# Creation of quark–gluon plasma droplets with three distinct geometries

## Abstract

Experimental studies of the collisions of heavy nuclei at relativistic energies have established the properties of the quark–gluon plasma (QGP), a state of hot, dense nuclear matter in which quarks and gluons are not bound into hadrons1,2,3,4. In this state, matter behaves as a nearly inviscid fluid5 that efficiently translates initial spatial anisotropies into correlated momentum anisotropies among the particles produced, creating a common velocity field pattern known as collective flow. In recent years, comparable momentum anisotropies have been measured in small-system proton–proton (p+p) and proton–nucleus (p+A) collisions, despite expectations that the volume and lifetime of the medium produced would be too small to form a QGP. Here we report on the observation of elliptic and triangular flow patterns of charged particles produced in proton–gold (p+Au), deuteron–gold (d+Au) and helium–gold (3He+Au) collisions at a nucleon–nucleon centre-of-mass energy $$\sqrt {s_{{\mathrm{NN}}}}$$ = 200 GeV. The unique combination of three distinct initial geometries and two flow patterns provides unprecedented model discrimination. Hydrodynamical models, which include the formation of a short-lived QGP droplet, provide the best simultaneous description of these measurements.

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## Data availability

All raw data for this study were collected using the PHENIX detector at Brookhaven National Laboratory. Data tables for the results reported in this paper and other findings of this study are publicly available on the PHENIX website (https://www.phenix.bnl.gov/phenix/WWW/info/data/ppg216_data.html) or from the corresponding author upon reasonable request.

## References

1. 1.

Arsene, I. et al. Quark gluon plasma and color glass condensate at RHIC? The perspective from the BRAHMS experiment. Nucl. Phys. A 757, 1–27 (2005).

2. 2.

Back, B. B. et al. The PHOBOS perspective on discoveries at RHIC. Nucl. Phys. A 757, 28–101 (2005).

3. 3.

Adams, J. et al. Experimental and theoretical challenges in the search for the quark gluon plasma: the STAR Collaboration’s critical assessment of the evidence from RHIC collisions. Nucl. Phys. A 757, 102–183 (2005).

4. 4.

Adcox, K. et al. Formation of dense partonic matter in relativistic nucleus-nucleus collisions at RHIC: experimental evaluation by the PHENIX collaboration. Nucl. Phys. A 757, 184–283 (2005).

5. 5.

Heinz, U. & Snellings, R. Collective flow and viscosity in relativistic heavy-ion collisions. Annu. Rev. Nucl. Part. Sci. 63, 123–151 (2013).

6. 6.

Khachatryan, V. et al. Observation of long-range near-side angular correlations in proton-proton collisions at the LHC. J. High Energy Phys. 09, 091 (2010).

7. 7.

Chatrchyan, S. et al. Observation of long-range near-side angular correlations in proton-lead collisions at the LHC. Phys. Lett. B 718, 795–814 (2013).

8. 8.

Abelev, B. et al. Long-range angular correlations on the near and away side in p-Pb collisions at = 5.02 TeV. Phys. Lett. B 719, 29–41 (2013).

9. 9.

Aad, G. et al. Observation of associated near-side and away-side long-range correlations in = 5.02 TeV proton-lead collisions with the ATLAS detector. Phys. Rev. Lett. 110, 182302 (2013).

10. 10.

Adare, A. et al. Quadrupole anisotropy in dihadron azimuthal correlations in central d + Au collisions at = 200 GeV. Phys. Rev. Lett. 111, 212301 (2013).

11. 11.

Adare, A. et al. Measurement of long-range angular correlation and quadrupole anisotropy of pions and (anti)protons in central d + Au collisions at = 200 GeV. Phys. Rev. Lett. 114, 192301 (2015).

12. 12.

Dusling, K., Li, W. & Schenke, B. Novel collective phenomena in high-energy proton-proton and proton-nucleus collisions. Int. J. Mod. Phys. E 25, 1630002 (2016).

13. 13.

Nagle, J. L. & Zajc, W. A. Small system collectivity in relativistic hadron and nuclear collisions. Preprint at https://arXiv.org/abs/1801.03477v2 (2018).

14. 14.

