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Ulf-G. Meißner

Publications and source records attributed to Ulf-G. Meißner.

At least 127 records · Page 7Linked to original sources
arXiv2023

On the role of isospin violation in the pion-nucleon $σ$-term

In recent years, a persistent tension between phenomenological and lattice QCD determinations of the pion-nucleon $σ$-term $σ_{πN}$ has developed. In particular, lattice-QCD calculations have matured to the point that isospin-violating effects need to be included. Here, we point out that the standard conventions adopted in both fields are incompatible, with the data-driven extraction based on the charged-pion mass, but lattice-QCD conventions relying on the mass of the neutral pion to define the isospin limit. The corresponding correction amounts to $Δσ_{πN}=3.1(5)\,\text{MeV}$ when evaluated in chiral perturbation theory with low-energy constants determined from a Roy-Steiner analysis of pion-nucleon scattering as well as $σ_{πN}$ itself. It reduces the tension with lattice QCD, and should be included in the comparison to phenomenological determinations. We also update the extraction from pionic atoms accounting for the latest measurement of the width of pionic hydrogen, $σ_{πN}=59.0(3.5)\,\text{MeV}$, and provide the corresponding set of scalar couplings of the nucleon.

hep-ph↗
arXiv2023

New insights into the pole parameters of the $Λ(1380)$, the $Λ(1405)$ and the $Σ(1385)$

A coupled-channel S- and P-wave next-to-leading order chiral-unitary approach for strangeness $S=-1$ meson-baryon scattering is extended to include the new data from the KLOE and AMADEUS experiments as well as the $Λπ$ mass distribution of the $Σ(1385)$. The positions of the poles on the second Riemann sheet corresponding to the $Σ(1385)$ pole and the $Λ(1380)$ and $Λ(1405)$ poles as well as the couplings of these states to various channels are calculated. We find that the resonance positions and branching ratios are on average determined with about 20\% higher precision when including the KLOE and AMADEUS data. Additionally, for the first time, the correlations between the parameters of the poles are investigated and shown to be relevant. We also find that the $Σ(1385)$ has negligible influence on the properties of the $Λ$ states given the available data. Still, we identify isospin-1 cusp structures in the present solution in light of new measurements of $π^\pmΛ$ line-shapes by the Belle collaboration.

nucl-th↗
arXiv2023

The electromagnetic fine-structure constant in primordial nucleosynthesis revisited

We study the dependence of the primordial nuclear abundances as a function of the electromagnetic fine-structure constant $α$, keeping all other fundamental constants fixed. We update the leading nuclear reaction rates, in particular the electromagnetic contribution to the neutron-proton mass difference pertinent to $β$-decays, and go beyond certain approximations made in the literature. In particular, we include the temperature-dependence of the leading nuclear reactions rates and assess the systematic uncertainties by using four different publicly available codes for Big Bang nucleosynthesis. Disregarding the unsolved so-called lithium-problem, we find that the current values for the observationally based $^{2}$H and $^{4}$He abundances restrict the fractional change in the fine-structure constant to less than $2\%$, which is a tighter bound than found in earlier works on the subject.

hep-th↗
arXiv2023

Ab initio study of nuclear clustering in hot dilute nuclear matter

We present a systematic ab initio study of clustering in hot dilute nuclear matter using nuclear lattice effective field theory with an SU(4)-symmetric interaction. We introduce a method called light-cluster distillation to determine the abundances of dimers, trimers, and alpha clusters as a function of density and temperature. Our lattice results are compared with an ideal gas model composed of free nucleons and clusters. Excellent agreement is found at very low density, while deviations from ideal gas abundances appear at increasing density due to cluster-nucleon and cluster-cluster interactions. In addition to determining the composition of hot dilute nuclear matter as a function of density and temperature, the lattice calculations also serve as benchmarks for virial expansion calculations, statistical models, and transport models of fragmentation and clustering in nucleus-nucleus collisions.

