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Ulf-G. Meißner

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

At least 19 recordsLinked to original sources
arXiv2026

Nucleon electromagnetic form-factors in a minimal Gari-Krümpelmann model including two-meson continua

The Gari--Krümpelmann (GK) model provided one of the first semiphenomenological descriptions of the electromagnetic nucleon form-factors over a wide range of momentum transfer. It combined vector-meson dominance at low and intermediate momentum transfer with perturbative-QCD constraints at large momentum transfer. The present work extends this framework by including the dispersive two-pion continuum explicitly in the isovector channel. The two-pion contribution is taken from modern dispersive analyses and embedded into the intrinsic high-$Q^2$ structure of the GK ansatz. The physical $ρ$ dynamics is then interpreted as part of the correlated two-pion spectral function. Similarly, we include in the isoscalar spectral function the correlated $K\bar K$ and $ρπ$ continua, where the former also includes the $ϕ$-meson pole. The direct coupling terms remain essential for the transition to the asymptotic regime. This gives a cleaner separation between long-range continuum dynamics, coherent vector-meson contributions, and direct quark-current dynamics. With just 7 parameters, we can describe the large body on form-factor data and differential cross sections from electron-proton scattering in the space-like region.

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arXiv2026

Elastic deuteron-deuteron scattering in the ${}^5S_2$ channel within Nuclear Lattice Effective Field Theory

We calculate low-energy deuteron-deuteron scattering in the spin-quintet $^{5}S_2$ channel using nuclear lattice effective field theory. The calculation combines chiral interactions at next-to-next-to-next-to-leading order, implemented through wavefunction matching, with the adiabatic projection method. Because the radial cluster basis develops small norm-matrix eigenvalues at large Euclidean projection time, we investigate two stabilization procedures: Tikhonov regularization and projection onto well-resolved norm eigenmodes. The two procedures yield consistent Coulomb-subtracted phase shifts within their statistical and numerical uncertainties. A Coulomb-modified effective-range analysis gives ${}^5a_{dd} = (11.99 \pm 0.22)$ fm and ${}^5r_{dd} = (3.44 \pm 0.79)$ fm. The phase shifts are more negative, and the scattering length is substantially larger than in previous calculations, corresponding to a stronger effective repulsion in the $^{5}S_2$ channel. These results provide a first nuclear-lattice benchmark for deuteron-deuteron scattering and establish a starting point for future coupled-channel calculations of the deuteron-induced reactions relevant to big-bang nucleosynthesis.

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arXiv2026

Extraction of the pion-nucleon coupling constant using the effective-range expansion with the left-hand cut

We apply the generalized effective-range expansion of Phys. Rev. Lett. 135, 011903(2025), which incorporates the left-hand cut from one-pion exchange, to low-energy neutron-proton scattering in the $^1S_0$ and $^3S_1$ channels. The amplitude zero for the center-of-mass momentum near 0.35 GeV in the $^1S_0$ channel is naturally accommodated within this framework. We extract the pole position, scattering length, effective range, and the pseudoscalar pion-nucleon coupling constant $g_{πN}^2/(4π)$ at different expansion orders. The low-energy parameters are stable and consistent with established values, while $g_{πN}^2/(4π)$ exhibits larger uncertainties. The extraction of $g_{πN}^2/(4π)$ is data-driven, relying on the analytic constraints from the left-hand cut and phase-shift data within the one-pion-exchange approximation. Despite larger uncertainties compared to high-precision extractions, the consistency with established values demonstrates that this framework can probe the left-hand-cut singularity.

hep-ph↗
arXiv2026

Three-body unitary determination of the $f_1(1285)$ and $f_1(1420)$ pole positions

