arXiv subjects

Christian S. Fischer

Publications and source records attributed to Christian S. Fischer.

At least 19 recordsLinked to original sources

arXiv2026

Spectra of light and heavy mesons with $J \le 5$ in a relativistic Bethe-Salpeter approach

We extend the range of application of the relativistic Dyson-Schwinger/Bethe-Salpeter approach from previously discussed mesons with total angular momentum $J \le 3$ to the ones with $J=4,5$. On a technical level, the new element is the general Dirac tensor representations for the latter which, to our knowledge, are presented here for the first time. As a first application, we provide an exploratory spectrum for these mesons in a rainbow-ladder truncation of Dyson-Schwinger and Bethe-Salpeter equations. We discuss the merits and limitations of this truncation and explore the shape of the heavy-quark potential corresponding to the underlying effective running coupling. With our predictions for the masses of ground state mesons with quantum numbers $J^{PC}=3^{--}, 4^{++}, 5^{--}$ we identify Regge trajectories in channels where the interaction model can be trusted on a semi-quantitative level. In other channels, discrepancies with experiments confirm the well-known need to go beyond rainbow-ladder in the Dyson-Schwinger/Bethe-Salpeter approach by using more sophisticated interactions.

hep-ph↗

arXiv2026

Hadronic vacuum polarization contribution to $a_μ$ from functional methods with strong and electromagnetic isospin breaking

We present a continuum-QCD determination of the leading-order hadronic vacuum-polarization contribution to the anomalous magnetic moment of the muon within the Dyson-Schwinger and Bethe-Salpeter equation framework. The calculation incorporates pion back-reaction, a fully dressed quark-photon vertex with a dynamically generated $ρ$-resonance structure in the timelike region, and both strong and electromagnetic isospin breaking treated self-consistently at the quark level. We obtain $a_μ^{\mathrm{HVP,LO}}(u+d+s+c)|_{\mathrm{ISB}} = 709.7 \times 10^{-10}$, in good agreement with recent lattice-QCD determinations, and find an isospin-breaking shift of $Δa_μ^{\mathrm{HVP,LO}} = 4.5 \times 10^{-10}$ ($0.6\%$), demonstrating that isospin-breaking effects, while quantitatively modest, are not negligible. Including the bottom-quark contribution and an indicative estimate of the systematic uncertainties, we obtain our final result, $a_μ^{\mathrm{HVP,LO}}(u+d+s+c+b)|_{\mathrm{ISB}} = (710.0 \pm 14.5) \times 10^{-10}$.

hep-ph↗

arXiv2026

Phase structure of strong interaction matter from Functional QCD

In this contribution to the Encyclopedia of Nuclear Physics, we aim to provide a pedagogical introduction to the functional approach to QCD at finite temperature and chemical potential. We briefly outline the general framework and address its complementarity to other first-principle approaches to non-perturbative QCD. We discuss selected results obtained with Dyson-Schwinger equations (DSE) and the functional renormalisation group (fRG) in the context of a general physics perspective on the QCD phase diagram. This article is specifically aimed at students and non-practitioners of functional methods alike and may serve as a short guide to further literature.

hep-ph↗

arXiv2026

Chiral symmetry restoration effects onto the meson spectrum from a Dyson-Schwinger and Bethe-Salpeter approach

Light meson spectra are studied in a Dyson-Schwinger/Bethe-Salpeter approach to QCD. By varying the interaction strength of three sets of models for the quark-antiquark interaction, the transition from the chiral symmetric to the chirally broken regime in the vacuum is studied. The simplest type of these models leads to degenerate meson spectra for a large domain of the strength parameter. The more sophisticated and thus more realistic models show significantly smaller parameter domains for which degenerate meson spectra are obtained. The underlying mechanism for obtaining and then lifting degeneracies is traced back to the location of the quark propagators' poles, in particular, whether they are beyond or within the domain of integration in the Bethe-Salpeter equation. In view of this mechanism the potential relation of the obtained degeneracies to the dynamical emergence of symmetries is discussed, adding thereby another point of view on the conjectured chiral spin symmetry of QCD in the temperature domain right above the crossover.

