Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Article
  • Published:

Probing dense baryon-rich matter with virtual photons

Nature Physics volume 15, pages 1040–1045 (2019)Cite this article

Subjects

Abstract

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.

This is a preview of subscription content, access via your institution

Access options

Buy this article

  • Purchase on Springer Link
  • Instant access to full article PDF
Buy now

Prices may be subject to local taxes which are calculated during checkout

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.

Similar content being viewed by others

Data availability

All data shown in figures are publicly available from HEPdata (https://hepdata.net/).

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.

References

  1. Andronic, A., Braun-Munzinger, P., Redlich, K. & Stachel, J. Decoding the phase structure of QCD via particle production at high energy. Nature 561, 321–330 (2018).

    Article  ADS  Google Scholar 

  2. Borsanyi, S. et al. Is there still any T c mystery in lattice QCD? Results with physical masses in the continuum limit III. J. High Energy Phys. 2010, 73 (2010).

    Article  ADS  Google Scholar 

  3. Bazavov, A. et al. Chiral crossover in QCD at zero and non-zero chemical potentials. Phys. Lett. B 795, 15–21 (2019).

    Article  ADS  MathSciNet  Google Scholar 

  4. Roberts, C. D. Perspective on the origin of hadron masses. Few Body Syst. 58, 5–17 (2017).

    Article  ADS  Google Scholar 

  5. Shuryak, E. V. Quark-gluon plasma and hadronic production of leptons, photons and psions. Phys. Lett. B 78, 150–153 (1978).

    Article  ADS  Google Scholar 

  6. Kroll, N. M., Lee, T. D. & Zumino, B. Neutral vector mesons and the hadronic electromagnetic current. Phys. Rev. 157, 1376–1399 (1967).

    Article  ADS  Google Scholar 

  7. McLerran, L. D. & Toimela, T. Photon and dilepton emission from the quark–gluon plasma: some general considerations. Phys. Rev. D 31, 545–602 (1985).

    Article  ADS  Google Scholar 

  8. Gale, C. & Kapusta, J. I. Dilepton radiation from high temperature nuclear matter. Phys. Rev. C 35, 2107–2116 (1987).

    Article  ADS  Google Scholar 

  9. Rapp, R. & Wambach, J. Chiral symmetry restoration and dileptons in relativistic heavy ion collisions. Adv. Nucl. Phys. 25, 1 (2000).

    Google Scholar 

  10. Pisarski, R. D. Phenomenology of the chiral phase transition. Phys. Lett. B 110, 155–158 (1982).

    Article  ADS  Google Scholar 

  11. Ding, H. T., Kaczmarek, O. & Meyer, F. Thermal dilepton rates and electrical conductivity of the QGP from the lattice. Phys. Rev. D 94, 034504 (2016).

    Article  ADS  Google Scholar 

  12. Hohler, P. M. & Rapp, R. Is ρ-meson melting compatible with chiral restoration?. Phys. Lett. B 731, 103–109 (2014).

    Article  ADS  Google Scholar 

  13. Jung, C., Rennecke, F., Tripolt, R. A., von Smekal, L. & Wambach, J. In-medium spectral functions of vector- and axial-vector mesons from the functional renormalization group. Phys. Rev. D 95, 036020 (2017).

    Article  ADS  Google Scholar 

  14. Specht, H. J. Thermal dileptons from hot and dense strongly interacting matter. AIP Conf. Proc. 1322, 1–10 (2010).

    ADS  Google Scholar 

  15. Rapp, R. & van Hees, H. Thermal dileptons as fireball thermometer and chronometer. Phys. Lett. B 753, 586–590 (2016).

    Article  ADS  Google Scholar 

  16. Heinz, U. W. & Lee, K. S. The rho peak in the dimuon spectrum as a clock for fireball lifetimes in relativistic nuclear collisions. Phys. Lett. B 259, 162–168 (1991).

    Article  ADS  Google Scholar 

  17. Barz, H. W., Friman, B. L., Knoll, J. & Schulz, H. Production of phi, rho and omega mesons in the hadronization of a quark–gluon plasma. Phys. Lett. B 254, 315–319 (1991).