We study the $I^G(J^{PC})=0^+(1^{++})$ $K\bar Kπ$ system in an infinite-volume three-body unitary framework, focusing on the pole content of the region of the $f_1(1285)$ and $f_1(1420)$ resonances. The coupled $πa_0$-$K\bar K^*$ amplitude is constructed in the spectator-isobar representation, where the one-particle-exchange interaction required by three-body unitarity automatically incorporates the triangle-singularity mechanism. The short-range three-body interaction is constrained by fitting the $0^+(1^{++})$ component of the BESIII $K^0_SK^0_Sπ^0$ invariant-mass distribution in the $J/ψ\toγ(K^0_SK^0_Sπ^0)$ decay. Analytically continuing the fitted amplitude to the relevant unphysical Riemann sheets, we find two robust poles: \begin{align} \sqrt{s_{f_1(1285)}}&= \left(1277\pm2\pm1\right) -\mathrm{i}\left(12\pm1\pm0\right)\text{MeV}\,,\notag\\ \sqrt{s_{f_1(1420)}}&= \left(1435\pm2\pm7\right) -\mathrm{i}\left(40\pm2\pm1\right)\text{MeV}\,.\notag \end{align} The pole trajectories indicate that the $f_1(1285)$ originates from dressing a bare state introduced in the potential. In contrast, the $f_1(1420)$ is predominantly dynamically generated, and a single-channel analysis traces it to an $S$-wave $K\bar K^*$ quasi-bound state mixed with the nearby bare state, supporting its hadronic-molecule interpretation. We also find an additional pole deeper in the complex plane in the best-fit amplitude on the same Riemann sheet as the $f_1(1285)$. This additional pole is generated by the $P$-wave $πa_0$ contact interaction alone. It has a sizable cutoff and two-body-input dependence, and leaves little visible imprint on the physical lineshape. Finally, we provide a detailed and pedagogical appendix on how three-body cuts affect the solution of the integral equation.

hep-ph↗
arXiv2026

Precise determination of the properties of $X(3872)$ and of its isovector partner $W_{c1}$

We perform a simultaneous fit to BESIII data on $e^+e^-\to γ(D^0{\bar{D}^{0}}π^0/J/ψπ^+π^-)$ and LHCb data on $B^+\to K^+(J/ψπ^+π^-)$ to precisely determine the properties of the $X(3872)$, with full consideration of three-body effects from $D^*\to Dπ$ decay, respecting both analyticity and unitarity. The $X(3872)$ is determined to be a quasi-bound state with a significance of $2.7\,σ$, representing the most precise determination to date. Its pole is located at $\left(-160^{+57}_{-74}-125^{+23}_{-38}\,i\right) \rm keV$, relative to the nominal $D^0\bar{D}^{*0}$ threshold. Moreover, we confirm the presence of an isovector partner state, $W_{c1}$. It is found as a virtual state at $\left(3.1\pm0.7+ 1.3^{+1.9}_{-0.6}\,i\right)\ \rm MeV$ relative to the $D^+ D^{*-}$ threshold on an unphysical Riemann sheet, strongly supporting a molecular nature of both $X(3872)$ and $W_{c1}$. As a highly nontrivial prediction we show that the $W_{c1}$ leads to nontrivial lineshapes around 3.88 GeV in $B^0\to K^0 X(3872)\to K^0 D^0\bar D^0π^0$ and $K^0J/ψπ^+π^-$ -- thus the scheme presented here can be tested further by improved measurements.

hep-ph↗
arXiv2026

QCD $θ$-vacuum energy and axion properties

At low energies, the strong interaction is governed by the Goldstone bosons associated with the spontaneous chiral symmetry breaking, which can be systematically described by chiral perturbation theory. In this paper, we apply this theory to study the $θ$-vacuum energy density and hence the QCD axion potential up to next-to-leading order with $N$ non-degenerate quark masses. By setting $N=3$, we then derive the axion mass, self-coupling, topological susceptibility and the normalized fourth cumulant both analytically and numerically, taking the strong isospin breaking effects into account. In addition, the model-independent part of the axion-photon coupling, which is important for axion search experiments, is also extracted from the chiral Lagrangian supplemented with the anomalous terms up to $\mathcal{O}(p^6)$.

hep-ph↗
arXiv2026

Vector charmonium(-like) states in the energy range of 4.1-4.6 GeV

The spectrum of vector charmonium(-like) states in the 4.1\dash4.6~GeV energy region exhibits a long-standing tension between inclusive and exclusive measurements. While the inclusive $R$-value indicates only conventional vector charmonia such as $ψ(4160)$ and $ψ(4415)$, exclusive $e^+e^-$ cross sections reveal additional structures whose parameters strongly depend on the observed final states when fitted with Breit--Wigner functions. This puzzling pattern suggests that coupled-channel and threshold effects play an essential role. In this work, we develop a unified coupled-channel framework for the $1^{--}$ resonances in this energy region. The framework incorporates the $S$-wave open-charm channels $D\bar{D}_1$, $D^*\bar{D}_1$, and $D^*\bar{D}_2^*$ constrained by heavy-quark spin symmetry, optional bare poles associated with $ψ(4160)$ and $ψ(4415)$, and final-state interactions in the $Z_c$ channels. We perform simultaneous fits to the BESIII cross sections for $e^+e^-\to J/ψπ^+π^-$, $h_cπ^+π^-$, $D\bar{D}^*π$, $D^*\bar{D}^*π$, $J/ψη$, and $χ_{c0}ω$, together with invariant-mass distributions exhibiting the $Z_c(3900)$ and $Z_c(4020)$ structures. The benchmark models differ in the number of bare seed states and the fitting strategy. We show that even the purely dynamical scheme without bare charmonia captures the gross features of the analyzed distributions. The inclusion of bare compact states improves the fit quality but does not change the conclusion that the measured line shapes can be understood in terms of strong coupled-channel effects with dynamically generated poles. We also discuss possible heavy-quark spin partners of the exotic $1^{--}$ states.

