Nuclear Theory·nucl-th

Monday·June 15, 2026

9 papers—3 primary·6 cross-listed

04
Anomalies, Topology, and Hadron Structure in QCD
Ismail Zahed🇺🇸
Quantum Chromodynamics (QCD) provides a remarkable realization of how quantum effects reshape the symmetries of a classical field theory. The axial anomaly links chirality to gauge-field topology and underlies the resolution of the $U (1)_{A}$ problem, while the trace anomaly generates the intrinsic QCD scale through dimensional transmutation and accounts for most of the mass of hadrons and hence visible matter. Together, these quantum effects reveal the central role of gluonic dynamics and vacuum structure in strong-interaction physics. In this review, we discuss the theoretical foundations and physical consequences of anomalous symmetry breaking in QCD. We examine anomalous Ward identities, the topological structure of gauge fields, instantons, topological susceptibility, and the realization of chiral and scale symmetries in the QCD vacuum. We review their role in the generation of hadron masses, the $η^{'}$ mass, and the resolution of the $U (1)_{A}$ problem, and discuss their manifestations in polarized deep inelastic scattering, nucleon spin structure, and modern studies of hadron structure. Particular emphasis is placed on recent developments connecting vacuum topology, the flavor-singlet axial charge, topological screening, and the proton spin problem. Our aim is to provide a unified perspective on how quantum anomalies connect vacuum structure, hadron properties, and partonic observables, bridging nonperturbative dynamics and perturbative QCD.
↳ hep-phhep-thnucl-th1 citation
05
Hadron spectra of finite-density QC $_{2}$ D
Kei Iida🇯🇵 · Etsuko Itou🇯🇵 · Kotaro Murakami🇯🇵 · Daiki Suenaga🇯🇵
We investigate the chemical-potential dependence of hadron spectra in two-color QCD using first-principles lattice simulations. We compute two-point correlation functions for all allowed hadronic operators by newly including the contributions from disconnected diagrams, and extract the corresponding effective masses. In the meson sector, the mass hierarchy in the hadronic phase (normal vacuum) is found to be $m_{π} ≲ m_{η} < m_{σ} (noisy) < m_{ρ} \sim m_{ω} ≪ m_{a_{1}}$ , which is similar to that in three-color QCD. In the superfluid phase, this hierarchy is modified, and with increasing density it changes to $m_{σ} (noisy) < m_{a_{1}} < m_{ρ} < m_{π} \sim m_{η} (noisy) ≪ m_{ω} (noisy)$ . In the diquark sector, the ordering remains as $m_{N G} ≲ m_{I = 0, S} < m_{I = 1, A V} < m_{I = 0, P S} ≲ m_{I = 0, V}$ in both phases, and the Nambu--Goldstone mode associated with spontaneous breaking of $U (1)_{B}$ is confirmed to be nearly massless. Furthermore, by comparing correlators for chiral partners, we find indications of chiral symmetry restoration at high density.
↳ hep-lathep-phnucl-exnucl-th2 citations
06
Meson molecules in strong magnetic fields: non-monotonic evolution of the charged pion and kaon energies
Toru Kojo🇯🇵
In strong magnetic fields, charged quarks occupy the lowest Landau level, leading to an effective dimensional reduction of hadronic dynamics. This dimensional reduction naturally generates a hierarchy of scales, separating fast intra-meson quark dynamics from slow collective meson motion; this motivates a Born-Oppenheimer description of meson-meson systems. Our Born-Oppenheimer analysis shows that the infrared behavior is controlled by the interplay between dimensional reduction and the structure of the meson-meson interaction, leading to three distinct regimes: scattering-dominated, molecular, and compact multiquark states. Charged pseudoscalar mesons such as $π_{+}$ and $K_{+}$ provide a particularly interesting realization of this framework, as their lattice spectra at large magnetic fields suggest the emergence of loosely bound states near the boundary between scattering and molecular regimes. Our results suggest that strong magnetic fields provide a useful laboratory for exploring the emergence and classification of hadronic bound states.
