Nuclear Theory·nucl-th

Fri·Oct 16, 2026Friday·October 16, 2026

10 papers—4 primary·6 cross-listed

01
Four-dimensional QCD equation of state with a critical surface from contours of constant entropy density
Tristan Gyure · Hitansh Shah · Lorenzo Formaggio · Francesco Di Clemente · Mauricio Hippert · Claudia Ratti · Volodymyr Vovchenko
We present a new lattice-QCD-based equation of state in the four-dimensional space of $(T, μ_{B}, μ_{Q}, μ_{S})$ which features a critical surface. This is obtained by generalizing the expansion of constant entropy density contours across the multiple conserved charges of QCD. Since this expansion permits the presence of a first-order phase transition, this is the first lattice QCD based equation of state in four dimensions with a critical surface. We parametrize the chemical potential space using spherical coordinates $(μ, θ, φ)$ , reducing the problem to a one-dimensional extrapolation in $(T, μ)$ at fixed angles that, together, span the full 4D space. By integrating the extrapolated entropy density, we reconstruct the pressure, from which we obtain the conserved charge densities, their susceptibilities and the speed of sound. Our results agree quantitatively with other lattice QCD constructions at low chemical potentials in the crossover region, and predict the emergence of a first-order phase transition at higher chemical potentials. We study equation of state properties in detail under heavy-ion collision (HIC) conditions for the strangeness and electric charge densities ( $n_{S} = 0, n_{Q} = 0.4 n_{B}$ ), where we find the CP location at $(T_{c}, μ_{B, c}, μ_{Q, c}, μ_{S, c}) = (115 \pm 7, 646 \pm 72, - 12 \pm 3, 117 \pm 16)$ MeV. In particular, we find a strong non-monotonic dependence of baryon number susceptibilities along the chemical freeze-out line at $s_{N N} ≲ 7.7$ GeV. Finally, we outline further applications of the 4D equation of state, including charge-neutral matter relevant to neutron stars and large isospin density relevant to cosmic trajectories in the early Universe.
nucl-th
02
Effective shear viscosity from the friction term in relativistic multi-fluid hydrodynamics
Christian Spieles · Marcus Bleicher
Within a three-fluid description of heavy-ion collisions, we investigate how nuclear friction can be used to model shear stress. In this multi-fluid approach, the individual fluids (projectile, target and fireball) are each treated as locally equilibrated ideal fluids at all times, while the local energy-momentum tensor of the superposed system exhibits characteristic anisotropies in its Landau rest frame. From the corresponding non-vanishing shear stress densities and the gradients of the Landau velocity field, we infer an ``effective shear viscosity'' according to the Navier-Stokes relation. We present the time evolution of the resulting values of $η_{eff}$ and the generated entropy density in nuclear collisions at different beam energies ( $1.23 A GeV \leq E_{lab} \leq 29.7 A GeV$ ). The resulting dimensionless ratio $η_{eff} / s$ is determined dynamically and self-consistently within the non-equilibrium friction model. We discuss the relation to previous calculations using alternative methods in hydrodynamics and to estimates based on microscopic transport models.
nucl-th
03
Algebraic coefficients of fractional parentage for three particles in the oscillator Jacobi basis
A.Stepšys · S.Mickevičius · D.Germanas
We present a closed-form construction of the orbital coefficients of fractional parentage (CFP) of three identical particles in the translationally invariant harmonic oscillator (HO) Jacobi basis. These are usually obtained by numerically diagonalizing a permutation operator in every energy-angular-momentum block. We show this diagonalization is unnecessary. Dressing the class operator $P_{13} + P_{23}$ with the single unitary $W = H^{1} Φ$ -- a fixed complexification followed by the HO bracket with mass parameter $d = 1$ -- turns it into the label swap times an integer. Every CFP is then the single ket $∣ K_{i, s} ⟩ = 2 Q^{s} W^{†} ∣ i ⟩$ -- a 2-scaled, sign-decorated column of the $d = 1$ bracket restricted to one $e_{2}$ -parity of the labels -- whose norm, $1 + s ⟨ i ∣ J ∣ i ⟩$ , is itself a single matrix element. The same $W$ carries label parity into swap symmetry, $W Q^{s} W^{†} = R^{s}$ , making the bookkeeping collapse. The eigenvalues are exact integers $\pm 2$ and $\pm 1$ labeling the $S (3)$ symmetry classes. The construction is uniform in $E$ , $L$ and symmetry class, reducing CFP-specific work from $O (n^{3})$ to $O (n^{2})$ in block dimension. An eigensolver remains only inside the bracket engine supplying $H^{1}$ , where all residual numerical error originates. We implement this on an SU(3)-basis bracket engine, checking all 1301 blocks up to $E = 50$ (912 366 CFPs, largest block $2907 \times 2907$ ): the $S (3)$ class assignment is exact everywhere. The residual numerical error (entirely the bracket engine's) stays below $1 0^{- 8}$ for $E \leq 35$ , restoring to $\leq 1 0^{- 22}$ via a quadruple-precision build beyond that. We further validate against independently generated antisymmetrization data of a production three-nucleon code, including the $^{3}$ He ground-state energy.