hep-ph↗

arXiv2026

Phase structure and observables at high densities from first principles QCD

We provide a short review of the progress made in the past decade with functional QCD in the description of the phase structure of QCD. We summarise the most important technical aspects of the framework, discuss strategies for truncations and address the problem of systematic error estimates. We detail efforts to gauge the approach systematically with lattice QCD at zero chemical potential, also including the physics of the Columbia plot at non-physical quark masses. Our main focus is, however, the high density regime of QCD. We address the predictive power of the functional approach for the appearance of new phases beyond the chiral crossover regime for chemical potentials $μ_B/T\geq 4.5$. The onset of this regime may be signalled by a critical end point of the crossover line but may also involve a moat regime or the emergence of an instability that indicates an inhomogeneous phase. Respective results include estimates for the location of the onset of new phases, and predictions for their experimental signatures.

hep-ph↗

arXiv2026

Extracting freeze-out conditions in beam energy scan via functional QCD

Fluctuations of conserved charges provide a link between high quality theoretical results and precision measurements of heavy ion collisions. We compare results for ratios of the lowest order baryon number susceptibilities from functional QCD approaches to proton number cumulants extracted from experiments. We find that they meet at a specific temperature and chemical potential for each collision energy. This is indicative of the respective freeze-out point. From this self-consistent determination of the freeze-out parameters we extract a prediction for the kurtosis on the freeze-out line. We find quantitative agreement with experimental data where available, despite comparing apples (baryons) with oranges (protons). At a collision energy around 5 GeV, our kurtosis exhibits a peak structure indicative of the critical end point of QCD.

hep-ph↗

arXiv2025

Quark mass dependence of a QCD critical point and structure of the Columbia plot

We study the quark-mass dependence of the QCD critical point at varying bare up/down quark masses with fixed strange quark mass. We explore the corresponding second-order critical surface in the three-dimensional Columbia plot and study the extension of the associated crossover hyperplane at real and imaginary baryon chemical potential. To this end, we employ a by now well-tested combination of lattice Yang--Mills theory and a (truncated) version of Dyson--Schwinger equations at $N_f=2+1$ quark flavours. We find evidence for a positive curvature of the second order surface at (large) real chemical potential and a crossover region for imaginary chemical potential. Our results support the notion of a tricritical point at finite chemical potential in the chiral limit.

hep-ph↗

arXiv2025

Structure of open-flavour four-quark states in the charm and bottom region

We present quantitative results for masses and the internal structure of four-quark states with two heavy quarks, i.e. $QQ'\bar{q}\bar{q}'$ with $Q,Q'\in \{c,b\}$ and $q,q' \in \{u,d,s\}$, and $J^{P} \in {1^+}$. The composition of these states in terms of meson-meson and diquark-antidiquark pairs, extracted from a relativistic four-body Faddeev-Yakubowski equation, is dynamically determined from underlying QCD forces. We find states at energy levels in very good agreement with lattice QCD and, where available, with experimental states. Their internal structure, most notably between the $T^+_{cc}$, $T_{bc}$ and $T^-_{bb}$, show significant and sizeable variations.

hep-ph↗

arXiv2025

Functional methods for hadron spectroscopy

We summarize recent results for four-quark states with hidden and open flavor obtained from a four-quark Bethe-Salpeter/Faddeev-Yakubowsky equation. The approach dynamically predicts the leading internal two-body clusters such as meson-meson, hadroquarkonium or diquark-antidiquark components. For hidden-flavor states, the meson-meson or hadroquarkonium configurations are dominant, while the diquark contributions are small. For open-flavor states, the situation is different and depends on the quark content: The $T_{bb}^-$ is a mixture of various components, where the diquark-antidiquark components are dominant, whereas the $T_{cc}^+$ is practically entirely dominated by $DD^\ast$ due to its proximity to the threshold. We also provide details on the analytic continuations and extrapolations to extract states above thresholds.