hep-ph↗
arXiv2026

Reaction Cross Sections and $α$-Cluster Geometry in $^{12}$C and Be Isotopes

Reaction cross sections $σ_{\rm R}$ are widely used to infer matter radii, yet their sensitivity to nuclear structure beyond radial one-body distributions is less well understood. We combine complete $A$-body nucleon configurations sampled from \textit{ab initio} nuclear lattice effective field theory (NLEFT) with event-by-event Monte Carlo Glauber calculations, thereby retaining the many-body correlations encoded in NLEFT. Using a fixed binary-collision prescription determined by the measured energy- and isospin-dependent total nucleon-nucleon cross sections, the calculations capture the overall magnitudes and energy dependence simultaneously for the available data on $^{12}$C and $^{9}$Be projectiles on carbon and hydrogen. Controlled randomization of angular correlations at fixed matter root-mean-square radius and spherically averaged one-body radial density produces only a weak change in $σ_{\rm R}$ for $^{12}$C but approximately a $10\%$ increase for $^{9}\mathrm{Be}+{}^{1}\mathrm{H}$. The calculations also capture the measured rise--plateau--sharp-rise--reduction trend across $^{7,9\text{--}12}$Be, a distinctive pattern reflecting the evolution of cluster and halo structures along the isotopic chain. These results show that $σ_{\rm R}$ retains sensitivity to intrinsic many-body geometry beyond a single inferred matter radius, opening a route to studies of exotic $α$-cluster geometries and spatial nucleon correlations through reaction cross sections.

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arXiv2026

Amplified Isospin Breaking from Coupled-Channel Dynamics Near Threshold

We identify the dynamical origin of amplified isospin breaking in near-threshold states. Using a pole-residue-based measure defined directly from the pole couplings, rather than from decay observables that are also affected by different final-state kinematics, we show that threshold proximity of the pole alone is insufficient: large isospin breaking requires a sizable interaction in the companion isospin channel. Applied to the $X(3872)$, the observed large isospin breaking points to a nearby partner pole, $W_{c1}$, as predicted in previous studies. In addition, we find that the $W_{c1}$ pole, which predominantly affects the line shapes near the charged threshold in the physical case, does not evolve into the pure $I=1$ eigenstate in the isospin limit; instead, that eigenstate is continuously connected to a more distant shadow pole.

hep-ph↗
arXiv2026

Deformation effects on reaction observables of beryllium nuclei from ab initio densities

We combine three-dimensional intrinsic densities from ab initio nuclear lattice effective field theory with a deformed Glauber model to study high-energy reactions of {7-12}Be. To connect the correlated many-body configurations to the core-plus-neutron reaction formalism without imposing a single-particle orbital, we introduce a configuration-resolved prescription that identifies the spatially outermost valence neutron after the two-cluster decomposition. For Be projectiles on 12C and 9Be targets at 790 MeV/A, explicit orientation averaging lowers the calculated reaction cross section of 11Be by up to approximately 50 mb relative to a calculation with the spherically averaged density. The deformed calculation reproduces the pronounced increase from 10Be to the established one-neutron halo nucleus 11Be for both targets. We further calculate the momentum distribution of the fragments after the one-neutron removal reaction of 11Be + 9Be , finding good agreement in shape with the measurement at 63 MeV/A and providing a prediction at 790 MeV/A. These results quantify how intrinsic deformation and weak binding are transmitted from microscopic many-body densities to reaction observables.

