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Physics Nuclear Theory

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Wednesday 7 October

Nuclear Theory · Cross-listed from hep-th

The hidden sunrise in the energy-energy correlator

The energy-energy correlator (EEC) is one of a handful of collider observables that can be computed analytically to high orders. As an energy-weighted cross-section, it exposes features of quantum field theory that scattering amplitudes do not, and it can be compared directly to data. As with scattering amplitudes, the analytic expressions require special functions beyond polylogarithms. In Henn et al., the EEC was computed to next-to-next-to-leading order (NNLO) for $\mathcal{N}=4$ super Yang-Mills (SYM) theory, and the result is expressed in terms of both harmonic polylogarithms (HPLs) and one two-fold integral, which contains elliptic curves. In this work, we report a complete result including the remaining elliptic sector, which is closely related to the sunrise Feynman integrals. We find the $j$-invariants of the EEC and the sunrise agree identically under a Möbius map, and thus the elliptic sector in the EEC lives on the $\Gamma_1(6)$ modular curve of the sunrise integral. We then express the answer in terms of iterated Eisenstein integrals, which can be evaluated to high precision quickly. The analytic form also allows a first study of the EEC Landau bootstrap, and the understanding of its function space in $\mathcal{N}=4$ SYM offers a concrete handle on the elliptic sector of the EEC in QCD. Many of the technical results in this paper were completed with AI under human supervision.
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Nuclear Theory · Cross-listed from hep-ph

On the leptonic decays of the $\tau$

In view of potential future improvements in the branching fractions of leptonic $\tau$ decays $\tau\to\ell\nu_\tau\bar\nu_\ell$, $\ell=e,\mu$, it is timely to derive the corresponding Standard-Model predictions beyond next-to-leading order. We provide such benchmark values for the fully inclusive decay widths, profiting from higher-order corrections that have been studied in the $\overline{\text{MS}}$ scheme up to three-loop order in the case of muon decay. To obtain complete next-to-next-to-leading-order results, we perform a numerical calculation of the two-loop coefficients in the on-shell scheme, including an evaluation of the hadronic effects, and compare the leptonic results to the known $\overline{\text{MS}}$ coefficients where available. Our final results are presented in the $\overline{\text{MS}}$ scheme, to be able to also include an estimate of the three-loop contribution. In addition, we comment on radiative and rare decays, whose consideration is critical for the conversion between $\overline{\text{MS}}$ and on-shell results.
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Nuclear Theory · Cross-listed from gr-qc

Impact of site-dependent noise configurations on Einstein Telescope science at low frequency

We investigate how site-dependent Newtonian noise configurations affect the scientific performance of the Einstein Telescope (ET), a third-generation gravitational-wave observatory. We compare the baseline triangular and 2L detector geometries, with arm lengths of 10km and 15km, respectively, under three site-dependent noise scenarios constructed from measurements in the Euregio Meuse-Rhine region, Lausitz, and Sardinia. The differences among the adopted sensitivities are concentrated mainly below 10Hz and thus have the greatest impact on science cases that rely on the early inspiral of light objects, or on massive systems whose characteristic frequencies lie in the low-frequency band. We quantify these effects for four ET science cases: (1) detection and parameter estimation of intermediate-mass black-hole binaries; (2) post-Newtonian tests of General Relativity based on the early inspiral; (3) black-hole ringdown spectroscopy with massive remnants; and (4) early-warning and sky-localization capabilities for binary neutron-star mergers. Using Bayesian parameter estimation, we find that improved low-frequency sensitivity can significantly enhance ET's scientific performance across all four cases. Within the configurations considered here, a 2L network in which at least one of the two L-shaped interferometers attains a noise level comparable to that assumed for the Sardinian site generally provides better performance than the triangular configurations. Conversely, strong degradation of the sensitivity below 10Hz can substantially reduce, and in some regimes offset, the scientific gains expected from increasing the arm length from 10km to 15km. These results highlight the importance of site selection and low-frequency noise mitigation, alongside network geometry, for fully realizing ET's scientific potential in the largely unexplored frequency band below 10Hz.
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All authors Francesco Crescimbeni, Matteo Di Giovanni, Davide Rozza, Andrea Contu, Alessandro Cardini, Domenico D'Urso, Carlo Giunchi, Luca Naticchioni, Marco Olivieri, Mike Lindner, Andreas Rietbrock, Paolo Pani

Nuclear Theory · Cross-listed from hep-ph

An effective macroscopic description of early-stage chemical equilibration in high-energy heavy-ion collisions

