## Final Report Summary - HICATLHC (Phenomenology of Heavy Ion Collisions at RHIC and the LHC)

Relativistic heavy-ion collisions are experiments carried out at the Rela- tivistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory, and the Large Hadron Collider (LHC) at CERN. In these experiments, the nuclei of heavy atoms are accelerated to incredible speeds and allowed to collide. During such a collision there exists, for the briefest instant, a system at the highest temperature ever achieved, similar to the conditions shortly after the Big Bang. The goal is to create and study a new form of matter called the Quark-Gluon Plasma (QGP), which is expected to exist at such temperatures. This project comprises theoretical work aimed at characterizing the properties of this new form of matter by analyzing the various debris that remain at the end of each collision, in order to better understand the fundamental laws of nature.

A surprising insight from these experiments is that the QGP medium that is created behaves as an almost perfect liquid — that is, a fluid with very little friction or dissipation. Following this insight, a crucial question is to determine the actual value of transport properties of the QGP fluid, and how they depend on temperature. Such transport properties include shear viscosity and bulk viscosity, and this project has addressed these questions on multiple fronts.

Most of what is seen exiting a collision are particles called hadrons, and they provide information about what happened to the system as a whole. They have been used to study the QGP shear viscosity. However, there are two problems to overcome before a precise value can be determined.

First, the final state of the debris from a collision depends not only on the properties of the QGP, but also on the earliest stages of the collision, which are not yet well understood. However, by making a map of which specific properties of the initial state determine which particular aspects of the observed particles, one can potentially disentangle these two effects, and isolate the effects of the QGP [5].

Second, the effects of bulk viscosity can appear to interfere with the effects of shear viscosity, making it difficult to pin down one without knowing the other. However, it turns out that the effects of bulk viscosity are not quite the same as shear viscosity, and these differences can potentially be used to determine each [2].

Besides the hadrons that make up the bulk of the system, experiments also detect other types of particles. In particular, they have detectors that can see photons — particles of light — as well as particles called leptons. Although they are more rare than hadrons, the advantage of these types of particles is that they interact with the QGP much more weakly than the constituents that make up the bulk of the system itself. That is, once they are created, they exit with almost no modification. Because of this, they can potentially carry direct information about the system at early times. This is in contrast to the aforementioned hadrons, which continue to interact and evolve until the system breaks up, and the state of the system at earlier times must be inferred.

However, there exist significant uncertainties which must first be overcome, and a sustained research effort will be required in order to take full advantage of these observations. Much progress has been made in this project regarding an understanding of how these particles are created, as well as how their properties depend on shear and bulk viscosity [1, 3, 6, 7].

Another fascinating question that can be asked is “what is the smallest drop of liquid”. It is expected that if you make a system smaller and smaller, it becomes more difficult to create a liquid, and the characteristic fluid behavior should cease. This has been studied by colliding ions of smaller and smaller size, and the results will shed light on how a QGP is created [4].

Finally, after a long shutdown for upgrades, the LHC has just begun colli- sions at an even higher energy. Making predictions for the results of these new collisions will help build confidence in our current understanding [8].

References

[1] G. Vujanovic, J. F. Paquet, G. S. Denicol, M. Luzum, B. Schenke, S. Jeon and C. Gale, Nucl. Phys. A 932, 230 (2014) [arXiv:1404.3714 [hep-ph]].

[2] J. B. Rose, J. F. Paquet, G. S. Denicol, M. Luzum, B. Schenke, S. Jeon and C. Gale, Nucl. Phys. A 931, 926 (2014) [arXiv:1408.0024 [nucl-th]].

[3] G. Vujanovic, J. F. Paquet, G. S. Denicol, M. Luzum, B. Schenke, S. Jeon and C. Gale, Nucl. Phys. A 931, 701 (2014) [arXiv:1408.1098 [nucl-th]].

[4] I. Kozlov, M. Luzum, G. S. Denicol, S. Jeon and C. Gale, Nucl. Phys. A 931, 1045 (2014) [arXiv:1412.3147 [nucl-th]].

[5] F. G. Gardim, J. Noronha-Hostler, M. Luzum and F. Grassi, Phys. Rev. C 91, no. 3, 034902 (2015) [arXiv:1411.2574 [nucl-th]].

[6] J. F. Paquet, C. Shen, G. S. Denicol, M. Luzum, B. Schenke, S. Jeon and C. Gale, arXiv:1509.06738 [hep-ph].