Nagle, J. L. et al. Exploiting intrinsic triangular geometry in relativistic 3He + Au collisions to disentangle medium properties. Phys. Rev. Lett. 113, 112301 (2014).

15. 15.

Miller, M. L., Reygers, K., Sanders, S. J. & Steinberg, P. Glauber modeling in high energy nuclear collisions. Annu. Rev. Nucl. Part. Sci. 57, 205–243 (2007).

16. 16.

Alver, B. & Roland, G. Collision geometry fluctuations and triangular flow in heavy-ion collisions. Phys. Rev. C 81, 054905 (2010); erratum 82, 039903 (2010).

17. 17.

Gale, C., Jeon, S. & Schenke, B. Hydrodynamic modeling of heavy-ion collisions. Int. J. Mod. Phys. A 28, 1340011 (2013).

18. 18.

Habich, M., Nagle, J. L. & Romatschke, P. Particle spectra and HBT radii for simulated central nuclear collisions of C + C, Al + Al, Cu + Cu, Au + Au, and Pb + Pb from = 62.4−2760 GeV. Eur. Phys. J. C 75, 15 (2015).

19. 19.

Dusling, K. & Venugopalan, R. Azimuthal collimation of long range rapidity correlations by strong color fields in high multiplicity hadron-hadron collisions. Phys. Rev. Lett. 108, 262001 (2012).

20. 20.

Dumitru, A. et al. The ridge in proton-proton collisions at the LHC. Phys. Lett. B 697, 21–25 (2011).

21. 21.

Dumitru, A. & Giannini, A. V. Initial state angular asymmetries in high energy p+A collisions: spontaneous breaking of rotational symmetry by a color electric field and C-odd fluctuations. Nucl. Phys. A 933, 212–228 (2015).

22. 22.

Lappi, T., Schenke, B., Schlichting, S. & Venugopalan, R. Tracing the origin of azimuthal gluon correlations in the color glass condensate. J. High Energy Phys. 01, 061 (2016).

23. 23.

Aidala, C. et al. Measurement of long-range angular correlations and azimuthal anisotropies in high-multiplicity p + Au collisions at = 200 GeV. Phys. Rev. C 95, 034910 (2017).

24. 24.

Aidala, C. et al. Measurements of azimuthal anisotropy and charged-particle multiplicity in d + Au collisions at = 200, 62.4, 39, and 19.6 GeV. Phys. Rev. C 96, 064905 (2017).

25. 25.

Adare, A. et al. Measurements of elliptic and triangular flow in high-multiplicity 3He + Au collisions at = 200 GeV. Phys. Rev. Lett. 115, 142301 (2015).

26. 26.

Voloshin, S. & Zhang, Y. Flow study in relativistic nuclear collisions by Fourier expansion of azimuthal particle distributions. Z. Phys. C 70, 665–672 (1996).

27. 27.

Ollitrault, J.-Y., Poskanzer, A. M. & Voloshin, S. A. Effect of nonflow and flow fluctuations on elliptic flow methods. Nucl. Phys. A 830, 279c–282c (2009).

28. 28.

Adare, A. et al. Centrality categorization for R p(d)+A in high-energy collisions. Phys. Rev. C 90, 034902 (2014).

29. 29.

Shen, C., Paquet, J.-F., Denicol, G. S., Jeon, S. & Gale, C. Collectivity and electromagnetic radiation in small systems. Phys. Rev. C 95, 014906 (2017).

30. 30.

Kovtun, P., Son, D. T. & Starinets, A. O. Viscosity in strongly interacting quantum field theories from black hole physics. Phys. Rev. Lett. 94, 111601 (2005).

31. 31.

Adare, A. et al. Quantitative constraints on the opacity of hot partonic matter from semi-inclusive single high transverse momentum pion suppression in Au + Au collisions at = 200 GeV. Phys. Rev. C 77, 064907 (2008).

32. 32.

Welsh, K., Singer, J. & Heinz, U. W. Initial state fluctuations in collisions between light and heavy ions. Phys. Rev. C 94, 024919 (2016).

33. 33.

Weller, R. D. & Romatschke, P. One fluid to rule them all: viscous hydrodynamic description of event-by-event central p + p, p + Pb and Pb + Pb collisions at = 5.02 TeV. Phys. Lett. B 774, 351–356 (2017).