The early stage of high-energy heavy-ion collisions undergoes transient dynamics, evolving non-perturbatively from a gluon-rich to a chemically equilibrated quark-gluon plasma. We develop an effective hydrodynamic approach that casts the microscopic quark equilibration dynamics as the relaxation evolution of an effective macroscopic bulk viscous pressure. The evolution of this effective bulk viscous pressure captures quark chemical equilibration and the breaking of conformal symmetry, thereby providing a smooth onset of the lattice QCD equation of state after a few fm/$c$ in the hydrodynamic evolution. We investigate phenomenological effects of this macroscopic description on final-state observables in $\mathrm{Pb}+\mathrm{Pb}$, $\mathrm{O}+\mathrm{O}$, and $\mathrm{p}+\mathrm{Pb}$ collisions at the Large Hadron Collider.
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Nuclear Theory · Cross-listed from hep-ph

Two-pion exchange potential in $D^{(*)}D^{(*)}$ system

Motivated by the recent HAL QCD lattice results on the $D$ - $D^*$ potential at nearly physical point $m_\pi=146.4$ MeV, we theoretically examine effects from the two-pion exchange in (semi-)long-range parts of the $DD^*$ system. We employ the framework of heavy-meson chiral perturbation theory with heavy-quark spin symmetry. The pion exchanges up to next-to-next-to-leading order (N$^2$LO) are taken into account. In order to separate the (semi-)long-range contributions, we apply the dispersion-relation method to one-loop diagrams. As a result, we find that the lattice data on the $D$ - $D^*$ potential in the regime of $0.5\, {\rm fm} \lesssim r$ is reproduced by adjusting the unknown $O(p^2)$ couplings. In particular, it turns out that contributions from isospin-independent N$^2$LO triangle diagrams play a central role in reproducing the lattice data of the form $V^{DD^*}\sim \left({\rm e}^{-m_\pi r}/r\right)^2$ in $1.0\, {\rm fm} \lesssim r\lesssim 2.0\, {\rm fm}$. We also present predictions of other $D^{(*)}$ - $D^{(*)}$ potentials, where the similar two-pion exchange tail is predicted. Our findings provide useful information on the (semi-)long-range regime of the $D$ - $D^*$ potential focusing on two-pion exchanges.
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Nuclear Theory · Cross-listed from astro-ph.HE

Composition $g$-modes and $f$-modes of neutron stars with gravitationally coupled dark matter: degeneracy with the symmetry energy

Composition gravity ($g$) modes have been proposed as a probe of dark matter in neutron stars, on the basis of one-fluid models in which the dark matter is in chemical equilibrium with the neutrons. We computed in full general relativity the $g$- and $f$-modes of stars in which fermionic dark matter couples to the nucleons only through gravity, and inferred the dark mass fraction from NICER and GW170817 data with a prior that comprises $63$ nucleonic equations of state (EOSs) from Skyrme and relativistic mean-field functionals, among them the models of Dutra et al. and of the compilation of Sun, Bhattiprolu and Lattimer. The data do not prefer dark matter and bound its mass fraction at $0.12 - 0.26$ ($95\%$) for particle masses of $0.5$–$2$~GeV. Coupled only through gravity, the dark matter reaches the buoyancy that restores a $g$-mode only through the gravitational field: a dark core holding $10\%$ of the mass raises the fundamental $g$-mode by $10-15\%$ at fixed EOS, several times less than in the one-fluid picture. The larger effect is indirect: a star with a dark core needs a stiffer nucleonic EOS, and since the $g$-mode is set by the density dependence of the symmetry energy, its frequency reflects the EOS that the star requires rather than the dark matter it contains; at fixed radius and symmetry energy a core holding $10\%$ of the mass is equivalent to a change of the slope $L$ of $9-12$~MeV. The $f$-mode, which follows the mean density, behaves differently: a dark core breaks the relation between the $f$-mode and the tidal deformability by about one percent, far outside the posterior spread of nucleonic stars.
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Nuclear Theory · Cross-listed from hep-lat

Bjorken sum rule down to the elastic region from the lattice

We present a lattice determination of the Bjorken sum rule via a Feynman-Hellmann calculation of the polarised forward Compton amplitude. We evaluate the lowest moment of the nucleon's polarised structure function, $g_1$, which we calculate over the range $0.5\lesssim Q^2 \lesssim 8$ GeV$^2$, covering both non-perturbative and perturbative regions. We compare our calculated moments with the Bjorken sum rule, and discuss implications for the calculation of higher-twist effects, as well as evaluations of $\alpha_s(Q^2)$ from hadronic quantities to complement phenomenological and other lattice methods.
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All authors J. A. Crawford, A. Hannaford-Gunn, R. Horsley, P. E. L. Rakow, T. G. Schar, G. Schierholz, H. Stüben, R. D. Young, J. M. Zanotti

