arXiv subjects

M. Beyer

Publications and source records attributed to M. Beyer.

At least 55 records · Page 3Linked to original sources

arXiv2003

Light front field theory of quark matter at finite temperature

A light front field theory for finite temperature and density is currently being developed. It will be used here to describe the transition region from quark matter to nuclear matter relevant in heavy ion collisions and in the early universe. The energy regime addressed is extremely challenging, both theoretically and experimentally. This is because of the confinement of quarks, the appearance of bound states and correlations, special relativity, and nonlinear phenomena that lead to a change of the vacuum structure of quantum chromodynamics. In the region of the phase transition it eventually leads to a change of the relevant degrees of freedom. We aim at describing this transition from quarks to hadronic degrees of freedom in a unified microscopic approach.

hep-ph↗

arXiv2003

Light clusters in nuclear matter of finite temperature

We investigate properties and the distribution of light nuclei (A<4) in symmetric nuclear matter of finite temperature within a microscopic framework. For this purpose we have solved few-body Alt-Grassberger-Sandhas type equations for quasi-nucleons that include self-energy corrections and Pauli blocking in a systematic way. In a statistical model we find a significant influence in the composition of nuclear matter if medium effects are included in the microscopic calculation of nuclei. If multiplicities are frozen out at a certain time (or volume), we expect significant consequences for the formation of light fragments in a heavy ion collision. As a consequence of the systematic inclusion of medium effects the ordering of multiplicities becomes opposite to the law of mass action of ideal components. This is necessary to explain the large abundance of $α$-particles in a heavy ion collision that are otherwise largely suppressed in an ideal equilibrium scenario.

nucl-th↗

arXiv2003

Light front thermal field theory at finite temperature and density

We investigate quark matter at finite temperature and finite chemical potential as an example for a relativistic many-particle quantum system. Special relativity is realized through the front form that allows for a Hamiltonian formulation of a statistical operator. Utilizing our previous results we generalize the present formulation of a relativistic thermal field theory to include a finite chemical potential. As an application we use the Nambu-Jona-Lasinio model to investigate the gap equation and chiral restoration.

hep-ph↗

arXiv2003

Neutron Charge Radius: Relativistic Effects and the Foldy Term

The neutron charge radius is studied within a light-front model with different spin coupling schemes and wave functions. The cancellation of the contributions from the Foldy term and Dirac form factor to the neutron charge form factor is verified for large nucleon sizes and it is independent of the detailed form of quark spin coupling and wave function. For the physical nucleon our results for the contribution of the Dirac form factor to the neutron radius are insensitive to the form of the wave function while they strongly depend on the quark spin coupling scheme.

hep-ph↗

arXiv2003

On the stability of three-body bound states on the light front

We investigate the stability of the relativistic three-boson system with a zero range force in the light front form. In particular we study the dependence of the system on an invariant cut-off. We discuss the conditions for the relativistic Thomas collapse. Finally, we fix the parameters of the model introducing a scale.

nucl-th↗

arXiv2002

Few-quark correlation in hot matters

We address the issue of three-quark clusters in a medium of finite temperatures and densities. We use the light front approach and utilize a Green function formalism. We consider first the isolated case using a zero-range interaction and introducing an invariant cut-off and then medium effects. The Mott lines are shown for different values of the cut-off.

hep-ph↗

arXiv2002

Three Quark Clusters in Hot and Dense Nuclear Matter

We present a relativistic in-medium three-body equation to study correlations in hot and dense quark matter. The equation is solved for a zero-range force for parameters close to the phase transition of QCD.

nucl-th↗

arXiv2001

Correlations in hot and dense quark matter

We present a relativistic three-body equation to investigate three-quark clusters in hot and dense quark matter. To derive such an equation we use the Dyson equation approach. The equation systematically includes the Pauli blocking factors as well as the self energy corrections of quarks. Special relativity is realized through the light front form. Presently we use a zero-range force and investigate the Mott transition.

hep-ph↗

arXiv2001

Three- and Four-body correlations in nuclear matter

Few-nucleon correlations in nuclear matter at finite densities and temperatures are explored. Using the Dyson equation approach leads to effective few-body equations that include self energy corrections and Pauli blocking factors in a systematic way. Examples given are the nucleon deuteron in-medium reaction rates, few-body bound states including the $\ga$-particle, and $\ga$-particle condensation.

nucl-th↗

arXiv2001

In-medium nucleon-nucleon potentials in configuration space

Based on the thermodynamic Green function approach two-nucleon correlations in nuclear matter at finite temperatures are revisited. To this end, we derive phase equivalent effective $r$-space potentials that include the effect of the Pauli blocking at a given temperature and density. These potentials enter into a Schrödinger equation that is the $r$-space representation of the Galitskii-Feynman equation for two nucleons. We explore the analytical structure of the equation in the complex $k$-plane by means of Jost functions. We find that despite the Mott effect the correlation with deuteron quantum numbers are manifested as antibound states, i.e., as zeros of the Jost function on the negative imaginary axis of the complex momentum space. The analysis presented here is also suited for Coulombic systems.

