-23100, University Cyprus (Cyprus), CNRS/IN2P3 IPN Orsay (France), AS CR M100481202, GACR 13-06759S, MSMT LM2015049, PO VVV CZ.02.1.01/0.0/0.0/16_013/0001677 and MSMT LTT17003 NPI CAS Rez (Czech Republic).

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All authors have contributed equally to the publication, being variously involved in the design and the construction of the detectors, writing software, calibrating subsystems, operating the detectors and acquiring data, and finally analysing the processed data.

Correspondence to J. Stroth.

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