Spin and polarization are central to precision tests of fundamental physics and for interpreting radiation from astrophysical sources and ultraintense laser-matter experiments. Here, focusing on the fundamental process of nonlinear Compton scattering, we demonstrate that a key assumption underlying current strong-field quantum electrodynamics models, i.e., that emission can be treated as an instantaneous random event sampled from a local differential rate, is inconsistent once emission angles, electron spin, and/or photon polarization are resolved. Namely, even in strictly constant and uniform fields, the resulting fully differential distribution is sign indefinite, yielding negative inferred probabilities. The physical reason is that the photon emission probability builds up over a finite length of the electron trajectory, the formation region, during which the electron direction changes by roughly the same small angle that defines the radiation cone. Therefore, we put forward a new method where we integrate over this formation region analytically to obtain a physically consistent electron spin and photon polarization model. We show that the implementation of our model is compatible with existing Monte Carlo and particle-in-cell workflows. Simulations of a GeV-class electron-laser collision accessible at current petawatt facilities and of emission in a pulsarlike magnetic field are shown to reveal spin and polarization patterns that differ even qualitatively from state-of-the-art local models. In particular, our new model predicts substantial angle-dependent circular photon polarization where the well-known collinear-emission approach yields none, and a pronounced helicity bias in the recoiling electrons absent from current predictions. These findings have direct implications for upcoming strong-field QED experiments and for interpreting polarized radiation from extreme astrophysical environments.

We investigate the spin correlations between production and decay in the process where dark photons $A^{\prime }$ are produced in pseudoscalar meson decays, for example, ${\pi }^0,\eta \to \gamma A^{\prime }$, and then decay to fermion pairs, $A^{\prime }\to f\overline{f}$. This process is the focus of many experimental searches, but spin correlations are typically ignored. We derive analytic results that allow us to quickly scan parameter space and quantify the error made in neglecting spin correlations. In particular, we define a discrepancy parameter and find that this parameter is always less than $\frac{1}{6\sqrt{3}}\approx 9.6\%$, which provides a rough measure of the size of spin correlation effects. However, these effects may be enhanced or suppressed by cuts in realistic experimental analyses, and so we also consider two representative examples, including current FASER analyses and possible future searches at SHiP. We find that the effects of spin correlations are negligible for existing FASER analyses, but can significantly reduce event rates at upcoming SHiP searches.

We construct a hybrid equation of state (EoS) by smoothly interpolating the EoS in the hadron resonance gas at low temperatures to that in the ideal parton gas at high temperatures, and employ it to study the properties of the quantum chromodynamics (QCD) matter under a finite magnetic field and nonzero chemical potential. In this work, we neglect the anomalous magnetic moment effects of both charged and neutral particles. Our results show that the thermodynamic observables such as the entropy density, the pressure, the energy density, the trace anomaly, and the specific heat at constant volume are sensitive to both the finite magnetic field and chemical potential. As the chemical potential increases from zero, these quantities rise in both the hadronic and quark-gluon plasma phases. In contrast, introducing a magnetic field suppresses them at low temperatures but enhances them at high temperatures. Furthermore, nonzero chemical potential and magnetic field introduce nontrivial modifications to the squared speed of sound. Both effects increase its value near the critical temperature while reducing it at lower temperatures. When both the chemical potential and the magnetic field are present, their influences superimpose, leading to more intricate changes in the thermodynamic behavior. Finally, we compare our results with the lattice QCD data for the quadratic fluctuations of conserved charges and their correlations. The model successfully reproduces the temperature dependence of these observables at $eB=0$ and $0.04{\mathrm{GeV}}^2$. However, at the stronger field strength $eB=0.14{\mathrm{GeV}}^2$, the model underestimates the magnitudes while still capturing the overall temperature trend.

Within the standard $3\nu$ framework, we discuss updated bounds on the leading oscillation parameters related to the $({\nu }_1,{\nu }_2)$ states, namely, the squared mass difference $\delta m^2=m_2^2-m_1^2$ and the mixing parameter ${\sin }^2{\theta }_{12}$. A previous global analysis of 2024 oscillation data estimated $\delta m^2$ and ${\sin }^2{\theta }_{12}$ with fractional $1\sigma$ errors of about 2.3% and 4.5%, respectively. First we update the analysis by applying the latest SNO+ (Sudbury Neutrino Observatory Plus) constraints that slightly shift the $(\delta m^2,{\sin }^2{\theta }_{12})$ best fits. Then we apply the constraints placed by the first Jiangmen Underground Neutrino Observatory results that significantly reduce the uncertainties of both parameters. Our updated global bounds (as of 2025) can be summarized in terms of a bivariate Gaussian as: $\mathit{\delta }{\mathit{m}}^{\mathbf{2}}\mathbf{/}{\mathbf{10}}^{\mathbf{-}\mathbf{5}}{\mathbf{eV}}^{\mathbf{2}}\mathbf{=}\mathbf{7.48}\mathbf{\pm }\mathbf{0.10}$ and ${\mathbf{sin}}^{\mathbf{2}}{\mathit{\theta }}_{\mathbf{12}}\mathbf{=}\mathbf{0.3085}\mathbf{\pm }\mathbf{0.0073}$ (with error correlation $\mathit{\rho }\mathbf{=}\mathbf{-}\mathbf{0.20}$), corresponding to $1\sigma$ uncertainties as small as 1.3% and 2.4%, respectively. We also comment on minor physical and statistical effects that, in the future, may contribute to lift the current mass-ordering degeneracy of $(\delta m^2,{\theta }_{12})$ estimates.