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arXiv2026

Form factors of the $ρ$-meson in chiral perturbation theory

We present the calculation of the electromagnetic form factors of the $ρ$-meson at one loop in Chiral Perturbation Theory. The power-counting-violating terms in the loop diagrams are subtracted by using infrared regularization, and the Ward identities are explicitly verified at the order considered. The results are compared to the recent calculations carried out in the framework of non-relativistic effective field theory.

hep-ph↗
arXiv2026

The anomalous long lifetime of $^{14}$C revealed by ab initio nuclear lattice EFT

The 5730-year half-life of $^{14}$C, the physical basis of radiocarbon dating, is anomalously long compared to typical nuclear-physics expectations. Its origin has remained a subject of debate for many decades. Here we report an \textit{ab initio} nuclear lattice effective field theory (NLEFT) calculation of $^{14}$C $β$ decay. Using systematically optimized interactions and transition operators consistently derived from chiral effective field theory, We obtain a result consistent with the Gamow-Teller matrix element $M_\text{GT}^\text{exp}\simeq 2\times 10^{-3}$ measured with the current uncertainty of $O(10^{-2})$. We first show that chiral interactions and weak currents beyond leading-order are essential for quenching the GT matrix element to the physical value, among which the optimization of three-nucleon forces is indispensable. We then illustrate that the quenching is deeply rooted in the ground-state structure of $^{14}$N as found in the nuclear shell model, where the competition between $S$- and $D$-wave components exists, sensitive to the interaction employed. The physical $^{14}$N ground state is found to be dominated by $D$-wave configurations, which constitutes the key factor for the quenching. The sensitivity of the decay matrix element to the fine-tuning of low-energy-constants is explored, revealing the prominent role of the $^3S_1$-channel two-nucleon contact force and the one-pion-exchange three-nucleon force. This work eliminates the gap between shell-model and \textit{ab initio} studies of $^{14}$C $β$ decay, provides a valid and straightforward explanation for the anomalous long lifetime of $^{14}$C, and turns NLEFT into a practical tool for the systematic study of nuclear transitions.

nucl-th↗
arXiv2026

Nuclear Charge Radius of $^9$Be from Muonic Atom Spectroscopy Using a Microcalorimeter

The $2p\to1s$ transition energy in muonic $^9$Be was measured using a metallic magnetic calorimeter, resulting in $E_{2p\to 1s}=33\,391.48(34)\,$eV. The result is 30 times more precise than the previous best measurement and enables the extraction of the corresponding nuclear charge radius $r_c($$^9$Be$)=2.5506(51)\,$fm. It is $2.4$ times more precise than the commonly used value based on electron scattering and differs from it by $2.3$ times the combined uncertainties. This measurement represents the first determination of a nuclear charge radius using muonic x-ray spectroscopy with microcalorimeters.

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arXiv2026

Full Uncertainty Quantification of Sign-Problem-Free Quantum Monte Carlo Methods and Nuclear Lattice Effective Field Theory Benchmarks

Sign-problem-free quantum Monte Carlo (QMC) methods provide one of the few polynomial-scaling routes to controlled, nonperturbative benchmarks of medium-mass and heavy nuclei. We present a detailed uncertainty analysis of the recently developed sign-problem-free spin-orbit lattice action LAT-OPT1 and use it to benchmark nuclear lattice effective field theory (NLEFT). We quantify various systematic uncertainties, finding that the cumulative many-body computational uncertainty in ground-state energies of doubly magic nuclei up to $^{100}$Sn is well below the percent level. In response to recent criticism of NLEFT benchmarks, we also revisit the relation between lattice transfer matrices, lattice Hamiltonians, Hartree--Fock variational bounds, finite-box and thermodynamic-limit calculations, and the continuum-limit behavior of regulated lattice interactions. We identify several conceptual and technical errors in the analysis of Ref.~\cite{Rothman2026_NuLattice}. These include (i) the comparison of inequivalent lattice transfer-matrix and lattice-Hamiltonian calculations, (ii) an inconsistent determination of correlation energies from comparisons of Hartree--Fock and full ground-state calculations with different boundary conditions, (iii) the attribution of nuclear saturation to lattice artifacts rather than to nonlocal smearing of interactions, a mechanism that can be demonstrated in continuous space, and (iv) an incorrect renormalization of short-range two-body interactions in the continuum limit. When the same regulated lattice theory, renormalization prescription, and finite-volume boundary conditions are used consistently and analyzed properly, the reported discrepancies and concerns about the corresponding published NLEFT results are resolved.

