arXiv subjects

Alfred Stadler

Publications and source records attributed to Alfred Stadler.

At least 19 recordsLinked to original sources

arXiv2026

Quark mass functions in Minkowski space

Using the Covariant Spectator Theory (CST), we calculate the dressed quark mass function and wave-function renormalization for the five quark flavors from up/down to bottom in both the spacelike and timelike regions of Minkowski space. The calculation employs a model dressed-gluon propagator fitted to lattice data in the spacelike region. The remaining free parameters of the quark self-energy are determined by fits to lattice results for the quark mass function including a constraint built into the wave-function renormalization. We perform these fits using lattice data from two different groups and find that the resulting mass predictions are within the range of the typical constituent masses used in quark models.

hep-ph↗

arXiv2026

Heavy and heavy-light mesons with arbitrary spin and parity in the Covariant Spectator Theory

This work generalizes the one-channel Covariant Spectator Theory (CST) formalism to describe quark-antiquark mesons of arbitrary spin-parity $J^P$. We also improve the quark-antiquark interaction kernel by incorporating the momentum dependence of the strong coupling. Within this framework, we perform global fits to the masses of experimentally established heavy and heavy-light mesons with $J^P=0^\pm, 1^\pm, 2^\pm,$ and $3^\pm$. With only eight adjustable parameters, the model yields an excellent global description of the observed quark-antiquark spectrum and predicts both unmeasured states and likely $J^P$ assignments for states with unconfirmed quantum numbers. In particular, our results support the identifications of the recently observed $B_c(1P)^+$ candidates as $0^+$, $1^+$, and $2^+$ states, and the $B_c^{*+}$ as the lowest $1^-$ state in the bottom-charm sector, as well as the $D_{s1}(2933)^+$ as axial-vector state in the charm-strange sector.

hep-ph↗

arXiv2026

Heavy and heavy-light tensor and axial-tensor mesons in the Covariant Spectator Theory

We present the first calculation of tensor and axial-tensor mesons with total spin $J\geq2$ within the Covariant Spectator Theory. We employ a refined quark-antiquark interaction kernel that incorporates the momentum dependence of the strong coupling, replacing the previously used constant term of the kernel. Global least-squares fits to the masses of experimentally established heavy and heavy-light meson states yield an excellent description of the mass spectrum for $J^P=0^\pm, 1^\pm, 2^\pm$, and $3^\pm$ using only eight adjustable parameters.

nucl-th↗

arXiv2024

Simple high-accuracy method for solving bound-state equations with the Cornell potential in momentum space

The well-known Cornell quark-antiquark potential in momentum space contains singularities both in its one-gluon-exchange (OGE) and linear confining parts, which prevents a direct use of the convenient Nystr\"om method to solve the corresponding bound-state integral equation for the meson masses. While it has been known for a long time how the Coulomb-type singularity in the OGE potential can be treated with a subtraction technique, only very complicated methods have been developed to deal with the stronger singularity in the linear potential. In this work, we present a simple subtraction method to remove this singularity from the kernel, such that the Nystr\"om method becomes applicable. Derivatives of the wave function, that appear as a result of the subtraction, are represented by means of interpolating functions, for which we found Lagrange polynomials to be very efficient. Test calculations show excellent agreement with exactly known energy eigenvalues. By increasing the number of integration points and the order of the Lagrange interpolation polynomials, extremely high accuracy can be achieved. This method can also be extended to relativistic Bethe-Salpeter type equations with singular kernels.

hep-ph↗

arXiv2022

Going to the light front with contour deformations

We explore a new method to calculate the valence light-front wave function of a system of two interacting particles, which is based on contour deformations combined with analytic continuation methods to project the Bethe-Salpeter wave function onto the light front. In this proof-of-concept study, we solve the Bethe-Salpeter equation for a scalar model and find excellent agreement between the light-front wave functions obtained with contour deformations and those obtained with the Nakanishi method frequently employed in the literature. The contour-deformation method is also able to handle extensions of the scalar model that mimic certain features of QCD such as unequal masses and complex singularities. In principle the method is suitable for computing parton distributions on the light front such as PDFs, TMDs and GPDs in the future.