↳ hep-phhep-latnucl-th1 citation
07
Thermal dileptons to probe the baryon-rich QCD matter in the forward region of LHC energy heavy-ion collisions
Motomi Oya🇯🇵 · Nicholas J. Benoit🇯🇵 · Chiho Nonaka🇯🇵 · Azumi Sakai🇯🇵 · Yorito Yamaguchi🇯🇵
We investigate thermal dilepton production from a quark-gluon plasma (QGP) with finite baryon chemical potential ( $μ_{B}$ ) in central Pb-Pb collisions at $s_{NN} = 5.02 TeV$ . Recent studies suggest that sizable baryon densities can be achieved at forward rapidity even at LHC energies. We incorporate finite $μ_{B}$ into a (3+1)-dimensional hydrodynamic framework and find that $μ_{B}$ exceeds 500 MeV around $η_{s} = 6$ during the medium evolution. Using this framework, we calculate thermal dilepton spectra over a wide rapidity range and evaluate the impact of finite $μ_{B}$ on dilepton production. A suppression of 3-4% is observed in the forward-rapidity region $5.2 < y < 7.2$ due to the reduced quark-antiquark abundance at finite baryon density. We further examine the effective temperature extracted from dilepton mass spectra in the intermediate-mass region $1.2 < M_{ℓℓ} < 2.6 GeV$ . The effective temperature remains strongly correlated with the underlying hydrodynamic temperature and retains sensitivity to the early high-temperature stage of the QGP evolution. These results demonstrate that forward-rapidity dileptons remain effective thermometers while providing sensitivity to finite baryon density at the LHC.
↳ hep-phhep-exnucl-th0 citations
08
Electric charge fluctuations from lattice QCD in the continuum limit
Szabolcs Borsányi🇩🇪 · Zoltán Fodor🇩🇪 · Jana N. Guenther🇩🇪 · Paolo Parotto🇮🇹 · Attila Pásztor🇭🇺 · Claudia Ratti🇺🇸 · Volodymyr Vovchenko🇺🇸 · Chik Him Wong🇩🇪
Electric charge fluctuations $χ_{n}^{Q}$ allow comparisons between theory and experiment, but are elusive on the lattice due to severe cutoff effects. We use a 4HEX action to obtain $χ_{2}^{Q}$ and, for the first time ever, $χ_{4}^{Q}$ in the continuum limit. We find disagreement with the hadron resonance gas (HRG) model, which we cannot explain with finite volume effects. We include light meson interactions in the HRG model via the S-matrix, reducing the tension for $χ_{4}^{Q}$ , but worsening the agreement for $χ_{2}^{Q}$ . We propose measuring the ratio $χ_{4}^{Q} / χ_{2}^{Q}$ at the LHC to investigate this tension.
↳ hep-latnucl-th0 citations
09
The unintuitive SU(3) flavor and chiral limits of hadron resonances
José Ramón Peláez🇪🇸 · Pablo Rabán🇪🇸 · Jacobo Ruiz de Elvira🇪🇸
Contrary to naive expectations, poles used to define hadron resonances rigorously in the physical world may not evolve continuously to become degenerate in the SU(3) $_{F}$ and chiral limits of QCD. Instead, other shadow poles, usually ignored, may be the ones that degenerate and characterize the resonances in these limits. This feature is general, and we illustrate it first with the simple and familiar light-vector mesons, followed by the much-discussed light-scalar case. Their shadow poles and their degeneracy are found using the QCD low-energy effective theory unitarized to one loop.
↳ hep-phhep-exhep-latnucl-th3 citations

Affiliations

first authorsco-authorsvia INSPIRE

Anomalies, Topology, and Hadron Structure in QCD

Hadron spectra of finite-density QC2​D

Meson molecules in strong magnetic fields: non-monotonic evolution of the charged pion and kaon energies

Thermal dileptons to probe the baryon-rich QCD matter in the forward region of LHC energy heavy-ion collisions

Electric charge fluctuations from lattice QCD in the continuum limit

The unintuitive SU(3) flavor and chiral limits of hadron resonances

Affiliations

Hadron spectra of finite-density QC $_{2}$ D