Nuclear Theory · Cross-listed from hep-ph

Tensor-structure sum rules for spin-1 targets at all $Q^2$

We derive a family of sum rules for the tensor structure functions of targets with spin one or higher, rooted in exact Siegert relations. The mixed rank-two $TT$–$LT$ degeneracy combines amplitudes of opposite crossing parity and, under the superconvergence assumption, gives $\int_0^1\! d x\,[b_2(x,Q^2)-3b_4(x,Q^2)]=0$ in the Hoodbhoy–Jaffe–Manohar (HJM) basis. At large $Q^2$, the protected moment agrees with Detmold's target-mass-complete twist-two OPE; in the strict Bjorken projection, and if $b_4$ is neglected, it reduces to the Efremov–Teryaev (ET)-type condition $\int_0^1\! d x\,x\,b_1(x)=0$, whose measured-range footprint is consistent with the HERMES data. Two further rank-two combinations connect crossing-even amplitudes and determine the corresponding subtraction functions through weighted HJM moments. The protected combination can be isolated through separated tensor $LT$ and $TT$ responses, providing a direct experimental strategy for JLab and, ultimately, a future EIC.
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Nuclear Theory

Novel chiral-wobbling excitation mode in atomic nuclei

The properties of high-spin band structures in $^{74}$Br nucleus are investigated using the triaxial projected shell model (TPSM) and particle triaxial rotor (PTR) approaches. It has already been demonstrated that lowest three bands observed in $^{74}$Br have a characteristic feature of a chiral-wobbler. The first excited band is the chiral partner of the yrast band and the second excited band is the wobbling band based on the yrast state. We have analyzed the third excited band using the TPSM and PTR calculated quantities, and a unique character of this band is revealed. It is predicted that this band is the chiral partner of the second excited band, and is also a wobbling mode based on the first excited band.
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All authors Kouser Qureshie, S. P. Rouoof, J. A. Sheikh, Nazira Nazir, G. H. Bhat, S. Jehangir, N. A. Rather, R. J. Guo, S. Y. Wang, C. Liu, Q. B. Chen, S. Frauendorf

Nuclear Theory · Cross-listed from astro-ph.HE

Axion effects on hybrid stars: earlier hadron-quark phase transition and larger quark-matter cores

We investigate the effects of axions on the properties of hybrid stars and the possible hadron-quark phase transition in their cores within the Maxwell construction. The hadronic phase is described by the improved isospin- and momentum-dependent interaction (ImMDI) model, while the quark phase is modeled by a three-flavor Nambu–Jona-Lasinio (NJL) model with an axion-modified Kobayashi-Maskawa-'t Hooft (KMT) interaction. Based on the obtained equation of state, we analyze the properties of hybrid-star matter, including the speed of sound and the polytropic index, as well as the structural characteristics of hybrid stars, including the mass-radius relation and the properties of the quark core. Our study shows that the axion field $a/f_a$ slightly stiffens the equation of state of quark matter but significantly shifts the hadron-quark phase transition to lower baryon densities. These two effects compete in influencing the equation of state of hybrid-star matter, and may even soften it, which is different from the effect of the vector coupling interaction. Consequently, the axion field has only a weak influence on the maximum mass of hybrid stars, but significantly increases the mass and radius of the quark core, especially for weak vector coupling. These results indicate that, within the present Maxwell construction, the primary role of the axion is to promote an earlier hadron-quark transition at low densities and to enlarge the quark core.
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All authors Yu-Heng Liu, He Liu, Hong-Ming Liu, Min Ju, Xu-Hao Wu, Peng-Cheng Chu

Tuesday 6 October

Nuclear Theory

The Fock representation of the Coulomb interaction. II. Three-body bound states

The Fock representation of the unscreened Coulomb interaction, a block of separable terms with analytic form factors and strengths, is included in the momentum-space Faddeev kernel of three-body bound states beside the separable nuclear terms. A bound-state kernel evaluates every pair interaction at negative pair energies, where the representation is exact and convergent, so the Coulomb force enters without a screening radius and without a limiting procedure. With this representation the $^{3}$H–$^{3}$He splitting of the CD~Bonn potential is in very good agreement with the exact Faddeev result; in the $S$-wave model space the Coulomb-distorted form factors give about half the Fock value. The Feshbach–Schur projection of the Pauli-forbidden states acts on the forbidden eigenstates of the nuclear-plus-Coulomb pair Hamiltonian, the Coulomb-dressed states obtained inside the extended separable problem, as the eigenstate condition requires. The Coulomb displacements of the cluster-model states of $^{6}$Li, $^{9}$Be and $^{12}$C and of the $^{9}$Be–$^{9}$B mirror pair are given. The eigenstate condition is tested in $^{9}$Be and $^{12}$C by replacing the Coulomb-dressed forbidden states with those of the Coulomb-subtracted interaction and with the oscillator functions of the orthogonality-condition model.
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Nuclear Theory · Cross-listed from hep-ph