    Article  ADS  Google Scholar 

  18. Adamova, D. et al. Enhanced production of low mass electron pairs in 40-AGeV Pb–Au collisions at the CERN SPS. Phys. Rev. Lett. 91, 042301 (2003).

    Article  ADS  Google Scholar 

  19. Agakishiev, G. et al. Low mass e+e− pair production in 158/A-GeV Pb–Au collisions at the CERN SPS, its dependence on multiplicity and transverse momentum. Phys. Lett. B 422, 405–412 (1998).

    Article  ADS  Google Scholar 

  20. Arnaldi, R. et al. First measurement of the rho spectral function in high-energy nuclear collisions. Phys. Rev. Lett. 96, 162302 (2006).

    Article  ADS  Google Scholar 

  21. Adare, A. et al. Dielectron production in Au + Au collisions at \(\sqrt{SNN} = 200\ {\mathrm{Gev}}\). Phys. Rev. C 93, 014904 (2016).

    Google Scholar 

  22. Adamczyk, L. et al. Measurements of dielectron production in Au + Au collisions at \(\sqrt{SNN} = 200\ {\mathrm{Gev}}\) from the STAR experiment. Phys. Rev. C 92, 024912 (2015).

    Google Scholar 

  23. Adam, J. et al. Measurements of dielectron production in Au + Au collisions at \(\sqrt{SNN} = 27,39\), and 62.4 GeV from the STAR experiment. Preprint at https://arxiv.org/abs/1810.10159 (2018).

  24. Herrmann, M., Friman, B. L. & Nörenberg, W. Properties of rho mesons in nuclear matter. Nucl. Phys. A 560, 411–436 (1993).

    Article  ADS  Google Scholar 

  25. Chanfray, G. & Schuck, P. The Rho meson in dense matter and its influence on dilepton production rates. Nucl. Phys. A 555, 329–353 (1993).

    Article  ADS  Google Scholar 

  26. Peters, W., Post, M., Lenske, H., Leupold, S. & Mosel, U. The spectral function of the rho meson in nuclear matter. Nucl. Phys. A 632, 109–127 (1998).

    Article  ADS  Google Scholar 

  27. Wilson, W. K. et al. Inclusive dielectron cross-sections in p + p and p + d interactions at beam energies from 1.04-GeV to 4.88-GeV. Phys. Rev. C 57, 1865–1878 (1998).

    Article  ADS  Google Scholar 

  28. Porter, R. J. et al. Dielectron cross-section measurements in nucleus-nucleus reactions at 1-A/GeV. Phys. Rev. Lett. 79, 1229–1232 (1997).

    Article  ADS  Google Scholar 

  29. Agakishiev, G. et al. Origin of the low-mass electron pair excess in light nucleus-nucleus collisions. Phys. Lett. B 690, 118–122 (2010).

    Article  ADS  Google Scholar 

  30. Adamczewski-Musch, J. et al. Analysis of the exclusive final state npe+e− in quasi-free np reaction. Eur. Phys. J. A 53, 149–161 (2017).

    Article  ADS  Google Scholar 

  31. Agakichiev, G. et al. Dielectron production in C-12 + C-12 collisions at 2-AGeV with HADES. Phys. Rev. Lett. 98, 052302 (2007).

    Article  ADS  Google Scholar 

  32. Agakishiev, G. et al. Study of dielectron production in C + C collisions at 1-A-GeV. Phys. Lett. B 663, 43–48 (2008).

    Article  ADS  Google Scholar 

  33. Agakishiev, G. et al. Dielectron production in Ar + KCl collisions at 1.76A GeV. Phys. Rev. C 84, 014902 (2011).

    Article  ADS  Google Scholar 

  34. Bratkovskaya, E. L. et al. System size and energy dependence of dilepton production in heavy-ion collisions at 1-2 GeV/nucleon energies. Phys. Rev. C 87, 064907 (2013).

    Article  ADS  Google Scholar 

  35. Weil, J., van Hees, H. & Mosel, U. Dilepton production in proton-induced reactions at SIS energies with the GiBUU transport model. Eur. Phys. J. A 48, 111–126 (2012).