nucl-th↗
arXiv2023

Emergent geometry and duality in the carbon nucleus

The carbon atom provides the backbone for the complex organic chemistry composing the building blocks of life. The physics of the carbon nucleus in its predominant isotope, $^{12}$C, is similarly full of multifaceted complexity. Some nuclear states of $^{12}$C can be preferentially treated as a collection of independent particles held by the mean field of the nucleus, while other states behave more as a collection of three alpha-particle clusters. But these two pictures are not mutually exclusive, and some states can be described in either fashion$^{1,2}$. In this work, we provide the first model-independent tomographic scan of the three-dimensional geometry of the nuclear states of $^{12}$C using the {\it ab initio} framework of nuclear lattice effective field theory. We find that the well-known but enigmatic Hoyle state is composed of a "bent-arm" or obtuse triangular arrangement of alpha clusters. We identify all of the low-lying nuclear states of $^{12}$C as having an intrinsic shape composed of three alpha clusters forming either an equilateral triangle or an obtuse triangle. From these basic structural formations, the various nuclear states correspond to different rotational and vibrational excitations as well as either distortions or large-amplitude displacements of the alpha clusters. The states with the equilateral triangle formation also have a dual description in terms of particle-hole excitations in the mean-field picture. We compare our theoretical calculations with experimental data for binding energies, quadrupole moments, electromagnetic transitions, charge densities, and form factors. The overall agreement is good, and further studies using higher-fidelity interactions are planned.

nucl-th↗
arXiv2023

Revealing the nature of hidden charm pentaquarks with machine learning

We study the nature of the hidden charm pentaquarks, i.e. the $P_c(4312)$, $P_c(4440)$ and $P_c(4457)$, with a neural network approach in pionless effective field theory. In this framework, the normal $χ^2$ fitting approach cannot distinguish the quantum numbers of the $P_c(4440)$ and $P_c(4457)$. In contrast to that, the neural network-based approach can discriminate them. In addition, we also illustrate the role of each experimental data bin of the invariant $J/ψp$ mass distribution on the underlying physics in both neural network and fitting methods. Their similarities and differences demonstrate that neural network methods can use data information more effectively and directly. This study provides more insights about how the neural network-based approach predicts the nature of exotic states from the mass spectrum.

hep-ph↗
arXiv2023

Particle-dimer approach for the Roper resonance in a finite volume

We propose a new finite-volume approach which implements two- and three-body dynamics in a transparent way based on an Effective Field Theory Lagrangian. The formalism utilizes a particle-dimer picture and formulates the quantization conditions based on the self-energy of the decaying particle. The formalism is studied for the case of the Roper resonance, using input from lattice QCD and phenomenology. Finally, finite-volume energy eigenvalues are predicted and compared to existing results of lattice QCD calculations. This crucially provides initial guidance on the necessary level of precision for the finite-volume spectrum.

hep-lat↗
arXiv2023

Isospin-conserving hadronic decay of the ${D_{s1}(2460)}$ into ${D_sπ^+π^-}$

The internal structure of the charm-strange mesons $D_{s0}^*(2317)$ and $D_{s1}(2460)$ are subject of intensive studies. Their widths are small because they decay dominantly through isospin-breaking hadronic channels $D_{s0}^*(2317)^+\to D_s^+π^0$ and $D_{s1}(2460)^+\to D_s^{*+}π^0$. The $D_{s1}(2460)$ can also decay into the hadronic final states $D_s^+ππ$, conserving isospin. In that case there is, however, a strong suppression from phase space. We study the transition $D_{s1}(2460)^+\to D_s^+π^+π^-$ in the scenario that the $D_{s1}(2460)$ is a $D^*K$ hadronic molecule. The $ππ$ final state interaction is taken into account through dispersion relations. We find that the ratio of the partial widths of the $Γ(D_{s1}(2460)^+\to D_s^+π^+π^-)/Γ(D_{s1}(2460)^+\to D_s^{*+}π^0)$ obtained in the molecular picture is consistent with the existing experimental measurement. More interestingly, we demonstrate that the $π^+π^-$ invariant mass distribution shows a double bump structure, which can be used to disentangle the hadronic molecular picture from the compact state picture for the $D_{s1}(2460)^+$. Predictions on the $B_{s1}^0\to B_s^0π^+π^-$ are also made.

hep-ph↗
arXiv2023

The electromagnetic Sigma-to-Lambda transition form factors with coupled-channel effects in the space-like region