Medium modification of pseudoscalar meson structure: Distribution amplitudes and parton distribution functions

In this work, we present a comprehensive investigation of the in-medium properties and internal structure of pseudoscalar mesons within the light-front quark model (LFQM), employing both Gaussian and power-law (PL) wave functions. We incorporate in-medium quark properties obtained from the quark-meson coupling (QMC) model into the LFQM, thereby providing a unified description in terms of quark degrees of freedom. We investigate the distribution amplitudes (DAs) and parton distribution functions (PDFs) of $\pi^{\pm}$, $K^{\pm}$, $D^{\pm}$, and $B^{\pm}$ mesons in symmetric nuclear matter. We find that the evolved in-medium pion DA and PDF are strongly suppressed at intermediate $x$, indicating substantial medium-induced modifications to the meson structure. The signs and magnitudes of the modified Gegenbauer coefficients $a_n'$, the parameter $\xi$, and the Mellin moments of the DAs and PDFs in nuclear matter are found to be consistent with other theoretical predictions. In particular, the second Gegenbauer coefficient of the pion DA, $a_2^\pi$, increases substantially with nuclear density, from $0.096$ ($0.057$) in vacuum to $0.232$ ($0.202$) at normal nuclear density for the Gaussian (PL) wave function, respectively. This increase indicates a broader pion DA and an enhanced second Gegenbauer moment in the nuclear medium. We further calculate the pion PDF ratio $R_{\rho/0}^{\pi}$ and find that its $x$ dependence exhibits a behavior qualitatively similar to the observed European Muon Collaboration (EMC) effect, highlighting the important role of QCD evolution in shaping the medium-modified partonic structure. Finally, we compare the calculated free-space $\pi^{\pm}$ and $K^{\pm}$ DAs with available free-space lattice QCD results and find reasonable agreement.
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All authors Muhammad Ridwan, Parada Tobel Paraduan Hutauruk, Ahmad Jafar Arifi, Kazuo Tsushima

Nuclear Theory · Cross-listed from hep-ph

Compositeness of near-threshold exotic hadrons in systems with Coulomb and short-range interactions

We study the universal properties of near-threshold $s$-wave eigenstates in systems with Coulomb plus short-range interactions by analyzing their compositeness. Using a nonrelativistic effective field theory, we derive a relation between the compositeness of near-threshold states and the Coulomb scattering length, the Coulomb effective range, and the Bohr radius. We find that the long-range Coulomb interaction modifies the threshold behavior expected from the low-energy universality of purely short-range systems. When the Coulomb interaction is relatively weak compared with the short-range interaction, a remnant of the low-energy universality associated with the short-range interaction survives, leading to composite-dominant near-threshold eigenstates. In contrast, in the strong-Coulomb regime, this remnant disappears.
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Nuclear Theory

Probing alternating-parity bands in even-even nuclei within an axial quadrupole-octupole-hexadecapole collective model

An analytic quadrupole octupole hexadecapole (QOH) axially symmetric model is used to provide a coherent framework for describing both octupole vibrations and octupole deformations in even-even nuclei. The model offers analytical solutions for the energy spectra and electric E1, E2 and E3 transition probabilities. The spectroscopic features are governed by three physically meaningful parameters: the stiffness constant $c_2$, the hexadecapole deformation parameter $\phi_{02}$, and the sextic potential parameter $\alpha$, directly linked to $\phi_{01}^{\pi}$ which represents the interplay between the quadrupole and octupole deformations for both even and odd angular momentum states. The model is applied across a broad range of nuclei including Ra, Th, Sm, Ba, U, Gd, Nd, Rn, Pu, Xe, and Ce isotopes, effectively capturing the characteristic properties of alternating parity bands. Detailed calculations of E1, E2, and E3 transition matrix elements have been performed for $^{220,222}$Rn, $^{222,224,228}$Ra, and $^{144,146}$Ba yielding results in good agreement with available experimental data.
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Nuclear Theory

One-nucleon intermediate-state contribution to the pion-coupled four-quark condensate in the $N$–$J/\psi$ interaction