hep-ph↗

arXiv2019

Scattering amplitudes and contour deformations

We employ a scalar model to exemplify the use of contour deformations when solving Lorentz-invariant integral equations for scattering amplitudes. In particular, we calculate the onshell 2 -> 2 scattering amplitude for the scalar system. The integrals produce branch cuts in the complex plane of the integrand which prohibit a naive Euclidean integration path. By employing contour deformations, we can also access the kinematical regions associated with the scattering amplitude in Minkowski space. We show that in principle a homogeneous Bethe-Salpeter equation, together with analytic continuation methods such as the Resonances-via-Padé method, is sufficient to determine the resonance pole locations on the second Riemann sheet. However, the scalar model investigated here does not produce resonance poles above threshold but instead virtual states on the real axis of the second sheet, which pose difficulties for analytic continuation methods. To address this, we calculate the scattering amplitude on the second sheet directly using the two-body unitarity relation which follows from the scattering equation.

hep-ph↗

arXiv2019

Quark mass function from a OGE-type interaction in Minkowski space

We present results for the quark mass function in Minkowski space calculated from an interaction kernel that consists of an effective one-gluon-exchange and a constant interaction. We analyze the gauge dependence of our results and compare them in the spacelike region to the available lattice QCD data.

hep-ph↗

arXiv2018

Quark mass function from a one-gluon-exchange-type interaction in Minkowski space

We present first results for the quark mass function in Minkowski space in both the spacelike and timelike regions calculated from the same quark-antiquark interaction kernel used in the latest meson calculations using the Gross equation. This kernel consists of a Lorentz vector effective one-gluon-exchange-type interaction, a vector constant, and a mixed scalar-pseudoscalar covariant linear confining interaction that does not contribute to the mass function. We analyze the gauge dependence of our results, prove the gauge independence of the constituent quark mass and mass gap equation, and identify the Yennie gauge as the appropriate gauge to be used in CST calculations. We compare our results in the spacelike region to lattice QCD data and find good agreement.

hep-ph↗

arXiv2018

Heavy and heavy-light mesons in the Covariant Spectator Theory

The masses and vertex functions of heavy and heavy-light mesons, described as quark-antiquark bound states, are calculated with the Covariant Spectator Theory (CST). We use a kernel with an adjustable mixture of Lorentz scalar, pseudoscalar, and vector linear confining interaction, together with a one-gluon-exchange kernel. A series of fits to the heavy and heavy-light meson spectrum were calculated, and we discuss what conclusions can be drawn from it, especially about the Lorentz structure of the kernel. We also apply the Brodsky-Huang-Lepage prescription to express the CST wave functions for heavy quarkonia in terms of light-front variables. They agree remarkably well with light-front wave functions obtained in the Hamiltonian basis light-front quantization (BLFQ) approach, even in excited states.

hep-ph↗

arXiv2017

Covariant spectator theory of quark-antiquark bound states: Mass spectra and vertex functions of heavy and heavy-light mesons

We use the covariant spectator theory with an effective quark-antiquark interaction, containing Lorentz scalar, pseudoscalar, and vector contributions, to calculate the masses and vertex functions of, simultaneously, heavy and heavy-light mesons. We perform least-square fits of the model parameters, including the quark masses, to the meson spectrum and systematically study the sensitivity of the parameters with respect to different sets of fitted data. We investigate the influence of the vector confining interaction by using a continuous parameter controlling its weight. We find that vector contributions to the confining interaction between 0% and about 30% lead to essentially the same agreement with the data. Similarly, the light quark masses are not very tightly constrained. In all cases, the meson mass spectra calculated with our fitted models agree very well with the experimental data. We also calculate the mesons wave functions in a partial wave representation and show how they are related to the meson vertex functions in covariant form.

hep-ph↗

arXiv2017

Heavy and heavy-light mesons and the Lorentz structure of the quark-antiquark interaction

We solve a Minkowski-space integral equation, derived in the Covariant Spectator Theory, for quark-antiquark bound states describing heavy and heavy-light mesons. The equation's kernel contains a one-gluon exchange interaction and a covariant generalization of a linear confining potential with a mixed scalar, pseudoscalar, and vector Lorentz structure, characterized by a continuous mixing parameter. We investigate to what extent the Lorentz structure of the confining kernel can be determined by fitting the mixing parameter to the meson spectrum.

hep-ph↗

arXiv2017

Comparison of two Minkowski-space approaches to heavy quarkonia

In this work we compare mass spectra and decay constants obtained from two recent, independent, and fully relativistic approaches to the quarkonium bound-state problem: the Hamiltonian basis light-front quantization (BLFQ) approach, where light-front wave functions are naturally formulated; and, the covariant spectator theory (CST), based on a reorganization of the Bethe-Salpeter equation. Even though conceptually different, both solutions are obtained in Minkowski space. Comparisons of decay constants for more than ten states of charmonium and bottomonium show favorable agreement between the two approaches as well as with experiment where available. We also apply the Brodsky-Huang-Lepage prescription to convert the CST amplitudes into functions of light-front variables. This provides an ideal opportunity to investigate the similarities and differences at the level of the wave functions. Several qualitative features are observed in remarkable agreement between the two approaches even for the rarely addressed excited states. Leading twist distribution amplitudes as well as parton distribution functions of heavy quarkonia are also analyzed.