Using dispersion theory, the electromagnetic Sigma-to-Lambda transition form factors are expressed as the product of the pion electromagnetic form factor and the $Σ\barΛ\toππ$ scattering amplitudes with the latter estimated from SU(3) chiral perturbation theory including the baryon decuplet as explicit degrees of freedom. The contribution of the $K\bar{K}$ channel is also taken into account and the $ππ$-$K\bar{K}$ coupled-channel effect is included by means of a two-channel Muskhelishvili-Omnès representation. It is found that the electric transition form factor shows a significant shift after the inclusion of the $K\bar{K}$ channel, while the magnetic transition form factor is only weakly affected. However, the $K\bar{K}$ effect on the electric form factor is obscured by the undetermined coupling $h_A$ in the three-flavor chiral Lagrangian. The error bands of the Sigma-to-Lambda transition form factors from the uncertainties of the couplings and low-energy constant in three-flavor chiral perturbation theory are estimated by a bootstrap sampling method.

hep-ph↗
arXiv2023

$Λ\barΛ$ final-state interaction in the reactions $e^+e^-\to ϕΛ\bar Λ$ and $e^+e^- \to ηΛ\bar Λ$

Near-threshold $Λ\barΛ$ mass spectra for the reactions $e^+e^- \to ηΛ\barΛ$ and $e^+e^- \to ϕΛ\barΛ$ are investigated with an emphasis on the role played by the interaction in the $Λ\barΛ$ system. A variety of $Λ\barΛ$ potential models is employed that have been established in the analysis of data on $p\bar p\to Λ\barΛ$ in the past. It is shown that the near-threshold enhancement observed for the two $e^+e^-$ reactions can be reproduced by considering the $Λ\barΛ$ final-state interaction in the partial waves suggested by the helicity-angle analysis of the experiments. For $e^+e^- \to ηΛ\barΛ$ the same $Λ\barΛ$ $S$-wave interaction as in $e^+e^- \to Λ\barΛ$ is relevant and with it a consistent description of the pertinent measurements can be achieved. It is pointed out that a nonzero threshold cross section as observed for the latter reaction is not supported by the new $ηΛ\barΛ$ data.

nucl-th↗
arXiv2023

Ab initio calculation of charge symmetry breaking in $A=7$ and $8$ $Λ$-hypernuclei

The separation energies of the isospin triplet $^7_Λ\mathrm{He}$, $^7_Λ\mathrm{Li^{*}}$, $^7_Λ$Be, and the $T=1/2$ doublet $^8_Λ$Li, $^8_Λ$Be are investigated within the no-core shell model. Calculations are performed based on a hyperon-nucleon potential derived from chiral effective field theory at next-to-leading order. The potential includes the leading charge-symmetry breaking (CSB) interaction in the $Λ$N channel, whose strength has been fixed to the experimentally known difference of the $Λ$ separation energies of the mirror hypernuclei $^4_Λ\mathrm{He}$ and $^4_Λ\mathrm{H}$. It turns out that the CSB predicted for the $A=7$ systems is small and agrees with the splittings deduced from the empirical binding energies within the experimental uncertainty. In case of the $A=8$ doublet, the computed CSB is somewhat larger than the available experimental value. Using other experimental input for $A=4$ can change this prediction moving it closer to experiment.

nucl-th↗
arXiv2023

Determination of diffractive PDFs from global QCD analysis of inclusive diffractive DIS and dijet cross-section measurements at HERA

We present an updated set of {\tt SKMHS} diffractive parton distribution functions (PDFs). In addition to the diffractive deep-inelastic scattering (diffractive DIS) data sets, the recent diffractive dijet cross sections measurements by the H1 experiment from the HERA collider are added to the data sample. The new set of diffractive PDFs, entitled {\tt SKMHS23} and {\tt SKMHS23-dijet}, are presented at next-to-leading order (NLO) and next-to-next-to-leading order (NNLO) accuracy in perturbative QCD. Since the gluons directly contribute to jet production through the boson-gluon fusion process, the data on diffractive dijet production in inclusive DIS help to constrain the gluon density, allowing for the determination of both the quark and gluon densities with better accuracy. The NLO and NNLO theory predictions calculated using both {\tt SKMHS23} and {\tt SKMHS23-dijet} are compared to the analyzed data showing excellent agreements. The effect arising from the inclusion of diffractive dijet data and higher order QCD corrections on the extracted diffractive PDFs and data/theory agreements are clearly examined and discussed.

hep-ph↗
arXiv2023

Hyperon-nucleon interaction in chiral effective field theory at next-to-next-to-leading order