The density dependence of four-quark condensates is a major source of uncertainty in operator product expansion analyses of hadrons in nuclear matter. Their determination is particularly relevant to the $N$–$J/\psi$ interaction, whose long-range attraction appears to be well described by two-pion exchange in lattice QCD. We evaluate the pion-coupled four-quark condensates using one-nucleon intermediate-state insertions and incorporate them into the QCD sum rule analysis for the $J/\psi$ meson in nuclear matter. At normal nuclear matter density, we find a mass shift of $-1.75$ MeV, which is of the same order as the approximately $-5$ MeV attraction inferred from a two-pion exchange potential fitted to lattice QCD results. A previous factorization-based treatment required the introduction of unnaturally large phenomenological parameters to obtain a comparable shift. Our result shows that one-nucleon intermediate states provide a significant contribution to the in-medium four-quark condensates and improve the connection between the operator product expansion and the long-range $N$–$J/\psi$ interaction.
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Nuclear Theory · Cross-listed from hep-ph

Comparison of pair-source and correlation-function based Lévy analyses in Ar+Sc system at SPS energies

Non-Gaussian features in femtoscopic observables have been observed in high-energy collisions, yet their origin remains not fully understood. This study focuses on the phenomenological approach to femtoscopy, going beyond the Gaussian approximation by utilizing Lévy-stable source distributions. Using UrQMD simulations, three-dimensional analyses of pion sources are done in central Ar+Sc collisions, and the extracted parameters are studied as functions of transverse mass and collision energy. This analysis contributes to a hadronic baseline motivated by past and future research work in femtoscopy at the NA61/SHINE experiment. A direct methodological comparison is presented between parameters extracted from the projections of three-dimensional pair-source distribution and those obtained from the three-dimensional correlation function, reconstructed via Fast Fourier Transform.
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2610.08141 · 6 Oct · 12 pages, 7 figures, proceedings of the 18th Workshop on Particle Correlations and Femtoscopy (WPCF 2026), Budapest, Hungary, May 18 - 22, 2026, to be submitted to the International Journal of Modern Physics A, Thematic Issue on Paritcle Correlations and Femtoscopy

Nuclear Theory · Cross-listed from astro-ph.GA

Early galactic heavy element production - a phenomenological approach

The high abundances of heavy elements in early, extremely metal-poor stars is still an unsolved issue in Galactic Chemical Evolution (GCE). Standard approaches, such as mergers of neutron stars, have several difficulties, and several non-merger production channels are under discussion. Here, we suggest that pre-enriched inflow might explain the heavy elements seen in low metallicity stars in the Galaxy. To characterize the conditions of the origin of such a pre-enriched infall, we apply a heavy-element freeze-out approach. We characterize the initial distribution by Lagrange parameters which are nonequilibrium generalizations of temperature and chemical potentials. We then speculate about the origin of such a pre-enriched infall: The required nonequilibrium conditions might be found in cosmological fluctuations in the early Universe, and might be connected to the formation of primordial black holes.
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All authors B. Wehmeyer, D. Blaschke, F.K. Röpke, G. Röpke

Nuclear Theory

Influence of nuclear matter properties on heavy-ion sub-barrier fusion reactions

Interactions between two nuclei in nuclear reactions are calculated by applying the energy density method to nuclear energy density functionals. We particularly focus on the effects of nuclear mat- ter properties on fusion reactions by using the Korea-IBS-Daegu-SKKU (KIDS) density functional. Uncertainty of the equation of state in symmetric nuclear matter is accounted for by using different values of the incompressibility and the effect is explored with the 40Ca+40Ca system. Though not prominent, a systematic dependence on the incompressibility can be inferred from the fusion cross sections. The effect of asymmetric nuclear matter is described by adopting the symmetry energy parameters that are consistent with the neutron star data, but still have large uncertainties. In the application to the 48Ca+48Ca and 16O+208Pb systems, we observe a systematic dependence on the stiffness of the symmetry energy. We also consider the uncertainty of the effective mass of the nucleon in nuclear medium. We employ models that have isoscalar and isovector effective masses varying from minimum to maximum values in the allowed ranges. Contrary to incompressibility and symmetry energy, fusion cross sections are insensitive to the isoscalar and isovector effective masses. Without any phenomenological adjustment or modification of the model, fusion processes are described accurately by applying the interaction potentials derived from nuclear structure mod- els
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All authors Ki-Seok Choi, Hana Gil, Kouichi Hagino, W. Y. So, Chang Ho Hyun, K. S. Kim

Monday 5 October

Nuclear Theory · Cross-listed from hep-ph

Neutral Pseudoscalar Mesons, Hadron Light-by-Light Scattering, and the Muon Anomalous Magnetic Moment

