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The nonlinear high energy regime of Quantum Chromodynamics

Periodic Reporting for period 4 - CGCglasmaQGP (The nonlinear high energy regime of Quantum Chromodynamics)

Reporting period: 2021-04-01 to 2021-09-30

The work in this project revolves around understanding the strong nuclear force, one of the fundamental interactions of nature, in extreme circumstances of high energies, densities and temperatures. The goal is to understand and interpret, based on the fundamental theory, measurements at collider experiments such as the LHC, and at planned next generation electron-nucleus collision experiments. The theoretical framework in the project is Quantum Chromodynamics (QCD), the theory of the strong interaction, in the limit of weak coupling constants. While not directly leading to technological applications, the quest to scientifically understand the basic microscopic laws governing nature is an essential part of society.

An feature of QCD is that while it is unquestionably confirmed to be the correct theory of the strong interaction at presently accessible energy scales, it is calculationally very challenging to extract quantitative predictions from the theory. In different circumstances different approximations and effective descriptions are needed. In this project we concentrate on the so called Color Glass Condensate effective theory that is valid in the limit of very large collision energies between particles. In particular it provides a good framework to study the initial stages in the creation of deconfined quark-gluon matter in ultrarelativistic heavy ion collisions.

The overall objectives of the project are threefold. Firstly the aim is to advance to higher accuracy in calculations of processes where a dilute probe collides with the strong color field of a high energy nucleus, such as in deep inelastic scattering. Secondly, the quantum fluctuations around the strong color fields in the initial stages of a relativistic heavy ion collision are studied with a new numerical method, leading to a better understanding of the thermalization dynamics of these gluon fields. Thirdly, the project aims to improve the calculations of the initial conditions for fluid dynamical studies of the quark gluon plasma dynamics in heavy ion collisions, based on QCD.
We have computed cross sections for processes such as deep inelastic electron-proton scattering, and particle production at forward scattering angles in very high energy proton-nucleus collisions. Here our calculations have been one of the first to be performed at next-to-leding order accuracy in the QCD coupling constant, in the high energy limit where the nonlinearities in the strong color fields have been taken in to account. In particular, we have calculated so called "light cone wave functions" for processes with heavy quarks at one loop accuracy in perturbation theory, also solving a longstanding puzzle concerning quark mass renormalization in axial gauge field theory.

We have developed and used a new numerical algorithm to study the dynamics of the long distance sector of overoccupied gluonic systems far from equilibrium. In particular we have demonstrated the existence of particle-like excitations (quasiparticles) in this gluon plasma, and determining their properties. We have used this method to understand the interactions of heavy quarks and very energetic particles (jets) with the gluonic pre-equilibrium matter in a heavy ion collision.

We have constructed a model for the distribution of the fluctuating gluonic field in the transverse plane of the proton which depends on the energy in a way that is calculated from the first principles theory and is consistent with measurements in electron-ion collisions. Such parametrizations and models are crucial in order to correctly understand the spacetime development of the quark-gluon matter produced in heavy ion collisions.
The cross section calculations at next to leading order in the high energy limit of QCD are a significant improvement over the state of the art where practically all the phenomenological applications are still at leading order accuracy. In the future they will be extended to several new processes that can be measured in different experiments, and now for the first time calculated at this level of accuracy together in a consistent framework.

We have for the first time clearly demonstrated the existence of quasiparticles in a strongly overoccupied gluon field system, where a priori a particle-type description of the gluon field is not at all guaranteed to exist. In the future this will lead to a better understanding of thermalization dynamics in systems that cannot be described in terms of particles, because of their large density.

Our calculation of the distribution of the fluctuating gluonic field in the transverse plane of the proton was the first time in the literature that this has been calculated in a way that matches existing experimental data and has an energy dependence given by QCD theory. Previously such models have tyically been much more ad hoc and less constrained. In a similar vein, in the future the project will lead to a more controlled understanding of the initial phase of the spacetime development of the quark gluon matter created in heavy ion collisions.
The response of a dense gluon the system to an external perturbation as a function of time
Elliptic eccentricity for a proton with 3 hot spots, as a function of the hot spot radius.
The fluctuating gluon field in the transverse plane of a proton at different collision energies
Violation of the Large number of colors approximation for a six-point function of Wilson lines