A hyperon-nucleon potential for the strangeness $S=-1$ sector ($ΛN$, $ΣN$) up to third order in the chiral expansion is presented. SU(3) flavor symmetry is imposed for constructing the interaction, however, the explicit SU(3) symmetry breaking by the physical masses of the pseudoscalar mesons and in the leading-order contact terms is taken into account. A novel regularization scheme is employed which has already been successfully used in studies of the nucleon-nucleon interaction within chiral effective field theory up to high orders. An excellent description of the low-energy $Λp$, $Σ^- p$ and $Σ^+ p$ scattering data is achieved. New data from J-PARC on angular distributions for the $ΣN$ channels are analyzed. Results for the hypertriton and $A=4$ hyper-nuclear separation energies are presented. An uncertainty estimate for the chiral expansion is performed for selected hyperon-nucleon observables.

nucl-th↗
arXiv2022

Weak radiative decay $Λ_{c}^{+}\toΣ^{+}γ$ using light-cone sum rules

We calculate the decay width of the $Λ_{c}^{+}\toΣ^{+}γ$ using light-cone sum rules. For the initial quark radiation an effective Hamiltonian is constructed, where the internal quark line shrinks to a point. The final quark radiation is studied within the full theory. The leading twist light-cone distribution amplitudes of the $Σ^+$ serve as the non-perturbative input for the sum rules calculation, and the perturbative kernel is calculated at leading order. The branching fraction we obtain is ${\cal B}(Λ_{c}^{+}\toΣ^{+}γ)=1.03\pm 0.36 \times 10^{-4}$, which is below the recent upper limit $<2.6 \times 10^{-4}$ given by the Belle collaboration.

hep-ph↗
arXiv2022

Light baryon resonances from a coupled-channel study including $\mathbf{KΣ}$ photoproduction

The Jülich-Bonn dynamical coupled-channel approach is extended to include $KΣ$ photoproduction off the proton. Differential cross section and (double) polarization data for $K^+Σ^0$ and $K^0Σ^+$ are analysed simultaneously with the pion- and photon-induced production of $πN$, $ηN$, $KΛ$, and $KΣ$ final states, totaling more than 67,000 data points for center-of-mass energies $W<2.4$ GeV. Based on the fit results the spectrum of $N^*$ and $Δ$ resonances is extracted in terms of pole positions and residues. We discuss the impact of the $γp\to KΣ$ channels in detail and investigate the influence of recent polarization data for $ηp$ photoproduction.

nucl-th↗
arXiv2022

The proton radius and its relatives -- much ado about nothing?

I summarize the dispersion-theoretical analysis of the nucleon electromagnetic form factors. Special emphasis is given on the extraction of the proton charge radius and its relatives, the proton magnetic radius as well as the neutron magnetic radius. Some recent work on the hyperfine splitting in leptonic hydrogen and on radiative corrections to muon-proton scattering is also discussed. Some views on future studies are given.

hep-ph↗
arXiv2022

The reaction $πN \to ωN$ in a dynamical coupled-channel approach

A refined investigation on light flavor meson-baryon scatterings is performed using a dynamical coupled-channel approach, the Jülich-Bonn model, that respects unitartiy and analyticity constraints. The channel space of $πN$, $πΔ$, $σN$, $ρN$, $ηN$, $K Λ$ and $K Σ$ is extended by adding the $ωN$ final state. The spectra of $N^*$ and $Δ$ resonances are extracted in terms of complex poles of the scattering amplitudes, based on the result of a global fit to a worldwide collection of data, in the energy region from the $πN$ threshold to center-of-mass energy $z=2.3$ GeV. A negative value of the $ωN$ elastic spin-averaged scattering length is extracted, questioning the existence of bound states of the $ω$ meson in the nuclear matter.

nucl-th↗
arXiv2022

Resonance form factors from finite-volume correlation functions with the external field method

A novel method for the extraction of form factors of unstable particles on the lattice is proposed. The approach is based on the study of two-particle scattering in a static, spatially periodic external field by using a generalization of the Lüscher method in the presence of such a field. It is shown that the resonance form factor is given by the derivative of the resonance pole position in the complex plane with respect to the coupling constant to the external field. Unlike the standard approach, this proposal does not suffer from problems caused by the presence of the triangle diagram.

hep-lat↗