hep-ph↗

arXiv2017

Application of the Covariant Spectator Theory to the study of heavy and heavy-light mesons

As an application of the Covariant Spectator Theory (CST) we calculate the spectrum of heavy-light and heavy-heavy mesons using covariant versions of a linear confining potential, a one- gluon exchange, and a constant interaction. The CST equations possess the correct one-body limit and are therefore well-suited to describe mesons in which one quark is much heavier than the other. We find a good fit to the mass spectrum of heavy-light and heavy-heavy mesons with just three parameters (apart from the quark masses). Remarkably, the fit parameters are nearly unchanged when we fit to experimental pseudoscalar states only or to the whole spectrum. Because pseudoscalar states are insensitive to spin-orbit interactions and do not determine spin-spin interactions separately from central interactions, this result suggests that it is the covariance of the kernel that correctly predicts the spin-dependent quark-antiquark interactions

hep-ph↗

arXiv2017

Relativistic phenomenology of meson spectra with a covariant quark model in Minkowski space

In this work, we perform a covariant treatment of quark-antiquark systems. We calculate the spectra and wave functions using a formalism based on the Covariant Spectator Theory (CST). Our results not only reproduce very well the experimental data with a very small set of global parameters, but they also allow a direct test of the predictive power of covariant kernels.

hep-ph↗

arXiv2016

Covariant Spectator Theory of heavy-light and heavy mesons and the predictive power of covariant interaction kernels

The Covariant Spectator Theory (CST) is used to calculate the mass spectrum and vertex functions of heavy-light and heavy mesons in Minkowski space. The covariant kernel contains Lorentz scalar, pseudoscalar, and vector contributions. The numerical calculations are performed in momentum space, where special care is taken to treat the strong singularities present in the confining kernel. The observed meson spectrum is very well reproduced after fitting a small number of model parameters. Remarkably, a fit to a few pseudoscalar meson states only, which are insensitive to spin-orbit and tensor forces and do not allow to separate the spin-spin from the central interaction, leads to essentially the same model parameters as a more general fit. This demonstrates that the covariance of the chosen interaction kernel is responsible for the very accurate prediction of the spin-dependent quark-antiquark interactions.

hep-ph↗

arXiv2016

Chiral-symmetry breaking and pion structure in the Covariant Spectator Theory

We introduce a covariant approach in Minkowski space for the description of quarks and mesons that exhibits both chiral-symmetry breaking and confinement. In a simple model for the interquark interaction the quark mass function is obtained and used in the calculation of the pion form factor. We study the effects of the mass function and of the different quark pole contributions on the pion form factor.

hep-ph↗

arXiv2015

Charge-conjugation symmetric complete impulse approximation for the pion electromagnetic form factor in the Covariant Spectator Theory

The pion form factor is calculated in the framework of the charge-conjugation invariant Covariant Spectator Theory. This formalism is established in Minkowski space and the calculation is set up in momentum space. In a previous calculation we included only the leading pole coming from the spectator quark (referred to as the relativistic impulse approximation). In this paper we also include the contributions from the poles of the quark which interacts with the photon and average over all poles in both the upper and lower half planes in order to preserve charge conjugation invariance (referred to as the $C$-symmetric complete impulse approximation). We find that for small pion mass these contributions are significant at all values of the four-momentum transfer $Q^2$ but, surprisingly, do not alter the shape obtained from the spectator poles alone.

hep-ph↗

arXiv2015

Quarkonia and heavy-light mesons in a covariant quark model

Preliminary calculations using the Covariant Spectator Theory (CST) employed a scalar linear confining interaction and an additional constant vector potential to compute the mesonic mass spectra. In this work we generalize the confining interaction to include more general structures, in particular a vector and also a pseudoscalar part, as suggested by a recent study. A one-gluon-exchange kernel is also implemented to describe the short-range part of the interaction. We solve the simplest CST approximation to the complete Bethe-Salpeter equation, the one-channel spectator equation, using a numerical technique that eliminates all singularities from the kernel. The parameters of the model are determined through a fit to the experimental pseudoscalar meson spectra, with a good agreement for both quarkonia and heavy-light states.

hep-ph↗