hep-ph↗

arXiv2025

New Tool to Detect Inhomogeneous Chiral Symmetry Breaking

In this letter, we discuss a novel method to search for inhomogeneous chiral symmetry breaking in theories with fermions. The prime application we have in mind is QCD, but the method is also applicable for other theories, including solid-state applications. It is based on an extension of the chiral susceptibility to inhomogeneous phases and it works as a stability analysis. Our method allows us to determine when homogeneous solutions have the tendency to create spatially modulated condensates. As proof of principle, we apply this technique to a rainbow-ladder QCD model and find that its phase diagram contains an inhomogeneous region.

hep-ph↗

arXiv2025

Apparent convergence in functional glueball calculations

We scrutinize the determination of glueball masses in pure Yang-Mills theory from functional equations, i.e. Dyson-Schwinger and Bethe-Salpeter equations. We survey the state-of-the-art input (dressed propagators and vertices) with an emphasis on the stability of the results under extensions of the employed truncations and explore the importance of different aspects of the bound state equations, focusing on the three lightest glueballs with $J^{PC} = 0^{++}$ , $0^{-+}$ and $2^{++}$ . As an important systematic extension compared to previous calculations we include two-loop diagrams in the Bethe-Salpeter kernels. In terms of the glueball spectrum we find only marginal mass shifts compared to previous results, indicating apparent convergence of the system. As a by-product, we also explore gauge invariance within a class of Landau-type gauges.

hep-ph↗

arXiv2025

Axial-vector and scalar contributions to hadronic light-by-light scattering

We present results for single axial-vector and scalar meson pole contributions to the hadronic light-by-light scattering (HLbL) part of the muon's anomalous magnetic moment. In the dispersive approach to these quantities (in narrow width approximation) the central inputs are the corresponding space-like electromagnetic transition form factors. We determine these directly using a functional approach to QCD by Dyson-Schwinger and Bethe-Salpeter equations in the very same setup we used previously to determine pseudo-scalar meson exchange ($π$, $η$ and $η'$) as well as meson ($π$ and $K$) box contributions. Particular care is taken to preserve gauge invariance and to comply with short distance constraints in both the form factors and the HLbL tensor. Our result for the contributions from a tower of axial-vector states including short distance constraints is { $a_μ^{\text{HLbL}}[\text{AV-tower+SDC}] = 24.8 \,(6.1) \times 10^{-11}$}. For the combined contributions from $f_0(980), a_0(980), f_0(1370)$ and $a_0(1450)$ we find $a_μ^{\text{HLbL}}[\text{scalar}] = -1.6 \,(5) \times 10^{-11}$.

hep-ph↗

arXiv2024

Inhomogenuous instabilities at large chemical potential in a rainbow-ladder QCD model

In this work we continue our efforts to study the existence of a phase with an inhomogeneous, i.e., spatially varying, chiral condensate in QCD. To this end we employ a previously established method of stability analysis of the two-particle irreducible effective action in a truncation that corresponds to a rainbow-ladder approximation of the quark-gluon interaction of QCD. If the analysis is restricted to homogeneous phases, the phase diagram features a first-order chiral transition in the lower-temperature regime. Performing the stability analysis along the lower-chemical-potential border of the corresponding spinodal region, we find that below a certain temperature the homogeneous chirally symmetric solution is unstable against inhomogeneous condensation. We argue that this instability may persist to chemical potentials above the homogeneous first-order phase boundary, in which case it signals the existence of an inhomogeneous ground state. Our methodology is also applicable for more sophisticated truncations of the QCD effective action.

hep-ph↗

arXiv2024

Hidden-flavour four-quark states in the charm and bottom region

We discuss the spectrum and the internal composition of ground and excited four-quark states in the charm and bottom energy region. To this end we extend previous calculations within the framework of the relativistic four-body Faddeev-Yakubovsky equation to include quantum numbers with $J^{P C} = 0^{++} , 0^{-+} , 1^{--} , 1^{+-}$ and $1^{+ +}$ and study their internal composition in terms of heavy-light meson pairs, hadroquarkonia and diquark-antidiquark clusters. We observe similar patterns in the charm and bottom energy region with different compositions of the four-quark states depending on $J^{P C}$ quantum numbers. Most notably, we find that all states with $C \cdot P = +1$ are dominated by heavy-light meson contributions, whereas for axialvector states with $J^{P C} = 1^{+-}$ including the $Z_c (3900)$ we find a much more complicated picture depending on the flavour content. We systematically compare our results for the spectrum with existing experimental results and provide predictions for future analyses.