nucl-thphysics.comp-phNuclear Physics A .Volume 1077, 2027, 123…
04
$^{6}$ He structure from semiclassical 3D models based on $ab initio$ spatial correlations
Mengyao Huang · Tobias Frederico · Peng Yin · James P. Vary
We construct semiclassical 3D models based on $ab initio$ no-core shell model calculated two-body spatial correlations, which represent the preferred spatial configuration of nucleons for the ground state of $^{6}$ He. We demonstrate that the presence of the two valence neutrons distorts and flattens the $α$ core in $^{6}$ He, marking a clear departure from the tetrahedral configuration of free $^{4}$ He, and that the two valence neutrons and the center of mass of the $α$ cluster ( $α$ -CM) are roughly located at the three distinct vertices of an equilateral triangle. We compute the geometric parameters that commonly characterize $^{6}$ He structure, 1) $θ_{nn}$ , the opening angle of the two valence neutrons with respect to $α$ -CM, and 2) $r_{nn}$ , the root-mean-square separation between the two valence neutrons. Refining the model with uncertainty estimation, we obtain $θ_{nn} = 58. 5_{- 6.2}^{+ 5.4}$ degrees and $r_{nn} = 4.1 2_{- 0.43}^{+ 0.37}$ fm, agreeing with those calculated via a phenomenologically modified continuum-discretized coupled-channels method, while an interferometry experiment obtained a marginally higher $r_{nn}$ .
nucl-th
05
Machine-learning-based neutrino-event generation constrained by experimental measurements
Jose Luis Bonilla🇵🇱 · Krzysztof M. Graczyk🇵🇱 · Rwik Dharmapal Banerjee🇵🇱 · Beata E. Kowal🇵🇱 · Hemant Prasad🇵🇱 · Jan T. Sobczyk🇵🇱
We present a generative surrogate for charged-current $\nu_\mu$-nucleus scattering that can be calibrated to measured differential cross sections without event-level experimental samples. Starting from a theoretical interaction model, we fine-tune the surrogate to a MicroBooNE measurement and show that this also improves the description of observables not used in the fit. The calibrated model can also be applied to measurements from different neutrino fluxes without retuning, while retaining event-level predictions of outgoing-muon kinematics. Our results show that experimental cross sections can directly constrain generative event models while preserving their applicability across different observables and beam configurations.
↳ hep-phcs.LGhep-exnucl-ex+10 citations
06
The Sill distribution for all partial waves
Krzysztof Kyzioł · Francesco Giacosa
In high-energy physics, short-lived resonances appearing in scattering processes are described by energy-dependent spectral functions. The relativistic Breit--Wigner distribution (rBW), obtained as a relativistic extension of the standard Breit--Wigner form, is commonly employed owing to its simplicity. However, when the lower-energy threshold lies sufficiently close to the resonance peak, sizable deviations from the rBW line shape are expected. The Sill distribution provides a simple way to incorporate the physical threshold while preserving exact normalization, but in its original formulation it is inherently restricted to $s$ -wave decays. Here, we extend the Sill distribution to arbitrary partial waves. The correct $ℓ$ -wave threshold behaviour is implemented while exact normalization is retained through a simple modification of the mass term of the propagator. As a benchmark, the $p$ -wave Sill provides a very good description of the $Δ (1232)^{++}$ line shape up to 1.35 GeV and simultaneously reproduces its on-shell mass and width and its complex pole position, in agreement with independent determinations compiled by the PDG.
↳ hep-phhep-exnucl-th
07
Localized states of the non-Hermitian Schrödinger equation with a complex $1/ r^{2}$ potential: from Efimov states to bipolarons
Kyoka Tsunoda · Kazuki Yamamoto · Shimpei Endo
We study the non-Hermitian Schrödinger equation with a complex $1/ r^{2}$ potential, which bridges Hermitian few-body and non-Hermitian many-body physics: a real attractive potential describes Efimov bound states, whereas a purely imaginary one describes the medium-induced interaction between two heavy polarons at large separations. We solve this equation analytically, obtaining both the localized and continuum states, and compare the solutions with numerical and WKB results. As the imaginary part of the potential is increased, the energies of the Efimov bound states acquire imaginary parts and rotate counterclockwise in the complex plane. On approaching the purely imaginary potential, the excited states disappear sequentially, while the Efimov ground state is smoothly connected to a bipolaron state localized at short distance. Such a bipolaron is localized solely by the medium-induced imaginary interaction, without any attractive real potential. Solving the equation by finite differences, we find that each bipolaron state is necessarily accompanied by another localized state that oscillates on the scale of the mesh size and hence has no counterpart in the continuum limit. Each such pair coalesces at an exceptional point as the strength of the purely imaginary potential is varied, whereas the coalescence is lifted once a finite real part is introduced. We show that this behavior originates from a symmetry of the discretized Schrödinger equation that holds for any purely imaginary potential.
