Periodic Reporting for period 4 - collectiveQCD (Collectivity in small, srongly interacting systems)
Reporting period: 2023-08-01 to 2025-07-31
Curiosity is deeply rooted in the nature of human beings. It is curiosity that lets us do research "So that I may perceive whatever holds The world together in its inmost folds" (Johann Wolfgang von Goethe: "Faust - Der Tragödie erster Teil", translation by George Madison Priest from http://www.levity.com/alchemy/faust02.html(opens in new window)). To our current understanding the answer to this fundamental question is 'the strong force'. The strong force rules over quarks and gluons, that make up protons and neutrons (the building blocks of atomic nuclei).
The strong force has a peculiar property that sets it apart from all other forces: the force between quarks and gluons gets stronger when the particles move further apart. An important consequence of that is that quarks and gluons cannot exist alone, but are always bound into composite particles like protons and neutrons. However, when such particles are compressed and/or heated enough, the interaction becomes weak and the particles 'melt' to form a new state of matter known as the Quark-Gluon Plasma (QGP). Powerful accelerators like the Large Hadron Collider LHC at CERN in Geneva can collide heavy atomic nuclei at energies that are high enough to form a QGP for a very short time. Experiments have found out that the QGP behaves like a liquid, which came as a surprise since most scientists had expected it would behave like a gas. The really big surprise, however, came when the experiments found that sometimes collisions of protons at the LHC look a bit as if a QGP was formed there too. This was thought to be impossible because proton-proton collisions are much smaller in size and not nearly as dense as colliding heavy nuclei.
The main goal of this project was to better understand from the theory side how the QGP is formed and how its properties come about. In particular, why proton-proton and nucleus-nucleus collisions sometimes look alike.
To this end a new simulation tool called ALPACA was developed. It simulates the time evolution of systems consisting of many particles by treating the particles and their interactions individually. This is called a kinetic theory and ALPACA is a faithful representation of a theoretically controlled kinetic theory of the strong interaction at high temperatures. One conclusion from the project is that systems consisting of few particles undergoing only few interactions can mimic signatures of a fluid.
Another focus of the project were very energetic particles that can be used as complementary probes of the QGP. Here, it was shown that two apparently conflicting experimental observations in systems with few particles are in fact not in contradiction to the current theoretical understanding when the uncertainties are taken into account. Furthermore, it was investigated how hard particles can be used to identify the earliest signs of QGP formation.
The strong force has a peculiar property that sets it apart from all other forces: the force between quarks and gluons gets stronger when the particles move further apart. An important consequence of that is that quarks and gluons cannot exist alone, but are always bound into composite particles like protons and neutrons. However, when such particles are compressed and/or heated enough, the interaction becomes weak and the particles 'melt' to form a new state of matter known as the Quark-Gluon Plasma (QGP). Powerful accelerators like the Large Hadron Collider LHC at CERN in Geneva can collide heavy atomic nuclei at energies that are high enough to form a QGP for a very short time. Experiments have found out that the QGP behaves like a liquid, which came as a surprise since most scientists had expected it would behave like a gas. The really big surprise, however, came when the experiments found that sometimes collisions of protons at the LHC look a bit as if a QGP was formed there too. This was thought to be impossible because proton-proton collisions are much smaller in size and not nearly as dense as colliding heavy nuclei.
The main goal of this project was to better understand from the theory side how the QGP is formed and how its properties come about. In particular, why proton-proton and nucleus-nucleus collisions sometimes look alike.
To this end a new simulation tool called ALPACA was developed. It simulates the time evolution of systems consisting of many particles by treating the particles and their interactions individually. This is called a kinetic theory and ALPACA is a faithful representation of a theoretically controlled kinetic theory of the strong interaction at high temperatures. One conclusion from the project is that systems consisting of few particles undergoing only few interactions can mimic signatures of a fluid.
Another focus of the project were very energetic particles that can be used as complementary probes of the QGP. Here, it was shown that two apparently conflicting experimental observations in systems with few particles are in fact not in contradiction to the current theoretical understanding when the uncertainties are taken into account. Furthermore, it was investigated how hard particles can be used to identify the earliest signs of QGP formation.
A lot of the work has gone into setting up a computer simulation called ALPACA of the QGP based on a successful effective kinetic theory by Arnold, Moore and Yaffe. In kinetic theory the evolution of a system of particles interacting with each other is described by tracking the particles individually. It is a very powerful framework, but some caution is needed as it cannot be applied in all situations. In this case the particles are quarks and gluons, that make up the protons and neutrons in atomic nuclei. The interactions among them can be normal elastic scattering (like scattering of billard balls), but also processes where a quark (or a gluon) splits into a quark (or a gluon) and another gluon, or absorbs a gluon. An important problem was solved on the way: the scattering probability depends not only on the properties of the scattering particles, but also on the particles in the surroundings (even though they do not participate in the scattering). We managed to calculate the scattering probabilities knowing only the particles in the system and nothing else. This had not been achieved before. Simulating the absorption of a gluon also required developing a new technique. We then tested ALPACA to make sure that it reproduces known results in the cases where solutions existed before.
