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Markus H. Thoma

Publications and source records attributed to Markus H. Thoma.

At least 37 records · Page 2Linked to original sources
arXiv2008

What can we learn from electromagnetic plasmas about the quark-gluon plasma?

Ultra-relativistic electromagnetic plasmas can be used for improving our understanding of the quark-gluon plasma. In the weakly coupled regime both plasmas can be described by transport theoretical and quantum field theoretical methods leading to similar results for the plasma properties (dielectric tensor, dispersion relations, plasma frequency, Debye screening, transport coefficients, damping and particle production rates). In particular, future experiments with ultra-relativistic electron-positron plasmas in ultra-strong laser fields might open the possibility to test these predictions, e.g. the existence of a new fermionic plasma wave (plasmino). In the strongly coupled regime electromagnetic plasmas such as complex plasmas can be used as models or at least analogies for the quark-gluon plasma possibly produced in relativistic heavy-ion experiments. For example, pair correlation functions can be used to investigate the equation of state and cross section enhancement for parton scattering can be explained.

hep-ph↗
arXiv2008

Ratio of viscosity to entropy density in a strongly coupled one-component plasma

String theoretical arguments led to the hypothesis that the ratio of viscosity to entropy of any physical system has a lower bound. Strongly coupled systems usually have a small viscosity compared to weakly coupled plasmas in which the viscosity is proportional to the mean free path. In the case of a one-component plasma the viscosity as a function of the coupling strength shows a minimum. Here we show that the ratio of viscosity to entropy of a strongly coupled one-component plasma is always above the lower bound predicted by string theory.

physics.plasm-ph↗
arXiv2007

Reply to Comment on the paper ``Energy Loss of Charm Quarks in the Quark-Gluon Plasma: Collisional vs Radiative'' by Mishra et al

The comments raised in Ref. [1] by Mishra et al aim at two papers contained in Ref. [2]. We show that those comments on Ref. [2] pointed out by Mishra et al in Ref.[1] are not relevant and the concept used in Ref.[2] is consistent and in compliance with the classical approximation of the transport coefficients [3]. We would also like to note that most of the comments in Ref. [1] were meant for light quarks, but are not even appropriate for heavy quarks.

hep-ph↗
arXiv2007

Hard Thermal Photon Production in Relativistic Heavy Ion Collisions

The recent status of hard thermal photon production in relativistic heavy ion collisions is reviewed and the current rates are presented with emphasis on corrected bremsstrahlung processes in the quark-gluon plasma (QGP) and quark-hadron duality. Employing Bjorken hydrodynamics with an EOS supporting the phase transition from QGP to hot hadron gas (HHG), thermal photon spectra are computed. For SPS 158 GeV Pb+Pb collisions, comparison with other theoretical results and the WA98 direct photon data indicates significant contributions due to prompt photons. Extrapolating the presented approach to RHIC and LHC experiments, predictions of the thermal photon spectrum show a QGP outshining the HHG in the high-pT-region.

hep-ph↗
arXiv2007

Wakes in a Collisional Quark-Gluon Plasma

Wakes created by a parton moving through a static and infinitely extended quark-gluon plasma are considered. In contrast to former investigations collisions within the quark-gluon plasma are taken into account using a transport theoretical approach (Boltzmann equation) with a Bhatnagar-Gross-Krook collision term. Within this model it is shown that the wake structure changes significantly compared to the collisionless case.

hep-ph↗
arXiv2007

Energy gain of heavy quarks by fluctuations in the QGP

The collisional energy gain of a heavy quark due to chromo-electromagnetic field fluctuations in a quark-gluon plasma is investigated. The field fluctuations lead to an energy gain of the quark for all temperatures and velocities. The net effect is a reduction of the collisional energy loss by 15-40% for parameters relevant at RHIC energies.

hep-ph↗
arXiv2006

What Do Electromagnetic Plasmas Tell Us about Quark-Gluon Plasma?

Since the quark-gluon plasma (QGP) reveals some obvious similarities to the well-known electromagnetic plasma (EMP), an accumulated knowledge on EMP can be used in the QGP studies. After discussing similarities and differences of the two systems, we present theoretical tools which are used to describe the plasmas. The tools include: kinetic theory, hydrodynamic approach and diagrammatic perturbative methods. We consider collective phenomena in the plasma with a particular emphasis on instabilities which crucially influence temporal evolution of the system. Finally, properties of strongly coupled plasma are discussed.

nucl-th↗
arXiv2006

Wakes in the quark-gluon plasma

Using the high temperature approximation we study, within the linear response theory, the wake in the quark-gluon plasma by a fast parton owing to dynamical screening in the space like region. When the parton moves with a speed less than the average speed of the plasmon, we find that the wake structure corresponds to a screening charge cloud traveling with the parton with one sign flip in the induced charge density resulting in a Lennard-Jones type potential in the outward flow with a short range repulsive and a long range attractive part. On the other hand if the parton moves with a speed higher than that of plasmon, the wake structure in the induced charge density is found to have alternate sign flips and the wake potential in the outward flow oscillates analogous to Cerenkov like wave generation with a Mach cone structure trailing the moving parton. The potential normal to the motion of the parton indicates a transverse flow in the system. We also calculate the potential due to a color dipole and discuss consequences of possible new bound states and $J/ψ$ suppression in the quark-gluon plasma.