    Article  ADS  Google Scholar 

  36. Staudenmaier, J., Weil, J., Steinberg, V., Endres, S. & Petersen, H. Dilepton production and resonance properties within a new hadronic transport approach in the context of the GSI-HADES experimental data. Phys. Rev. C 98, 054908 (2018).

    Article  ADS  Google Scholar 

  37. Galatyuk, T., Hohler, P. M., Rapp, R., Seck, F. & Stroth, J. Thermal dileptons from coarse-grained transport as fireball probes at SIS energies. Eur. Phys. J. A 52, 131–139 (2016).

    Article  ADS  Google Scholar 

  38. Endres, S., van Hees, H., Weil, J. & Bleicher, M. Dilepton production and reaction dynamics in heavy-ion collisions at SIS energies from coarse-grained transport simulations. Phys. Rev. C 92, 014911 (2015).

    Article  ADS  Google Scholar 

  39. Lang, A. et al. Local thermodynamic properties and equilibration in relativistic heavy ion collisions. Phys. Lett. B 245, 147–152 (1990).

    Article  ADS  Google Scholar 

  40. Seck, F., Galatyuk, T., Rapp, R. & Stroth, J. Thermal dileptons as QCD matter probes at SIS. J. Phys. Conf. Ser. 1024, 012011 (2018).

    Article  Google Scholar 

  41. Bauswein, A., Goriely, S. & Janka, H.-T. Systematics of dynamical mass ejection, nucleosynthesis, and radioactively powered electromagnetic signals from neutron-star mergers. Astrophys. J. 773, 78 (2013).

    Article  ADS  Google Scholar 

  42. Most, E. R. et al. Signatures of quark-hadron phase transitions in general-relativistic neutron-star mergers. Phys. Rev. Lett. 122, 061101 (2019).

    Article  ADS  Google Scholar 

  43. Rapp, R. Hadro chemistry and evolution of (anti-) baryon densities at RHIC. Phys. Rev. C 66, 017901 (2002).

    Article  ADS  Google Scholar 

  44. Guenther, J. N. et al. The QCD equation of state at finite density from analytical continuation. Nucl. Phys. A 967, 720–723 (2017).

    Article  ADS  Google Scholar 

  45. Bass, S. A. et al. Microscopic models for ultrarelativistic heavy ion collisions. Prog. Part. Nucl. Phys. 41, 255–369 (1998).

    Article  ADS  Google Scholar 

  46. Floerchinger, S. & Wetterich, C. Chemical freeze-out in heavy ion collisions at large baryon densities. Nucl. Phys. A 890-891, 11–24 (2012).

    Article  ADS  Google Scholar 

  47. Agakishiev, G. et al. The High-Acceptance Dielectron Spectrometer HADES. Eur. Phys. J. A 41, 243–277 (2009).

    Article  ADS  Google Scholar 

  48. Adamczewski-Musch, J. et al. Centrality determination of Au + Au collisions at 1.23 A GeV with HADES. Eur. Phys. J. A 54, 85 (2018).

    Article  ADS  Google Scholar 

  49. Harabasz, S. Reconstruction of Virtual Photons from Au + Au Collisions at 1.23 GeV/u. PhD Thesis, Tech. Univ. Darmstadt and Jagiellonian Univ. Cracow (2017).

  50. Hocker, A. et al. TMVA, the Toolkit for Multivariate Data Analysis with ROOT. PoS(ACAT)040 (2007).

  51. Sellheim, P. Reconstruction of the Low-Mass Dielectron Signal in 1.23 A GeV Au + Au Collisions. PhD Thesis, Goethe-Univ. Frankfurt (2017).

  52. Fröhlich, I. et al. Design of the Pluto Event Generator. J. Phys. Conf. Ser. 219, 032039 (2010).

    Article  Google Scholar 

  53. Averbeck, R. et al. Neutral pions and eta mesons as probes of the hadronic fireball in nucleus-nucleus collisions around 1-A-GeV. Phys. Rev. C 67, 024903 (2003).

    Article  ADS  Google Scholar 

  54. Adamczewski-Musch, J. et al. Deep sub-threshold ϕ production in Au + Au collisions. Phys. Lett. B 778, 403–407 (2018).

