Browsing by Author "Müller, Berndt"

The production of heavy quarkonium in heavy ion collisions has been used as an important probe of the quark-gluon plasma (QGP). Due to the plasma screening effect, the color attraction between the heavy quark antiquark pair inside a quarkonium is significantly suppressed at high temperature and thus no bound states can exist, i.e., they ``melt". In addition, a bound heavy quark antiquark pair can dissociate if enough energy is transferred to it in a dynamical process inside the plasma. So one would expect the production of quarkonium to be considerably suppressed in heavy ion collisions. However, experimental measurements have shown that a large amount of quarkonia survive the evolution inside the high temperature plasma. It is realized that the in-medium recombination of unbound heavy quark pairs into quarkonium is as crucial as the melting and dissociation. Thus, phenomenological studies have to account for static screening, dissociation and recombination in a consistent way. But recombination is less understood theoretically than the melting and dissociation. Many studies using semi-classical transport equations model the recombination effect from the consideration of detailed balance at thermal equilibrium. However, these studies cannot explain how the system of quarkonium reaches equilibrium and estimate the time scale of the thermalization. Recently, another approach based on the open quantum system formalism started being used. In this framework, one solves a quantum evolution for in-medium quarkonium. Dissociation and recombination are accounted for consistently. However, the connection between the semi-classical transport equation and the quantum evolution is not clear.

In this dissertation, I will try to address the issues raised above. As a warm-up project, I will first study a similar problem: $\alpha$-$\alpha$ scattering at the $^8$Be resonance inside an $e^-e^+\gamma$ plasma. By applying pionless effective field theory and thermal field theory, I will show how the plasma screening effect modifies the $^8$Be resonance energy and width. I will discuss the need to use the open quantum system formalism when studying the time evolution of a system embedded inside a plasma. Then I will use effective field theory of QCD and the open quantum system formalism to derive a Lindblad equation for bound and unbound heavy quark antiquark pairs inside a weakly-coupled QGP. Under the Markovian approximation and the assumption of weak coupling between the system and the environment, the Lindblad equation will be shown to turn to a Boltzmann transport equation if a Wigner transform is applied to the open system density matrix. These assumptions will be justified by using the separation of scales, which is assumed in the construction of effective field theory. I will show the scattering amplitudes that contribute to the collision terms in the Boltzmann equation are gauge invariant and infrared safe. By coupling the transport equation of quarkonium with those of open heavy flavors and solving them using Monte Carlo simulations, I will demonstrate how the system of bound and unbound heavy quark antiquark pairs reaches detailed balance and equilibrium inside the QGP. Phenomenologically, my calculations can describe the experimental data on bottomonium production. Finally I will extend the framework to study the in-medium evolution of heavy diquarks and estimate the production rate of the doubly charmed baryon $\Xi_{cc}^{++}$ in heavy ion collisions.

In order to analyze the strongly interacting quark gluon plasma in heavy ion collisions, we study different probes by applying the gauge/gravity duality to facilitate our qualitative understandings on such a non-perturbative system. In this dissertation, we utilize a variety of holographic models to tackle many problems in heavy ion physics including the rapid thermalization, jet quenching, photon production, and anomalous effects led by external electromagnetic fields. We employ the AdS-Vaidya metric to study the gravitational collapse corresponding to the thermalization of a strongly coupled gauge theory, where we compute the approximated thermalization time and stopping distances of light probes in such a non-equilibrium medium. We further generalize the study to the case with a nonzero chemical potential. We find that the non-equilibrium effect is more influential for the probes with smaller energy. In the presence of a finite chemical potential, the decrease of thermalization times for both the medium and the light probes is observed.

On the other hand, we also investigate the anisotropic effect on the stopping distance related to jet quenching of light probes and thermal-photon production. The stopping distance and photoemission rate in the anisotropic background depend on the moving directions of probes.

The influence from a magnetic field on photoemission is as well investigated in the framework of the D3/D7 system, where the contributions from massive quarks are involved. The enhancement of photon production for photons generated perpendicular to the magnetic field is found. Given that the mass of massive quarks is close to the critical embedding, the meson-photon transition will yield a resonance in the spectrum. We thus evaluate flow coefficient $v_2$ of thermal photons in a 2+1 flavor strongly interacting plasma. The magnetic-field induced photoemission results in large $v_2$ and the resonance from massive quarks gives rise to a mild peak in the spectrum. Moreover, we utilize Sakai-Sugimoto model to analyze the chiral electric separation effect, where an axial current is generated parallel to the applied electric field in the presence of both the vector and axial chemical potentials. Interestingly, the axial conductivity is approximately proportional to the product of the vector chemical potential and the axial chemical potential for arbitrary magnitudes of the chemical potentials.

The quark gluon plasma (QGP) forms when matter governed by quantum chromodynamics (QCD) undergoes a transition at high temperature or high density from hadronic bound states to deconfined quarks and gluons. The QGP at high temperature is believed to be experimentally accessible in relativistic heavy-ion collisions, such as those done at the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Lab and in the near future at the Large Hadron Collider (LHC) at CERN. The results obtained so far reveal the production of energetic (hard) partons in the early stages of a heavy-ion collision which propagate through the plasma. Results also show that the QGP produced at RHIC is a nearly ideal fluid and that hard partons may generate conical, Mach-like, disturbances in the QGP.

This thesis uses theoretical methods to address how the QGP responds to a hard parton that propagates through the plasma and contains the first rigorous derivation of how a hard parton deposits energy and momentum in a QGP which lead to the formation of a Mach cone. A comparison of experimental results with the theory introduced in this thesis could shed light on important properties of the QGP such as its equation of state and transport coefficients like viscosity. I investigate this problem by evaluating the source of energy and momentum generated by the hard parton in the QGP. Formalisms are developed and applied for evaluating the source of energy and momentum in perturbation theory with three different methods: classical kinetic theory, finite temperature field theory, and by including the energy lost by the hard parton to radiation. Having obtained the source of energy and momentum generated by the hard parton, I evaluate the medium response using linearized hydrodynamics. My results show Mach cone formation in the medium. I compare the medium response for different viscosities and speeds of sound, from which I find the Mach cone weakens and broadens as viscosity is increased. By studying the time evolution of the medium response once the source of energy and momentum is turned off, which occurs in a heavy-ion collision during the hadronic phase, I find that the conical disturbance is enhanced relative to diffusive contributions over a time period of several fm/c.

Cutting edge research problems require the use of complicated and computationally expensive computer models. I will present a practical overview of the design and analysis of computer experiments in high energy nuclear and astro phsyics. The aim of these experiments is to infer credible ranges for certain fundamental parameters of the underlying physical processes through the analysis of model output and experimental data.

To be truly useful computer models must be calibrated against experimental data. Gaining an understanding of the response of expensive models across the full range of inputs can be a slow and painful process. Gaussian Process emulators can be an efficient and informative surrogate for expensive computer models and prove to be an ideal mechanism for exploring the response of these models to variations in their inputs.

A sensitivity analysis can be performed on these model emulators to characterize and quantify the relationship between model input parameters and predicted observable properties. The result of this analysis provides the user with information about which parameters are most important and most likely to affect the prediction of a given observable. Sensitivity analysis allow us to identify what model parameters can be most efficiently constrained by the given observational data set.

In this thesis I describe a range of techniques for the calibration and exploration of the complex and expensive computer models so common in modern physics research. These statistical methods are illustrated with examples drawn from the fields of high energy nuclear physics and galaxy formation.