Continuum Schwinger function methods\linebreak (CSM) are used to provide estimates of pseudoscalar meson pole contributions - $\mathsf P = \pi^0$, $\eta$, $\eta^\prime$, $\eta_c$, $\eta_b$ - to the muon anomalous magnetic moment. The analysis framework is benchmarked via successful applications to an array of other meson and nucleon properties. It uses the leading order CSM truncation, augmented by a parametrisation of non-Abelian anomaly effects, to deliver an internally consistent treatment of all the pseudoscalar–two-photon transition form factors required to complete such a calculation. Employing those form factor predictions, the following results are obtained (units $10^{-11}$): $a_\mu^{\pi^0} + a_\mu^{\eta} + a_\mu^{\eta^\prime} = 84.3(0.8)$ - a value lower than some other contemporary estimates; $a_\mu^{\eta_c} = 0.82(0.04)$; and a value of $a_\mu^{\eta_b}$ that is negligible. The total contribution is $\sum _{\mathsf P} a_\mu^{\mathsf P} = 85.1(0.8) / 10^{11}$. Our analysis suggests that this pole contribution to $a_\mu$ is still not known with precision better than 5%.
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All authors Cong-Cong Ye, Yu-Chen Hu, Minghui Ding, Craig D. Roberts

Nuclear Theory · Cross-listed from hep-ph

Born-Oppenheimer Effective Field Theory as a unified framework for all the XYZ exotics

We discuss some recent applications of the Born-Oppenheimer effective field theory (BOEFT) formalism, directly derived from QCD and grounded in scale separation and symmetries, to hadronic systems containing two heavy quarks. This formalism provides a unified description of both ordinary and exotic quarkonium states (i.e., hybrids, tetraquarks, pentaquarks, ...) dynamics via coupled Schrödinger equations, whose potentials are computable on the lattice. The aim is to unveil the elusive pattern of the XYZ exotic hadrons. Here, we apply the BOEFT formalism to the $\chi_{c1}(3872)$, $T_{cc}(3875)^+$, and their bottom counterparts. Moreover, we assess the threshold effects on the quarkonium spectrum and discuss the inclusive production cross section for $\chi_{c1}(3872)$.
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Nuclear Theory · Cross-listed from gr-qc

Gravitational-Wave Propagation in Quadratic Einstein–Cartan–Dirac Gravity: A Conditional Tensor-Sector Analysis

We study leading tensor propagation in a specified first-order Einstein–Cartan–Dirac theory with the parity-even quadratic torsion invariant $\zeta T_{\lambda\mu\nu}T^{\lambda\mu\nu}$. Eliminating the nondynamical Lorentz connection on the regular algebraic branch gives a metric description containing the Einstein–Hilbert term, the Dirac action, and a $\zeta$-dependent axial–axial fermion contact interaction. We evaluate the transverse-traceless (TT) Hessian under an explicit factorized mean-field closure, with vanishing connected-current TT response, vanishing first- and second-order TT response of the factorized current scalar, and zero intrinsic TT anisotropic stress. Under these imposed conditions, the leading two-derivative tensor action has $Q_T=M_{\rm Pl}^{2}$ and $c_T=1$, with no leading dispersion or propagation-induced polarization splitting. These conditions do not follow from isotropy of the one-point axial current, and the present calculation does not derive them from a microscopic fermionic state. The result is therefore conditional and does not establish a general Einstein–Cartan–Dirac no-go theorem. We identify the connected-current TT response that must be calculated to extend the result beyond this closure. Published NANOGrav summaries are used only to check consistency with the GR-like prediction; no new timing-data likelihood or constraint on $\zeta$ is obtained.
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All authors Zhi-Fu Gao, Biao-Peng Li, Na Wang, Hui Wang, Guo-Qiang Jin, Zhoujian Cao, Luiz C. Garcia de Andrade

Nuclear Theory · Cross-listed from astro-ph.HE

Radial Modes of Hot Neutrino-Trapped Hybrid Stars

We study the fundamental radial oscillation mode of hot hadronic and hybrid stars under conditions relevant to proto-neutron stars and neutron-star merger remnants. The stellar backgrounds are constructed from finite-temperature equations of state with a covariant density functional for hadronic matter and three-flavor 2SC quark matter at high density. We consider two isentropic configurations in both neutrino-transparent matter and with trapped electron neutrinos at fixed electron-lepton fraction. For trapped matter we compare a Gibbs-type construction with global electron-lepton-number conservation to a locally constrained Maxwell-type prescription; in both cases the fixed-entropy stellar equation of state contains an extended coexistence region and no sharp interface requiring conversion rate-dependent junction conditions. We find that the fundamental radial frequency follows the corresponding purely hadronic sequence up to the onset of deconfinement and then decreases relative to it. Neutrino trapping shifts this dynamical signature to larger stellar masses and central pressures, reflecting the delayed appearance of quark matter. The Maxwell-type prescription moves the onset of deconfinement to still higher central pressure and produces a more rapid decrease of the mode frequency near the stability limit. We quantify the deconfinement effect through the fractional frequency shift between hybrid and hadronic stars at equal gravitational mass. These results show that the fundamental radial mode provides a sensitive dynamical probe of the interplay among entropy, lepton trapping, phase-transition construction, and quark deconfinement in hot compact stars.
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Nuclear Theory