34. 34.

Lin, Z.-W., Ko, C. M., Li, B.-A., Zhang, B. & Pal, S. A multi-phase transport model for relativistic heavy ion collisions. Phys. Rev. C 72, 064901 (2005).

35. 35.

Orjuela Koop, J. D., Adare, A., McGlinchey, D. & Nagle, J. L. Azimuthal anisotropy relative to the participant plane from a multiphase transport model in central p + Au, d + Au, and 3He + Au collisions at = 200 GeV. Phys. Rev. C 92, 054903 (2015).

36. 36.

Mace, M., Skokov, V. V., Tribedy, P. & Venugopalan, R. Hierarchy of azimuthal anisotropy harmonics in collisions of small systems from the color glass condensate. Phys. Rev. Lett. 121, 052301 (2018).

37. 37.

Nagle, J. L. & Zajc, W. A. Assessing saturation physics explanations of collectivity in small collision systems with the IP-Jazma model. Preprint at https://arXiv.org/abs/1808.01276v1 (2018).

38. 38.

Adare, A. et al. Pseudorapidity dependence of particle production and elliptic flow in asymmetric nuclear collisions of p+Al, p+Au, d+Au, and 3He+Au at $$\sqrt {s_{_{NN}}}$$ = 200 GeV. Preprint at https://arXiv.org/abs/1807.11928v1 (2018).

39. 39.

Adcox, K. et al. PHENIX central arm tracking detectors. Nucl. Instrum. Meth. A 499, 489–507 (2003).

40. 40.

Aidala, C. et al. The PHENIX forward silicon vertex detector. Nucl. Instrum. Meth. A 755, 44–61 (2014).

41. 41.

Adcox, K. et al. PHENIX detector overview. Nucl. Instrum. Meth. A 499, 469–479 (2003).

42. 42.

Poskanzer, A. M. & Voloshin, S. A. Methods for analyzing anisotropic flow in relativistic nuclear collisions. Phys. Rev. C 58, 1671–1678 (1998).

## Acknowledgements

We thank the staff of the Collider-Accelerator and Physics Departments at Brookhaven National Laboratory and the staff of the other PHENIX participating institutions for their vital contributions. We acknowledge support from the Office of Nuclear Physics in the Office of Science of the Department of Energy, the National Science Foundation, Abilene Christian University Research Council, Research Foundation of SUNY, and Dean of the College of Arts and Sciences, Vanderbilt University (USA), Ministry of Education, Culture, Sports, Science, and Technology and the Japan Society for the Promotion of Science (Japan), Conselho Nacional de Desenvolvimento Cientfico e Tecnológico and Fundação de Amparo à Pesquisa do Estado de São Paulo (Brazil), Natural Science Foundation of China (People’s Republic of China), Croatian Science Foundation and Ministry of Science and Education (Croatia), Ministry of Education, Youth and Sports (Czech Republic), Centre National de la Recherche Scientifique, Commissariat à l'Énergie Atomique, and Institut National de Physique Nucléaire et de Physique des Particules (France), Bundesministerium für Bildung und Forschung, Deutscher Akademischer Austausch Dienst, and Alexander von Humboldt Stiftung (Germany), NKFIH, EFOP, the New National Excellence Program (ÚNKP) and the J. Bolyai Research Scholarships (Hungary), Department of Atomic Energy and Department of Science and Technology (India), Israel Science Foundation (Israel), Basic Science Research Program through NRF of the Ministry of Education (Korea), Physics Department, Lahore University of Management Sciences (Pakistan), Ministry of Education and Science, Russian Academy of Sciences, Federal Agency of Atomic Energy (Russia), VR and Wallenberg Foundation (Sweden), the US Civilian Research and Development Foundation for the Independent States of the Former Soviet Union, the Hungarian American Enterprise Scholarship Fund, and the US–Israel Binational Science Foundation.

## Author information

All PHENIX collaboration members contributed to the publication of these results in a variety of roles including detector construction, data collection, data processing, and analysis. A subset of collaboration members prepared this manuscript, and all authors had the opportunity to review the final version.

Correspondence to J. L. Nagle.

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### Competing interests

The authors declare no competing interests.

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