hep-ph↗
arXiv2006

A model of the effect of collisions on QCD plasma instabilities

We study the effect of including a BGK collisional kernel on the collective modes of a QCD plasma which has a hard-particle distribution function which is anisotropic in momentum space. We calculate dispersion relations for both the stable and unstable modes and show that the addition of hard particle collisions slows the rate of growth of QCD plasma unstable modes. We also show that for any anisotropy there is an upper limit on the collisional frequency beyond which no instabilities exist. Estimating a realistic value for the collisional frequency for alpha_s ~ 0.2 - 0.4 we find that for the large-anisotropy case which is relevant for the initial state of matter generated by free streaming in heavy-ion collisions that the collisional frequency is below this critical value.

hep-ph↗
arXiv2005

Structure Functions and Pair Correlations of the Quark-Gluon Plasma

Recent experiments at RHIC and theoretical considerations indicate that the quark-gluon plasma, present in the fireball of relativistic heavy-ion collisions, might be in a liquid phase. The liquid state can be identified by characteristic correlation and structure functions. Here definitions of the structure functions and pair correlations of the quark-gluon plasma are presented as well as perturbative results. These definitions might be useful for verifying the quark-gluon-plasma liquid in QCD lattice calculations.

hep-ph↗
arXiv2005

Complex Plasmas as a Model for the Quark-Gluon-Plasma Liquid

The quark-gluon plasma, possibly created in ultrarelativistic heavy-ion collisions, is a strongly interacting many-body parton system. By comparison with strongly coupled electromagnetic plasmas (classical and non-relativistic) it is concluded that the quark-gluon plasma could be in the liquid phase. As an example for a strongly coupled plasma, complex plasmas, which show liquid and even solid phases, are discussed briefly. Furthermore, methods based on correlation functions for confirming and investigating the quark-gluon-plasma liquid are presented. Finally, consequences of the strong coupling, in particular a cross section enhancement in accordance with experimental observations at RHIC, are discussed.

hep-ph↗
arXiv2004

The Quark-Gluon-Plasma Liquid

The quark-gluon plasma close to the critical temperature is a strongly interacting system. Using strongly coupled, classical, non-relativistic plasmas as an analogy, we argue that the quark-gluon plasma is in the liquid phase. This allows to understand experimental observations in ultrarelativistic heavy-ion collisions and to interpret lattice QCD results. It also supports the indications of the presence of a strongly coupled QGP in ultrarelativistic heavy-ion collisions.

hep-ph↗
arXiv2004

Screening of a Moving Parton in the Quark-Gluon Plasma

The screening potential of a parton moving through a quark-gluon plasma is calculated using the semi-classical transport theory. An anisotropic potential showing a minimum in the direction of the parton velocity is found. As consequences possible new bound states and J/psi dissociation are discussed.

hep-ph↗
arXiv2003

Strongly Coupled Plasmas in High-Energy Physics

One of the main activities in high-energy and nuclear physics is the search for the so-called quark-gluon plasma, a new state of matter which should have existed a few microseconds after the Big Bang. A quark-gluon plasma consists of free color charges, i.e. quarks and gluons, interacting by the strong (instead of electromagnetic) force. Theoretical considerations predict that the critical temperature for the phase transition from nuclear matter to a quark-gluon plasma is about 150 - 200 MeV. In the laboratory such a temperature can be reached in a so-called relativistic heavy-ion collision in accelerator experiments. Using the color charge instead of the electric charge, the Coulomb coupling parameter of such a system is of the order 10 - 30. Hence the quark-gluon plasma is a strongly coupled, relativistic plasma, in which also quantum effects are important. In the present work the experimental and theoretical status of the quark-gluon plasma physics will be reviewed, emphasizing the similarities and differences with usual plasma physics. Furthermore, the mixed phase consisting of free quarks and gluons together with hadrons (e.g. pions) will be discussed, which can be regarded as a complex plasma due to the finite extent of the hadrons.

hep-ph↗
arXiv2003

Strange Quark Matter in Neutron Stars? - New Results from Chandra and XMM

It has been predicted that quark and hybrid stars, containing strange quark matter in their core, have a significantly smaller radius than ordinary neutron stars. Preliminary X-ray observations of isolated neutron stars indicated a surprisingly small radius consistent with quark matter in the interior of the star. However, a new analysis of the data led to a radius corresponding to an ordinary neutron star. In the present talk we will discuss theoretical calculations of the mass-radius relation for quark and hybrid stars, taking into account medium effects in quark matter, and report on recent X-ray observations by Chandra and XMM and their latest interpretation.

astro-ph↗
arXiv2003

Quark Number Susceptibility, Thermodynamic Sum Rule, and Hard Thermal Loop Approximation

The quark number susceptibility, associated with the conserved quark number density, is closely related to the baryon and charge fluctuations in the quark-gluon plasma, which might serve as signature for the quark-gluon plasma formation in ultrarelativistic heavy-ion collisions. Beside QCD lattice simulations, the quark number susceptibility has been calculated recently using a resummed perturbation theory (Hard Thermal Loop resummation). In the present work we show, based on general arguments, that the computation of this quantity neglecting Hard Thermal Loop vertices contradicts the Ward identity and violates the thermodynamic sum rule following from the quark number conservation. We further show that the Hard Thermal Loop perturbation theory is consistent with the thermodynamic sum rule.

hep-ph↗