Next, we studied what happens in the effective kinetic theory when there is at most one scattering during the evolution of the system. We did this by directly solving the theory for this case and with the full simulation. In this set-up the system shows a behaviour characteristic for liquids, but it does not come close to being a QGP. We understand how this comes about in the theory, and this is important for understanding small collision systems like proton-proton. A surprising discovery was that under certain conditions a quantity used to characterise fluid-like behaviour can change sign in small systems. Measuring the sign is extremely challenging, but our work has encouraged some of our experimental colleagues to try to measure it. This also sparked a fruitful collaboration with another theoretical group.
The work with the kinetic theory was inspired by the traditional view on collisions of heavy nuclei and QGP formation, and we applied this picture to small systems where it is unclear whether a QGP can form. In an attempt to approach the small systems puzzle from the other side we also worked on extending a model for proton-proton collisions to collisions involving nuclei. We worked out how this can be done and also started writing the simulation software, but did not continue because the model was not working well enough already for proton-proton collisions.
The third building block in the project were energetic particles, which can serve as a complementary probe of the QGP. When such a particle travels through a QGP it interacts and loses some of its energy. We studied how such energetic particles can be used to detect earliest signs of processes that ultimately lead to QGP formation. Building on the experience with the kinetic theory we have a new picture the quarks and gluons produced in a collision of protons or small nuclei start to interact in a way similar to what happens in a large system. But before enough interactions have happened to form a QGP the systems flies apart. We have come up with suggestions for how even a single interaction of an energetic particle could perhaps me measured in experiments.
We have published our results in scientific journals and presented them at a number of international conferences. The software that we have written is freely available as soon as it is ready.
Next, we studied what happens in the effective kinetic theory when there is at most one scattering during the evolution of the system. We did this by directly solving the theory for this case and with the full simulation. In this set-up the system shows a behaviour characteristic for liquids, but it does not come close to being a QGP. We understand how this comes about in the theory, and this is important for understanding small collision systems like proton-proton. A surprising discovery was that under certain conditions a quantity used to characterise fluid-like behaviour can change sign in small systems. Measuring the sign is extremely challenging, but our work has encouraged some of our experimental colleagues to try to measure it. This also sparked a fruitful collaboration with another theoretical group.
The work with the kinetic theory was inspired by the traditional view on collisions of heavy nuclei and QGP formation, and we applied this picture to small systems where it is unclear whether a QGP can form. In an attempt to approach the small systems puzzle from the other side we also worked on extending a model for proton-proton collisions to collisions involving nuclei. We worked out how this can be done and also started writing the simulation software, but did not continue because the model was not working well enough already for proton-proton collisions.
The third building block in the project were energetic particles, which can serve as a complementary probe of the QGP. When such a particle travels through a QGP it interacts and loses some of its energy. We studied how such energetic particles can be used to detect earliest signs of processes that ultimately lead to QGP formation. Building on the experience with the kinetic theory we have a new picture the quarks and gluons produced in a collision of protons or small nuclei start to interact in a way similar to what happens in a large system. But before enough interactions have happened to form a QGP the systems flies apart. We have come up with suggestions for how even a single interaction of an energetic particle could perhaps me measured in experiments.
We have published our results in scientific journals and presented them at a number of international conferences. The software that we have written is freely available as soon as it is ready.
ALPACA is a novel and powerful simulation tool that has significantly improved the state of the art. There are still a few things to be added to it, but once that is done it will be a unique framework to consistently describe collisions at colliders covering the entire range in system size and the entire range in particle energy. It is also the first faithful implementation of a theoretically controlled kinetic theory of the strong interaction.
Having such a tool will help answering the questions like
* How is the QGP formed?
* Can it be formed in small systems such as proton-proton collisions?
Answers to these questions will help us to better understand the strong force. The strong force has many fascinating facets: it holds atomic nucei together, generates masses for the protons and neutrons, and describes how the universe evolved very shortly after the Big Bang. ALPACA will thus be a useful tool for a long time after the end of the project.
Having such a tool will help answering the questions like
* How is the QGP formed?
* Can it be formed in small systems such as proton-proton collisions?
Answers to these questions will help us to better understand the strong force. The strong force has many fascinating facets: it holds atomic nucei together, generates masses for the protons and neutrons, and describes how the universe evolved very shortly after the Big Bang. ALPACA will thus be a useful tool for a long time after the end of the project.