Nuclear structure functions and EMC effect in a relativistic deuteron

We investigate the longitudinal momentum distributions (LMDs) of the deuteron using a relativistic deuteron wave function derived from a realistic nucleon-nucleon (NN) interaction within the light-front framework. By convoluting the deuteron LMDs with the nucleon parton distribution functions (PDFs), we analyze the unpolarized, longitudinally polarized, and tensor-polarized structure functions of the deuteron. In addition, we also study the EMC effect in the weakly bound deuteron through the structure function ratio $F_{2}^{D}/(F_{2}^{p}+F_{2}^{n})$, which deviates from unity due to nuclear binding and Fermi motion effects.
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All authors Xianghui Cao, Bheemsehan Gurjar, Vladimir A. Karmanov, Yang Li

Nuclear Theory

Interplay between final-state nucleon distortion, nuclear transparency and intranuclear cascade models in neutrino event generators

We present an overview of the relativistic distorted-wave impulse approximation (RDWIA) for lepton-nucleus scattering, with emphasis on its use as an elementary-vertex model in event generators. In the intermediate-energy regime, neutrino-nucleus event generators typically follow a two-step scheme: an elementary neutrino-hadron interaction is first generated, and the outgoing hadrons are then propagated through the nuclear medium using an intranuclear cascade (INC) model. Using exclusive and inclusive $(e,e'p)$ and $(e,e')$ data for several nuclei, we assess how nuclear effects should be distributed between these two stages. In particular, we identify which effects should be included in the elementary-vertex description and which can be treated more effectively by the INC, while addressing the consistency required between both components. We also discuss nuclear transparency in quasielastic scattering. We show that it is directly connected to $(e,e'p)$ data below the two-nucleon knockout threshold; and depends on the full nuclear density distribution and the initial position of the nucleon, therefore, it varies substantially with the shell from which the nucleon is removed. We demonstrate that this shell dependence can be incorporated straightforwardly into the INC component of event generators.
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All authors R. González-Jiménez, A. Nikolakopoulos, J. McKean, K. Niewczas, T. Franco-Munoz, J. García-Marcos, M. Hooft, N. Jachowicz, G. D. Megias, J. M. Udías

Nuclear Theory

Exact resummation of hydrodynamics for arbitrary elastic scattering integral

We consider monoenergetic particles propagating in three spatial dimensions and undergoing elastic scattering off an external medium. For an arbitrary rotationally invariant scattering kernel, we derive an exact expression for the frequency- and wave-number-dependent diffusivity, thereby resumming the hydrodynamic gradient expansion to all orders. The result is an infinite continued fraction, with each successive level determined by the relaxation rate of a higher angular harmonic of the collision operator. Thus, while ordinary diffusion probes only the first angular harmonic, the resummed constitutive relation retains information about the full scattering kernel. We show how this microscopic information manifests itself in the analytic structure of the diffusivity and in the response to a localized external source. In particular, deviations from Fick's law decay exponentially with distance, at a rate determined by the nearest complex-wavenumber singularity of the inverse diffusivity. Thus, exponentially small departures from hydrodynamics retain detailed information about the underlying scattering process.
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Nuclear Theory

$^{86}$Ni: Possible doubly-magic nucleus with neutron halo

Self-consistent mean-field calculations with semi-realistic interactions have predicted that $N=58$ becomes a magic number near Ni, where the $n2s_{1/2}$ orbit is the highest occupied state. As the last neutrons occupy the shallowly bound $s$-orbit, $^{86}$Ni can be highly distinctive, being a doubly-magic nucleus and possessing a neutron halo simultaneously. With minimal ambiguity in the many-body wave function, it may provide a unique opportunity to investigate, e.g., reaction mechanisms in halo nuclei. As an example, we present differential cross sections for proton elastic scattering predicted via a folding potential without local approximations, and compare them with those obtained using an empirical potential.
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Sunday 4 October