    Article  ADS  Google Scholar 

Download references

Acknowledgements

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).

Author information

Author notes

  1. Deceased: S. Chernenko.

  2. Deceased: Y. Zanevsky.

  3. A full list of authors and affiliations appears in the online version of this paper.

Authors and Affiliations

  1. GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany

    J. Adamczewski-Musch, T. Galatyuk, M. Gumberidze, T. Heinz, R. Holzmann, I. Koenig, W. Koenig, B. W. Kolb, J. Markert, V. Pechenov, O. Pechenova, J. Pietraszko, E. Schwab, I. Selyuzhenkov, J. Stroth, C. Sturm, M. Traxler, C. Wendisch & P. Zumbruch

  2. Excellence Cluster ‘Origin and Structure of the Universe’, Garching, Germany

    O. Arnold, J. C. Berger-Chen, E. Epple, L. Fabbietti, K. Lapidus, D. M. Mihaylov, R. Münzer & J. Wirth

  3. Physik Department E62, Technische Universität München, Garching, Germany

    O. Arnold, J. C. Berger-Chen, M. Böhmer, E. Epple, L. Fabbietti, J. Friese, R. Gernhäuser, T. Kunz, K. Lapidus, L. Maier, S. Maurus, D. M. Mihaylov, R. Münzer, J. Siebenson & J. Wirth

  4. Institut für Kernphysik, Goethe-Universität, Frankfurt, Germany

    C. Behnke, C. Blume, I. Fröhlich, B. Kardan, M. Lorenz, J. Michel, C. Müntz, T. Scheib, H. Schuldes, P. Sellheim, S. Spies, H. Ströbele, J. Stroth, M. Szala & M. G. Wiebusch

  5. Institut de Physique Nucléaire, CNRS-IN2P3, Université Paris-Sud, Université Paris-Saclay, Orsay, France

    A. Belounnas, T. Hennino, B. Ramstein, P. Rosier & F. Scozzi

  6. Joint Institute of Nuclear Research, Dubna, Russia

    A. Belyaev, S. Chernenko, O. Fateev, A. Ierusalimov, A. Kurilkin, P. Kurilkin, V. Ladygin & Y. Zanevsky

  7. Smoluchowski Institute of Physics, Jagiellonian University of Cracow, Kraków, Poland

    J. Biernat, A. Dybczak, S. Harabasz, G. Korcyl, R. Lalik, K. N. Nowakowski, M. Palka, W. Przygoda, P. Salabura & P. Strzempek

  8. LIP—Laboratório de Instrumentação e Física Experimental de Partículas, Coimbra, Portugal

    A. Blanco, P. Bordalo, P. Fonte, C. Franco, L. Lopes, A. Mangiarotti, S. Ramos & L. Silva

  9. Nuclear Physics Institute, The Czech Academy of Sciences, Rez, Czech Republic

    L. Chlad, A. Kugler, P. Rodriguez-Ramos, Yu. G. Sobolev, O. Svoboda, P. Tlusty & V. Wagner

  10. II. Physikalisches Institut, Justus Liebig Universität Giessen, Giessen, Germany

    C. Deveaux, C. Höhne, T. Mahmoud & V. Metag

  11. Technische Universität Darmstadt, Darmstadt, Germany

    D. Dittert, T. Galatyuk, M. Gumberidze, S. Harabasz, G. Kornakov, A. Rost, F. Scozzi & F. Seck

  12. Institut für Strahlenphysik, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany

    J. Dreyer, R. Greifenhagen, B. Kämpfer, R. Kotte & L. Naumann

  13. Institute of Physics, Slovak Academy of Sciences, Bratislava, Slovakia

    P. Filip & S. Hlavac

  14. LabCAF. F. Física, Universidade de Santiago de Compostela, Santiago de Compostela, Spain

    J. A. Garzón

  15. Institute for Nuclear Research, Russian Academy of Science, Moscow, Russia

    M. Golubeva, F. Guber, A. Ivashkin, T. Karavicheva, A. Kurepin, S. Morozov, O. Petukhov, A. Reshetin, A. Sadovsky & E. Usenko