Recent anomalies in nuclear transitions observed by the ATOMKI Collaboration suggest the existence of a new boson with a mass of $\sim 17\mathrm{MeV}$. A theoretically consistent interpretation requires a framework that not only matches the kinematics but also reproduces the observed decay rates while satisfying stringent experimental constraints. Among various possibilities, an axial-vector or mixed vector–axial-vector mediator $Z^{\prime }$ emerges as the most viable candidate. However, getting such couplings for a light $Z^{\prime }$ gauge boson is a highly nontrivial task. In this work, we construct a gauged chiral, flavor specific $U(1{)}_X$ extensions of the Standard Model where the associated $Z^{\prime }$ boson acts as the 17 MeV particle. By employing a two Higgs doublet framework, we generate the necessary nonvanishing axial-vector couplings while ensuring gauge anomaly cancellation and consistent fermion mass generation. Focusing on the ${}^8\mathrm{Be}$ and ${}^4\mathrm{He}$ signals, we show that in this model the viable parameter space to resolve the ATOMKI anomalies is also consistent with a diverse set of experimental constraints, including atomic parity violation, beam dump experiments, meson decays, and neutrino nucleus and neutrino electron scatterings. Our results demonstrate that this framework offers a theoretically sound and phenomenologically robust solution to the ATOMKI anomaly.

We present the first predictive realization, to our knowledge, of vector freeze-in dark matter from a hidden non-Abelian $SU(2)$ gauge sector spontaneously broken by a Higgs triplet to a residual $U(1)$ symmetry with a massless dark photon mediator. A massive dark vector particle-antiparticle pair acquires small millicharges through a dimension-4 kinetic-mixing term with an induced coefficient $\epsilon$, generated by an effective dimension-5 operator involved the Higgs triplet. The dark sector interactions are governed by the hidden gauge coupling $g_D$, providing a weak connection to the Standard Model to realize the freeze-in dark matter production mechanism. Solving the two-temperature Boltzmann evolution including plasmon decay, we find a wide region of parameter space consistent with the observed relic abundance while satisfying astrophysical and cosmological constraints. This minimal framework connects non-Abelian vector dynamics with long-range dark forces and may be probed by upcoming sub-GeV dark matter direct-detection experiments.

The scotogenic model defines a framework for radiative neutrino masses and provides a viable dark matter candidate. Since the scotogenic dark matter is leptophilic, indirect searches appear as an especially interesting possibility. Here we discuss a variation of the usual model with a very distinct dark matter phenomenology. The scotogenic fermion singlets are naturally grouped into pseudo-Dirac pairs of mass of 0.1–1 TeV. We show that the lightest one constitutes a dark matter candidate that near threshold annihilates with a 90% branching ratio into neutrino pairs. The model gives the observed relic abundance consistent with the bounds from direct searches and with all neutrino and charged lepton data. We also show that, for a sub-MeV dark matter particle, the model suggests a scenario that could address the lithium problem.

The impact of astrophysical uncertainties in direct detection searches can vary significantly across particle dark matter models and detector targets, due to the different velocity and momentum dependencies of the scattering cross section. We address these uncertainties for all operators of the nonrelativistic effective field theory of dark-matter/nucleon interactions, making use of the Kullback-Leibler (KL) information divergence to measure the deviation of the true dark matter velocity distribution from the Maxwell-Boltzmann form. This approach quantifies how astrophysical uncertainties affect each operator in the effective theory, without assuming any specific functional form for the velocity distribution. While for some operators the uncertainties are smaller than 1 order of magnitude for entropically motivated deviations from the Maxwell-Boltzmann form, for other operators, these uncertainties can be as large as three orders of magnitude near threshold. Furthermore, we identify the dependence of the scattering rate for various operators of the effective theory with different velocity-weighted moments of the velocity distribution, functionally analogous to the mean, variance, or skewness. This provides new analytic insight into which features of the velocity distribution are most relevant to detect a given particle dark matter model. Our technique is general and could be applied to a broader class of physics problems where a physical observable depends on the statistical moments of an uncertain theoretical distribution.

We introduce classical, relativistic Lagrangians for point-particle analogs to the field theory description of the standard-model extension (SME) for Lorentz violation. Lagrangians of a form alternative to those derived and studied in previous works are in the spotlight. Interestingly, they have well-defined massless limits, which makes them suitable for describing classical-particle analogs of photons subject to Lorentz violation. We first deal with different types of Dirac fermion coefficients, followed by various configurations of the SME photon sector. The Lagrangians are accompanied by constraints that we treat properly using the techniques due to Dirac. The results encountered may find application in photon propagation through gravitational fields in the presence of spacetime symmetry violation. Connections to Finsler geometry are likely to exist.