[7] G. Vujanovic, G. S. Denicol, C. Shen, M. Luzum, B. Schenke, S. Jeon and C. Gale, arXiv:1510.00441 [nucl-th].

[8] J. Noronha-Hostler, M. Luzum and J. Y. Ollitrault, arXiv:1511.06289 [nucl- th].

A surprising insight from these experiments is that the QGP medium that is created behaves as an almost perfect liquid — that is, a fluid with very little friction or dissipation. Following this insight, a crucial question is to determine the actual value of transport properties of the QGP fluid, and how they depend on temperature. Such transport properties include shear viscosity and bulk viscosity, and this project has addressed these questions on multiple fronts.

Most of what is seen exiting a collision are particles called hadrons, and they provide information about what happened to the system as a whole. They have been used to study the QGP shear viscosity. However, there are two problems to overcome before a precise value can be determined.

First, the final state of the debris from a collision depends not only on the properties of the QGP, but also on the earliest stages of the collision, which are not yet well understood. However, by making a map of which specific properties of the initial state determine which particular aspects of the observed particles, one can potentially disentangle these two effects, and isolate the effects of the QGP [5].

Second, the effects of bulk viscosity can appear to interfere with the effects of shear viscosity, making it difficult to pin down one without knowing the other. However, it turns out that the effects of bulk viscosity are not quite the same as shear viscosity, and these differences can potentially be used to determine each [2].

Besides the hadrons that make up the bulk of the system, experiments also detect other types of particles. In particular, they have detectors that can see photons — particles of light — as well as particles called leptons. Although they are more rare than hadrons, the advantage of these types of particles is that they interact with the QGP much more weakly than the constituents that make up the bulk of the system itself. That is, once they are created, they exit with almost no modification. Because of this, they can potentially carry direct information about the system at early times. This is in contrast to the aforementioned hadrons, which continue to interact and evolve until the system breaks up, and the state of the system at earlier times must be inferred.

However, there exist significant uncertainties which must first be overcome, and a sustained research effort will be required in order to take full advantage of these observations. Much progress has been made in this project regarding an understanding of how these particles are created, as well as how their properties depend on shear and bulk viscosity [1, 3, 6, 7].

Another fascinating question that can be asked is “what is the smallest drop of liquid”. It is expected that if you make a system smaller and smaller, it becomes more difficult to create a liquid, and the characteristic fluid behavior should cease. This has been studied by colliding ions of smaller and smaller size, and the results will shed light on how a QGP is created [4].

Finally, after a long shutdown for upgrades, the LHC has just begun colli- sions at an even higher energy. Making predictions for the results of these new collisions will help build confidence in our current understanding [8].

References

[1] G. Vujanovic, J. F. Paquet, G. S. Denicol, M. Luzum, B. Schenke, S. Jeon and C. Gale, Nucl. Phys. A 932, 230 (2014) [arXiv:1404.3714 [hep-ph]].

[2] J. B. Rose, J. F. Paquet, G. S. Denicol, M. Luzum, B. Schenke, S. Jeon and C. Gale, Nucl. Phys. A 931, 926 (2014) [arXiv:1408.0024 [nucl-th]].

[3] G. Vujanovic, J. F. Paquet, G. S. Denicol, M. Luzum, B. Schenke, S. Jeon and C. Gale, Nucl. Phys. A 931, 701 (2014) [arXiv:1408.1098 [nucl-th]].

[4] I. Kozlov, M. Luzum, G. S. Denicol, S. Jeon and C. Gale, Nucl. Phys. A 931, 1045 (2014) [arXiv:1412.3147 [nucl-th]].

[5] F. G. Gardim, J. Noronha-Hostler, M. Luzum and F. Grassi, Phys. Rev. C 91, no. 3, 034902 (2015) [arXiv:1411.2574 [nucl-th]].

[6] J. F. Paquet, C. Shen, G. S. Denicol, M. Luzum, B. Schenke, S. Jeon and C. Gale, arXiv:1509.06738 [hep-ph].

[7] G. Vujanovic, G. S. Denicol, C. Shen, M. Luzum, B. Schenke, S. Jeon and C. Gale, arXiv:1510.00441 [nucl-th].

[8] J. Noronha-Hostler, M. Luzum and J. Y. Ollitrault, arXiv:1511.06289 [nucl- th].