  16. Institute of Theoretical and Experimental Physics, Moscow, Russia

    A. Lebedev

  17. National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Moscow, Russia

    S. Morozov & I. Selyuzhenkov

  18. Dipartimento di Fisica and INFN, Università di Torino, Turin, Italy

    Y. Parpottas & S. Spataro

  19. Department of Physics, University of Cyprus, Nicosia, Cyprus

    H. Tsertos

  20. ISEC Coimbra, Coimbra, Portugal

    P. Fonte

  21. Technische Universität Dresden, Dresden, Germany

    R. Greifenhagen & B. Kämpfer

  22. Frederick University, Nicosia, Cyprus

    Y. Parpottas

Consortia

The HADES Collaboration

  • J. Adamczewski-Musch
  • , O. Arnold
  • , C. Behnke
  • , A. Belounnas
  • , A. Belyaev
  • , J. C. Berger-Chen
  • , J. Biernat
  • , A. Blanco
  • , C. Blume
  • , M. Böhmer
  • , P. Bordalo
  • , S. Chernenko
  • , L. Chlad
  • , C. Deveaux
  • , D. Dittert
  • , J. Dreyer
  • , A. Dybczak
  • , E. Epple
  • , L. Fabbietti
  • , O. Fateev
  • , P. Filip
  • , P. Fonte
  • , C. Franco
  • , J. Friese
  • , I. Fröhlich
  • , T. Galatyuk
  • , J. A. Garzón
  • , R. Gernhäuser
  • , M. Golubeva
  • , R. Greifenhagen
  • , F. Guber
  • , M. Gumberidze
  • , S. Harabasz
  • , T. Heinz
  • , T. Hennino
  • , S. Hlavac
  • , C. Höhne
  • , R. Holzmann
  • , A. Ierusalimov
  • , A. Ivashkin
  • , B. Kämpfer
  • , T. Karavicheva
  • , B. Kardan
  • , I. Koenig
  • , W. Koenig
  • , B. W. Kolb
  • , G. Korcyl
  • , G. Kornakov
  • , R. Kotte
  • , A. Kugler
  • , T. Kunz
  • , A. Kurepin
  • , A. Kurilkin
  • , P. Kurilkin
  • , V. Ladygin
  • , R. Lalik
  • , K. Lapidus
  • , A. Lebedev
  • , L. Lopes
  • , M. Lorenz
  • , T. Mahmoud
  • , L. Maier
  • , A. Mangiarotti
  • , J. Markert
  • , S. Maurus
  • , V. Metag
  • , J. Michel
  • , D. M. Mihaylov
  • , S. Morozov
  • , C. Müntz
  • , R. Münzer
  • , L. Naumann
  • , K. N. Nowakowski
  • , M. Palka
  • , Y. Parpottas
  • , V. Pechenov
  • , O. Pechenova
  • , O. Petukhov
  • , J. Pietraszko
  • , W. Przygoda
  • , S. Ramos
  • , B. Ramstein
  • , A. Reshetin
  • , P. Rodriguez-Ramos
  • , P. Rosier
  • , A. Rost
  • , A. Sadovsky
  • , P. Salabura
  • , T. Scheib
  • , H. Schuldes
  • , E. Schwab
  • , F. Scozzi
  • , F. Seck
  • , P. Sellheim
  • , I. Selyuzhenkov
  • , J. Siebenson
  • , L. Silva
  • , Yu. G. Sobolev
  • , S. Spataro
  • , S. Spies
  • , H. Ströbele
  • , J. Stroth
  • , P. Strzempek
  • , C. Sturm
  • , O. Svoboda
  • , M. Szala
  • , P. Tlusty
  • , M. Traxler
  • , H. Tsertos
  • , E. Usenko
  • , V. Wagner
  • , C. Wendisch
  • , M. G. Wiebusch
  • , J. Wirth
  • , Y. Zanevsky
  •  & P. Zumbruch

Contributions

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.

Corresponding author

Correspondence to J. Stroth.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary figures.

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

The HADES Collaboration. Probing dense baryon-rich matter with virtual photons. Nat. Phys. 15, 1040–1045 (2019). https://doi.org/10.1038/s41567-019-0583-8

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s41567-019-0583-8

This article is cited by

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing