arXiv subjects

Bernard Metsch

Publications and source records attributed to Bernard Metsch.

15 recordsLinked to original sources

arXiv2025

Bounds on variations of the strange quark mass from Big Bang nucleosynthesis

We analyze the effect of a variation of the strange nucleon matrix element $\langle N| m_s \bar{s}s|N\rangle$ on the abundances of the light elements produced in the Big Bang. For that, we vary the nucleon mass in the leading eight reactions that involve neutrons, protons and the four lightest nuclei. We use various available Big Bang nucleosynthesis codes and find that the measured deuterium and $^4$He abundances set strict limits on the nucleon mass variations. This translates into an upper bound of possible variations of the strange quark mass, $\left| \Delta m_s/m_s \right| \leq 5.1\%$.

hep-ph↗

arXiv2020

Hadron-Hadron Interactions from $N_f=2+1+1$ Lattice QCD: The $ρ$-resonance

We present an investigation of the Rho-meson from Nf=2+1+1 flavour lattice QCD. The calculation is performed based on gauge configuration ensembles produced by the ETM collaboration with three lattice spacing values and pion masses ranging from 230 MeV to 500 MeV. Applying the Lüscher method phase shift curves are determined for all ensembles separately. Assuming a Breit-Wigner form, the Rho-meson mass and width are determined by a fit to these phase shift curves. Mass and width combined are then extrapolated to the chiral limit, while lattice artefacts are not detectable within our statistical uncertainties. For the Rho-meson mass extrapolated to the physical point we find good agreement with experiment. The corresponding decay width differs by about two standard deviations from the experimental value.

hep-lat↗

arXiv2006

Charmed baryons in a relativistic quark model

We calculate mass spectra of charmed baryons within a relativistically covariant quark model based on the Bethe-Salpeter-equation in instantaneous approximation. Interactions are given by a linearly rising three-body confinement potential and a flavor dependent two-body force derived from QCD instanton effects. This model has already been successfully applied to the calculation of light flavor baryon spectra and is now extended to heavy baryons. Within the same framework we compare the results to those obtained with the more conventional one-gluon-exchange potential.

hep-ph↗

arXiv2006

Semileptonic decays of baryons in a relativistic quark model

We calculate semileptonic decays of light and heavy baryons in a relativistically covariant constituent quark model. The model is based on the Bethe-Salpeter-equation in instantaneous approximation. It generates satisfactory mass spectra for mesons and baryons up to the highest observable energies. Without introducing additional free parameters we compute on this basis helicity amplitudes of electronic and muonic semileptonic decays of baryons. We thus obtain form factor ratios and decay rates in good agreement with experiment.

hep-ph↗

arXiv2006

Static observables of relativistic three-fermion systems with instantaneous interactions

We show that static properties like the charge radius and the magnetic moment of relativistic three-fermion bound states with instantaneous interactions can be formulated as expectation values with respect to intrinsically defined wavefunctions. The resulting operators can be given a natural physical interpretation in accordance with relativistic covariance. We also indicate how the formalism may be generalized to arbitrary moments. The method is applied to the computation of static baryon properties with numerical results for the nucleon charge radii and the baryon octet magnetic moments. In addition we make predictions for the magnetic moments of some selected nucleon resonances and discuss the decomposition of the nucleon magnetic moments in contributions of spin and angular momentum, as well as the evolution of these contributions with decreasing quark mass.

hep-ph↗

arXiv2005

Helicity amplitudes and electromagnetic decays of hyperon resonances

We present results for the helicity amplitudes of the lowest-lying hyperon resonances Y*, computed within the framework of the Bonn constituent-quark model, which is based on the Bethe-Salpeter approach. The seven parameters entering the model were fitted to the best known baryon masses. Accordingly, the results for the helicity amplitudes are genuine predictions. Some hyperon resonances are seen to couple more strongly to a virtual photon with finite Q^2 than to a real photon. Other Y*'s, such as the S_{01}(1670) Lambda resonance or the S_{11}(1620) Sigma resonance, couple very strongly to real photons. We present a qualitative argument for predicting the behaviour of the helicity asymmetries of baryon resonances at high Q^2.

nucl-th↗

arXiv2004

Quark model description of hadrons

In this contribution I will try to give an overview of what has been achieved in constituent quark models of mesons and baryons by a comparison of some selected results from various ansaetze with experimental data. In particular I will address the role of relativistic covariance, the nature of the effective quark forces, the status of results on electromagnetic and strong-decay observables beyond the mere mass spectra, as well as some unresolved issues in hadron spectroscopy.

hep-ph↗

arXiv2003

Structure of baryons in a relativistic quark model

Baryonic excitation spectra, electroweak and strong decay properties are discussed within a relativistically covariant constituent quark model based on the instantaneous approximation to the three-body Bethe-Salpeter equation.