hep-ph↗

arXiv2024

Space-like Electromagnetic Form Factors of Lambda- and Sigma-Baryons from Quark-Diquark Faddeev Equations

An important goal of ongoing and future experiments is to explore spectra and transition form factors of baryons with non-zero strangeness. Of particular interest is the transition form factor$γ^{(*)} Σ^0 \rightarrow Λ$ in the time-like momentum region that can be extracted from Dalitz decays. On the road towards a theoretical description of these form factors we extend a covariant dynamical quark-diquark model for the baryon Faddeev equation to the strange-quark sector. Based on an excellent description of the mass spectrum of selected baryon octet and decuplet states and reasonable results for the nucleon form factors we determine the elastic electromagnetic form factors of $Λ$ and $Σ^+, Σ^0, Σ^-$ hyperons in the space-like region as well as the ones for the octet transition $γ^{(*)} Σ^0 \rightarrow Λ$. We discuss qualitative and quantitative features of the diquark-quark picture and compare systematically with previous results from a three-body Faddeev approach and lattice data where available.

hep-ph↗

arXiv2024

QCD phase transitions in the light quark chiral limit

We investigate the order of the QCD chiral transition in the limit of vanishing bare up/down quark masses and variations of the bare strange-quark mass $0 \le m_{\mathrm{s}} \le \infty$. In this limit and due to universality long range correlations with the quantum numbers of pseudoscalar and scalar mesons may dominate the physics. In order to study the interplay between the microscopic quark and gluon degrees of freedom and the long range correlations we extend a combination of lattice Yang--Mills theory and a (truncated) version of Dyson--Schwinger equations by also taking back-reactions of mesonic degrees of freedom into account. Both this system and the meson backcoupling approach have been studied extensively in the past but this is the first work in a full $(2 + 1)$-flavor setup. Starting from the physical point, we determine the chiral susceptibilities for decreasing up/down quark masses and find good agreement with both lattice and functional renormalization group results. We then proceed to determine the order of the chiral transition along the left-hand side of the Columbia plot, for chemical potentials in the range $-(30 \,\textrm{MeV})^2 \le μ_q^2 \le (30 \,\textrm{MeV})^2$. We find a second-order phase transition throughout and no trace of a first-order region in the $N_{f} = 3$ corner of the Columbia plot. This result remains unchanged when an additional Goldstone boson due to a restored axial $\mathrm{U_A}(1)$ is taken into account.

hep-ph↗

arXiv2024

From imaginary to real chemical potential QCD with functional methods

We investigate the quality of the extrapolation procedure employed in Ref. [1] to extract the crossover line at real chemical potential from lattice data at imaginary potential. To this end we employ a functional approach that does not suffer from the sign problem. We utilize a well-studied combination of lattice Yang--Mills theory with a truncated set of Dyson--Schwinger equations in Landau gauge for $2 + 1$ quark flavors. This system predicts a critical endpoint at moderate temperatures and rather large (real) chemical potential with a curvature of the pseudo-critical transition line comparable to recent lattice extrapolations. We determine the light quark condensate and chiral susceptibility at imaginary chemical potentials and perform an analytic continuation along the lines described in [1]. We find that the analytically continued crossover line agrees very well (within one percent) with the explicitly calculated one for chemical potentials up to about 80 % of the one of the critical end point. The method breaks down in the region where the chiral susceptibility as a function of the condensate cannot any longer be well described by a polynomial.

hep-ph↗

arXiv2023

Nucleon to $Δ$ axial and pseudoscalar transition form factors

A symmetry-preserving continuum approach to the calculation of baryon properties in relativistic quantum field theory is used to predict all form factors associated with nucleon-to-$Δ$ axial and pseudoscalar transition currents, thereby unifying them with many additional properties of these and other baryons. The new parameter-free predictions can serve as credible benchmarks for use in analysing existing and anticipated data from worldwide efforts focused on elucidation of $ν$ properties.

hep-ph↗