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arXiv2026

Understanding the near-threshold structures in $e^+e^- $ annihilation from a unified $N \bar N$-interaction perspective

Near-threshold structures have been observed in the cross sections for $e^+e^- \to p \bar p$, $e^+e^- \to n \bar n$ and several non-baryonic final states in the vicinity of the $N \bar N$ thresholds. We investigate whether these structures can be understood as manifestations of a common $N \bar N$ final-state interaction. The strong $N \bar N$ interaction is taken from the chiral EFT description of the coupled ${}^3S_1$-${}^3D_1$ system constrained by low-energy $N \bar N$ scattering data. With this interaction fixed, the $p \bar p$ and $n \bar n$ cross sections are described by fitting only short-distance electromagnetic production sources, which are assumed to vary slowly over the near-threshold region. The resulting $N \bar N$ production amplitudes are then used as input for five inelastic hadronic channels. A simultaneous description of the near-threshold cross sections is obtained, indicating that the observed structures can be consistently interpreted as consequences of the same underlying $N \bar N$ dynamics, without introducing separate narrow resonances in individual channels.

hep-ph↗
arXiv2026

Ab initio $α$-$α$ scattering with high-fidelity chiral interactions

Low-energy $α$-$α$ scattering underlies stellar helium burning and sharply tests nuclear forces in the reaction regime. We present its first calculation using the high-fidelity N3LO chiral NLEFT interaction, incorporated through wave function matching, on a fine lattice, using the adiabatic projection method. On the fine lattice, the two-cluster norm matrix becomes severely ill-conditioned, and its direct inversion is unstable. We address this with Tikhonov regularization, extrapolating the regulator to zero, and confirm the result with an independent truncated singular-value decomposition. The S- and D-wave phase shifts agree with empirical analyses, extending the validation of this interaction from bound states and charge radii to scattering and providing a practical route to ab initio nuclear reactions on fine lattices

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arXiv2026

Emergence of the $π(1300)$ Resonance from Lattice QCD

The mass of the lightest hadron in nature, the pion, is one seventh of that of the nucleon and one tenth of the mass of its first excited state, the $π(1300)$. This enormous energy difference opens an interesting window into the confinement of quarks and the structure of the lightest hadrons. In this Letter, we provide the first calculation of resonance parameters of the $π(1300)$ from lattice quantum chromodynamics (QCD). For this purpose, recently derived state-of-the-art tools are adapted and applied both in the construction of three-hadron operators and for mapping finite-volume spectra to infinite-volume amplitudes, subsequently analytically continuing these to complex energies. For our heavy pion mass ensembles, we find a clear signal of the resonance. Making a simple assumption of vanishing pion mass dependence for the three-body force, but incorporating constraints from Chiral Perturbation Theory for all the two-body channels, enables a robust extrapolation to the physical point. Applying model averaging, we extract a pole position of $M_{π(1300)}=(1169\pm46)-i(62_{-62}^{+168})\,\MeV$ supporting values from phenomenology.

hep-lat↗
arXiv2026

Threshold cusp effects to measure masses of radiatively decaying hadrons: The $B_{s0}^*$ mass from the $Υϕ$ spectrum

Hadrons that decay predominantly into final states containing photons are notoriously difficult to detect at hadron colliders. Prominent examples are the yet-unobserved $B_{s0}^*$ and $B_{s1}$, the bottom partners of the $D_{s0}^*(2317)$ and $D_{s1}(2460)$, which are expected to exhibit exotic properties deviating from the conventional quark-model predictions of $\bar b s$ mesons. We propose a general, model-independent method to overcome this problem: when the target hadron has an attractive $S$-wave interaction with a companion hadron of precisely known mass, the line shape of a suitable final state develops a cusp at the pair threshold, or a peak just below it if the attraction binds, so that subtracting the companion mass returns the target mass, up to the binding energy in the latter case. As a proof of concept, a leading-order particle-dimer calculation of the $D\bar D_s K$ three-body system reproduces the $X(4274)$ structure in the LHCb $J/ψϕ$ distribution extracted from $B\to K J/ψϕ$, as a $D_{s0}^*\bar D_s$ threshold cusp driven by a nearby virtual-state pole, yielding $m_{D_{s0}^*}=(2322\pm6)$ MeV in agreement with its measured value and favoring $J^{PC}=0^{-+}$ for the $X(4274)$. Transferring the elastic three-body interaction to the bottom sector, we predict an analogous structure at the $B_{s0}^*\bar B_s$ threshold near $11.09$ GeV, making the $Υϕ$ invariant-mass distribution at the LHC a clean probe of the $B_{s0}^*$ mass.

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