hep-ph↗

arXiv2002

Electroweak form factors of non-strange baryons

We compute electroweak form factors of the nucleon and photon transition form factors of non-strange baryon resonances up to the third resonance region in a model with instanton-induced interaction. The calculation is based on the Bethe-Salpeter equation for three light constituent quarks and is fully relativistic (U. Löring {\it et al.}, Eur. Phys. J. {\bf A10}, 309 (2001)). Static nucleon properties and photon resonance couplings are in good agreement with experiment and the $Q^2$-behaviour of the experimentally known form factors up to large momentum transfer is accounted for.

hep-ph↗

arXiv2001

A relativistic quark model of baryons

On the basis of the three-particle Bethe-Salpeter equation we formulated a relativistic quark model for baryons. Assuming the propagators to be given by their free form with constituent quark masses and the interaction kernel by an instantaneous potential, which contains a string-like parameterization of confinement and a flavor dependent interaction motivated by instanton effects we can account for the major features in the baryon spectrum, such as the low position of the Roper resonance and the occurrence of approximate parity doublets apparent in the Nucleon- and Lambda-spectra.

hep-ph↗

arXiv2001

Parity Doublets from a Relativistic Quark Model

The Nucleon- and the Lambda-excitation spectrum exhibit parity doublets, i.e. states of the same spin but with opposite parity being almost degenerate in mass. It is shown that in a relativistic quark model with instantaneous interaction kernels, where confinement is implemented by a linearly rising potential and the major mass splittings are generated from an interaction based on instanton effects, this degeneracy occurs quite naturally: Once the parameters of the instanton induced interaction are fixed to reproduce the ground state octet-decuplet splittings, some states are selectively lowered to a position which is degenerate with states of opposite parity. Some observable consequences are briefly discussed.

hep-ph↗

arXiv2001

Weak Decays Of Heavy Mesons In A Covariant Quark Model

In this paper we investigate weak decays of heavy mesons in the framework of a covariant quark model, which is based on the Bethe-Salpeter equation in instantaneous approximation. Apart from a phenomenological confinement potential, a residual interaction induced by instantons is adopted. Masses and many decay observables of light mesons have already been described successfully in this model. An appropiate extension allows a unified description of light and heavy systems. Using a set of parameters which are fixed by the mass spectra, we evaluate the form factors of semileptonic decays of charmed and bottom mesons. In the heavy quark limit these can be reduced to the Isgur-Wise function, which is calculated. Finally the form factors are used to determine the non-leptonic decay rates of B mesons in the factorization approximation.

hep-ph↗

arXiv2000

A Relativistic Quark Model for Mesons with an Instanton-Induced Interaction

We present new results of a relativistic quark model based on the Bethe-Salpeter equation in its instantaneous approximation. Assuming a linearly rising confinement potential with an appropriate spinorial structure in Dirac space and adopting a residual interaction based on instanton effects, we can compute masses of the light mesons up to highest observed angular momenta with a natural solution of the $U_A(1)$ problem. The calculated ground states masses and the radial excitations describe the experimental results well. In this paper, we will also discuss our results concerning numerous meson decay properties. For processes like $π^+/K^+\to e^+ν_eγ$ and $0^-\toγγ$ at various photon virtualities, we find a good agreement with experimental data. We will also comment on the form factors of the $K_{\ell 3}$ decay and on the decay constants of the $π$, $K$ and $η$ mesons. For the sake of completeness, we will furthermore present the electromagnetic form factors of the charged $π$ and $K$ mesons as well as a comparison of the radiative meson decay widths with the most recent experimental data.

hep-ph↗

arXiv1997

A Relativistic Quark Model for Baryons

We present a relativistic quark model for baryons, based on the Bethe-Salpeter equation in instantaneous approximation. Confinement is implemented by an interaction kernel which essentially is a linearly rising potential with a spin-dependence chosen such as to minimize spin-orbit effects. The fine structure of the baryon spectrum follows from an effective quark-interaction based on instanton effects. Results for the spectra of all baryons build from u,d,s-quarks are presented. In particular it is found, that the present relativistic setup can account for the low position of Roper-like resonances in all sectors.

hep-ph↗

arXiv1997

Quark Structure of Light Mesons

On the basis of the Bethe-Salpeter Equation we developed a covariant constituent quark model, with confinement implemented by a linear potential and an instanton induced interaction explaining mass splittings and mixing of pseudoscalar mesons. In addition this interaction yields a scalar (essentially) flavour singlet state at approximately 1 GeV, considerably lower in mass than the corresponding octet states calculated around 1.4 GeV. The validity of the present approach was checked through various electroweak observables. The puzzling properties of scalar mesons is briefly discussed.

hep-ph↗