↳ cond-mat.quant-gasnucl-thquant-ph
08
Nuclear PDFs
K. Greve · T. Ježo · M. Klasen · A. Kusina · F.I. Olness · J. Wissmann
We summarize the current status of global analyses of model-independent nuclear parton distri- bution functions (nPDFs). A carefully conducted global analysis of nPDFs in the era of heavy-ion collisions at the LHC is key to gaining access to new information through the data measured by the collider experiments. This implies incorporating as large and diverse a dataset as possible, as well as the latest theoretical calculations, in order to capture a faithful representation of nuclear structure that reflects the current state of knowledge. On the other hand, the uncertainties in par- ticular must reflect what is not known. For many observables, nuclear uncertainties represent the most significant source of error in current theoretical models; nevertheless, they should not, where possible, be artificially constrained (for example, through parameterization bias). The quality of a global (n)PDF fit is often measured by the $χ^{2}$ per degree of freedom, which is approximately one for a fit that describes the data precisely without overfitting. Many additional aspects allow to characterize a good fit, e.g. the $χ^{2}$ distribution per experiment, the shape of the uncertainties, and the sensitivity to different types of data. We inspect various fit quality indicators for fits obtained by different groups. Morever we pay close attention to the $A$ -dependence. Traditionally, the A-dependence had a primarily functional role in global nPDF analyses and comparatively less attention has been paid to it when characterizing a fit. However, we have now entered the era of nuclei collisions at the LHC and need to investigate the $A$ -dependence properly.
↳ hep-phnucl-th
09
Extracting the Central Sound Speed of Neutron Stars through Intrinsic Stellar Structure Scaling with Reduced Equation-of-State Model Dependence
Jian-Hao Shi · Bao-Jun Cai · Yu-Gang Ma
Inferring neutron star (NS) core properties remains sensitive to the assumed equation of state (EOS). Using a nonlinear Walecka model with generalized $ω$ -meson self-interactions, we construct six EOS families, each containing $1 0^{3}$ samples subject to identical nuclear and astrophysical constraints. Despite satisfying these constraints, the families predict markedly different high-density behavior and core properties; higher-order $ω$ interactions can also generate interior sound-speed peaks within purely nucleonic matter. To reduce this model dependence, we use the intrinsic and perturbative analysis of the dimensionless Tolman--Oppenheimer--Volkoff equations (IPAD-TOV) to derive an EOS-independent estimator $Θ (X, Ψ)$ of the central squared sound speed $s_{c}^{2}$ , where $X = P_{c} / ε_{c}$ is the central EOS-parameter and $Ψ$ characterizes the stellar-mass response to central energy density. For canonical $1.4 M_{⊙}$ NSs, $Θ$ correlates linearly with $s_{c}^{2}$ across the families ( $R^{2} \approx 0.94$ - $0.99$ ). The family-dependent relations converge toward a common optimum: minimizing their dispersion yields $s_{c}^{2} \approx 0.523 \pm 0.044$ and $X \approx 0.17 3_{- 0.018}^{+ 0.020}$ for all six families, or $s_{c}^{2} \approx 0.496 \pm 0.092$ and $X \approx 0.16 7_{- 0.029}^{+ 0.030}$ when the family without $ω$ self-interaction is excluded. The corresponding radii are $11.7 3_{- 0.92}^{+ 1.08}$ and $11.9 2_{- 1.28}^{+ 1.76} km$ , respectively, with $95%$ confidence intervals. A unified scaling fitted to all EOS samples supports both extractions. These results show how common stellar-structure relations can constrain NS core properties with reduced EOS-model dependence, even when existing observations cannot distinguish among the underlying EOS descriptions.
↳ astro-ph.HEgr-qcnucl-th
10
Radiative transitions of hybrid mesons
Shahriyar Jafarzade · Eric S. Swanson
Radiative transitions of hybrid mesons probe nonperturbative gluodynamics, particularly the interplay between quark spin and gluonic excitations, and are of direct relevance to the search for exotic hadrons at Jefferson Lab. We calculate these transitions across the lowest charmonium-hybrid multiplet in a constituent gluon model based on Coulomb-gauge QCD in the heavy quark expansion. The results for $Y \to η_{c} γ$ and $Y \to χ_{c 0} γ$ overlap the width obtained in quenched lattice computations, whereas $η_{c 1} \to J / ψ γ$ is substantially below the corresponding lattice estimate.
↳ hep-phhep-exhep-latnucl-th

Affiliations

first authorsco-authorsvia INSPIRE