nucl-th↗

arXiv2001

Three-quark clusters at finite temperatures and densities

We present a relativistic three-body equation to study correlations in a medium of finite temperatures and densities. This equation is derived within a systematic Dyson equation approach and includes the dominant medium effects due to Pauli blocking and self energy corrections. Relativity is implemented utilizing the light front form. The equation is solved for a zero-range force for parameters close to the confinement-deconfinement transition of QCD. We present correlations between two- and three-particle binding energies and calculate the three-body Mott transition.

hep-th↗

arXiv2001

Weak annihilation in the rare radiative $B\to ργ$ decay

The ampitude of the $B\toργ$ decay induced by the flavour-changing neutral currents contains the penguin contribution and the weak-annihilation contribution generated by the 4-quark operators in the effective Hamiltonian. The penguin contribution is known quite well. We analyse the weak-annihilation which is suppressed by the heavy-quark mass compared to the penguin contribution. In the factorization approximation, the weak annihilation amplitude is represented in terms of the leptonic decay constants and the meson-photon matrix elements of the weak currents. The latter contain the $Bγ$, $ργ$ transition form factors and contact terms determined by the equations of motion. We calculate the $Bγ$ and $ργ$ form factors within the relativistic dispersion approach and obtain numerical estimates for the weak annihilation amplitude.

hep-ph↗

arXiv2001

Relativistic Quark Spin Coupling Effects in the Correlations Between Nucleon Electroweak Properties

We investigate the effect of different relativistic spin couplings of constituent quarks on nucleon electroweak properties. Within each quark spin coupling scheme the correlations between static electroweak observables are found to be independent of the particular shape of the momentum part of the nucleon light-front wave function. The neutron charge form factor is very sensitive to different choices of spin coupling schemes once the magnetic moment is fitted to the experimental value. However, it is found rather insensitive to the details of the momentum part of the three-quark wave function model.

nucl-th↗

arXiv2000

Few-Body Correlations in Fermi Systems

I present recent results on few-body correlations in nuclear matter at finite temperatures and densities. In particular I focus on the four-body correlations. The alpha-particle is the strongest bound state. The investigation is done based on effective in-medium few-body equations that consistently include self energy corrections and Pauli blocking effects. I address the question of four-body condensates as well.

nucl-th↗

arXiv2000

Violations of Lorentz Covariance in Light Front Quark Models

Electromagnetic form factors of the nucleon from relativistic quark models are analyzed: results from null-plane projection of the Feynman triangle diagram are compared with a Bakamjian-Thomas model. The magnetic form factors of the models differ by about 15% at spacelike momentum transfer 0.5 GeV^2, while the charge form factors are much closer. Spurious contributions to electromagnetic form factors due to violations of rotational symmetry are eliminated from both models. One method changes magnetic form factors by about 10%, whereas the charge form factors stay nearly the same. Another one changes the charge form factor of the Bakamjian-Thomas model by more than 50%.

hep-ph↗

arXiv2000

Medium corrections in the formation of light charged particles in heavy ion reactions

Within a microscopic statistical description of heavy ion collisions, we investigate the effect of the medium on the formation of light clusters. The dominant medium effects are self-energy corrections and Pauli blocking that produce the Mott effect for composite particles and enhanced reaction rates in the collision integrals. Microscopic description of composites in the medium follows the Dyson equation approach combined with the cluster mean-field expansion. The resulting effective few-body problem is solved within a properly modified Alt-Grassberger-Sandhas formalism. The results are incorporated in a Boltzmann-Uehling-Uhlenbeck simulation for heavy ion collisions. The number and spectra of light charged particles emerging from a heavy ion collision changes in a significant manner in effect of the medium modification of production and absorption processes.

nucl-th↗

arXiv2000

Formation of Few-Body Clusters in Nuclear Matter

Starting from a suitably modified three-body equation to include dominant medium effects such as self energy corrections and Pauli blocking I present results on several observables relevant for the formation of light clusters in a heavy ion collision at moderate energies. Properly taking into account the medium leads to the Mott effect, larger reaction rates, in turn, faster time scales for the deuteron life time and chemical relaxation time. Within a Boltzmann-Uehling-Uhlenbeck approach to simulate the heavy ion reactions, also the total number of deuterons and the energy spectrum are significantly changed due to medium effects in the elementary cross sections entering into the collision integrals.

nucl-th↗

arXiv2000

The alpha-particle in nuclear matter

Among the light nuclear clusters the alpha-particle is by far the strongest bound system and therefore expected to play a significant role in the dynamics of nuclei and the phases of nuclear matter. To systematically study the properties of the alpha-particle we have derived an effective four-body equation of the Alt-Grassberger-Sandhas (AGS) type that includes the dominant medium effects, i.e. self energy corrections and Pauli-blocking in a consistent way. The equation is solved utilizing the energy dependent pole expansion for the sub system amplitudes. We find that the Mott transition of an alpha-particle at rest differs from that expected from perturbation theory and occurs at approximately 1/10 of nuclear matter densities.

nucl-th↗