Nuclear Theory

Neutrino Emissivity in $\Delta$-direct-Urca Processes Inside Neutron Stars

Observed massive neutron stars may contain heavier baryons such as $\Delta$ isobars and hyperons in their interiors. In the presence of $\Delta$ isobars, the neutrino emitting nucleonic Urca processes can be accompanied by $\Delta$-direct-Urca processes involving nucleons and hyperons. Over the past three decades, significant theoretical effort has been dedicated to understanding the thermal evolution of neutron stars with the nucleonic and hyperonic Urca processes emerging as dominant cooling mechanisms. While recent studies have proposed the presence of $\Delta$ isobars in the dense core of neutron stars, the specific contribution of the $\Delta$-direct-Urca process to neutrino emissivity has not been explicitly quantified. In this work, we address this gap by deriving the neutrino emissivity associated with $\Delta$-direct-Urca processes in dense matter inside neutron stars. We show that once $\Delta$-direct-Urca processes become kinematically allowed, their emissivities are comparable to those of nucleonic-direct-Urca processes.
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Nuclear Theory · Cross-listed from astro-ph.HE

Fast Bayesian Updating of the Neutron-Star Equation of State with Neural Posterior and Evidence Estimation

We present two complementary neural routes for microscopic neutron-star EOS inference: truncated sequential neural posterior estimation with mixture importance sampling (TSNPE+MIS), which targets one observation, and amortized neural posterior estimation with importance sampling (A-NET+IS), which is reused across nuclear inputs and NICER sources. Both are corrected against the original prior and full likelihood, with the effective sample size as a diagnostic. We test nucleonic and hyperonic relativistic mean-field models with nuclear data, NICER, GW170817, and pQCD. For the fiducial analyses, both agree with UltraNest in the parameter posteriors and 90% mass-radius and tidal-deformability bands. TSNPE+MIS band-edge differences are only 0.024-0.033 km and 1.02-1.03%, while A-NET+IS differences are 0.019-0.050 km and 0.86-1.13%. TSNPE+MIS reduces the computing time by factors of 1.6-2.3; after training, A-NET+IS is 15-240 times faster per configuration. The frozen A-NET also analyzes three NICER sources absent from training, including the newly reported high-mass, compact PSR J1614-2230 posterior. For this source, the inferred $R_{1.4}$ agrees with UltraNest within 0.010 km for nucleonic matter and 0.009 km for hyperonic matter. We also introduce a Green-function Evidence Network (EN), a regression-based variant of Evidence Networks and, to our knowledge, the first Evidence Network applied to neutron-star EOS inference. After a one-time cost, its network queries return absolute evidences in about 0.03 s for new nuclear data and 3.5 s for a new NICER source; all 16 differences from UltraNest are below $1\sigma$, and independent importance-sampling evidences agree within the quoted EN uncertainties. Under the adopted likelihood, both evidence calculations favor nucleonic matter by Bayes factors of about 22:1-27:1.
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Nuclear Theory

Sensitivity of fission barrier heights to nuclear matter properties

We systematically investigate the correlations between nuclear fission properties and the surface energy coefficient $a_{\rm surf}$ extracted from semi-infinite nuclear matter. To this end, we employ the covariant density functional theory based on a point-coupling energy density functional (EDF). Taking $^{240}$Pu and $^{236}$U as representative examples, we employ Gaussian-process emulators to construct the joint posterior distributions of $a_{\rm surf}$ and selected fission observables, constrained by the ground state properties of the two nuclei. The results reveal a strong correlation between $a_{\rm surf}$ and the fission barrier heights. A sensitivity analysis further shows that the height of the outer fission barrier is governed predominantly by the isoscalar-vector derivative coupling constant $\delta_V$ in the point-coupling EDF, whereas that of the inner barrier is by the combined effects of $\delta_V$ and several mutually correlated coupling constants.
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Nuclear Theory

Rapidity-dependent charge dynamics in high-energy nuclear collisions from dynamical core-corona initialization

We develop a new model that provides a unified description of the equilibration and subsequent transport of conserved charges, including baryon number, electric charge, and strangeness, as well as the dynamical formation and evolution of the quark-gluon plasma medium in high-energy nuclear collisions. This is achieved by extending the state-of-the-art model of dynamical core-corona initialization to incorporate phenomenological source terms for conserved charges within a full (3+1)-dimensional hydrodynamic framework; the updated model is referred to as DCCI3. Consequently, the model simultaneously describes the core (equilibrium) and corona (nonequilibrium), including their mutual interactions. We present a detailed construction of the DCCI3 model and employ it in numerical simulations of Pb+Pb collisions at $\sqrt{s_{NN}}=2.76~\mathrm{TeV}$ to investigate the resulting dynamics of matter and conserved charges, with particular emphasis on the longitudinal structure. We find that each of the chemical potentials exhibits sizable fluctuations around midrapidity, which challenges the conventional assumption of vanishing values as an oversimplification. Moreover, the baryon chemical potential increases sharply with rapidity and becomes comparable to the temperature at forward rapidities. This suggests that the Rapidity Scan (RS) approach in high-energy nuclear collisions can serve as a complementary probe to the Beam Energy Scan (BES) in exploring the QCD phase diagram.
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