We apply score-based diffusion models to two-dimensional SU(2) lattice pure gauge theory with the Wilson action, extending recent work on U(1) gauge theories. The SU(2) manifold structure is handled through a quaternion parametrization. The model is trained on 10,000 configurations generated via hybrid Monte Carlo at a fixed coupling ${\beta }_0=2.0$ on an $8\times 8$ lattice, augmented to 20,000 samples via random gauge transformations. Through physics-conditioned sampling exploiting the linear $\beta$ dependence of the score function, we generate configurations at different values of the coupling without retraining; through the fully convolutional U-Net architecture with periodic boundary conditions, we generate configurations on lattices of different spatial extents. We validate our approach by comparing the average plaquette against exact analytical predictions. At the training lattice size ($8\times 8$), the model reproduces the exact plaquette with biases $|\mathrm{\Delta }|\le 0.001$ for $\beta \in [1.5,2.5]$ and $|\mathrm{\Delta }|<0.06$ across $\beta \in [1,4]$. For lattices sharing the training extent $L=8$ in at least one direction, biases remain below $\sim 0.003$ for $\beta \in [1.5,2.5]$, with larger deviations at higher couplings. This work demonstrates that diffusion models are a promising tool for non-Abelian gauge field generation and motivates further investigation toward higher-dimensional theories.

Recently, self-dualities based on saddle-point expansions have been proposed as a means to obtain qualitative nonperturbative information in scalar field theories. In this work, we test this proposition quantitatively by studying the phase transition for critical scalar ${\varphi }^4$ field theory in $1+1$ dimensions using a variational method. We find that saddle-point methods obtain quantitative agreement for the free energy, but differ on the order of 25% for the peak location of the correlation length.

We report our new lattice results for the sphaleron rate calculated within a thermal effective field theory of soft SU(N) gluons whose momenta are below the magnetic scale, where $N=2$, 3, for a wide range of temperatures spanning from 0.6 to ${10}^{15}\mathrm{GeV}$ at sufficiently large volumes. Comparing these results with sphaleron rates in a nonthermal SU(N) plasma where the infrared gluons are overoccupied, we estimate the typical thermalization time for these ultrasoft gluons during the early stages of reheating after inflation. We also calculate the thermal production rate of relativistic axions due to these nonperturbatively interacting soft gluons which shows a significant deviation from its perturbative estimate even at the electroweak scale.

We perform a comprehensive study of the properties of the Dirac eigenvalue spectrum in QCD as a function of temperature on the lattice. In addition to effects due to the interplay between interactions and disorder inherently present in a many-body system, the Dirac spectrum also contains crucial information about the effective restoration of different subgroups of almost exact two-flavor chiral symmetry in QCD. We calculate the infrared eigenvalues of the overlap Dirac operator on $2+1$ flavor QCD ensembles generated using domain wall fermion discretization, on a large volume lattice. From the normalized level spacing ratios, we identify those eigenvalues that have intermediate level statistics, distinctly different from the majority in the bulk spectrum that follow universal level fluctuations similar to a random matrix of Gaussian unitary type. We provide an explanation of these intermediate level ratios in terms of a specific random matrix model and quantify the correlation between these eigenstates and disorder in the gauge fields manifested in the renormalized Polyakov loop values. Whereas the existence of intermediate eigenmodes is intimately connected to the effective restoration of different subgroups of chiral symmetry close to the chiral crossover transition, their origin can be traced to random uncorrelated disorder at higher temperatures when the $U_A(1)$ is effectively restored. We also, for the first time, calculate the Thouless conductance for the Dirac spectrum that quantifies the structural rigidity of the eigenvectors, and use it as a diagnostic tool to understand the restoration of the anomalous $U_A(1)$ subgroup of chiral symmetry and localization driven by disorder.

We analyze the $K^{-}p\to K^{+}{\mathrm{\Xi }}^{-}$ and $K^{-}p\to K^0{\mathrm{\Xi }}^0$ processes in the energy region $1.8<\sqrt{s}<2.8\mathrm{GeV}$ within an effective Lagrangian approach. The $\mathrm{\Lambda }(1800)$ and $\mathrm{\Sigma }(2250)$ resonances, along with the ground states $\mathrm{\Sigma }$ and $\mathrm{\Lambda }$, are included. Additionally, a possible $J^P=3/2^{-}{\mathrm{\Sigma }}^{*}$ or ${\mathrm{\Lambda }}^{*}$ resonance with a mass around 1.9 GeV and a width of approximately 200 MeV is introduced to describe the structure at 2.0 GeV in the total cross section and reproducing the threshold behavior. The two possible solutions corresponding to ${\mathrm{\Sigma }}^{*}(3/2^{-})$ and ${\mathrm{\Lambda }}^{*}(3/2^{-})$ cannot be distinguished by the existing data. Predictions for the polarization of the final-state $\mathrm{\Xi }$ and the cross section of $K^{-}n\to K^0{\mathrm{\Xi }}^{-}$ are compared with the experimental data, we find that the results of solution-II with ${\mathrm{\Lambda }}^{*}(3/2^{-})$ are much better. We also discuss the possible interpretations of the introduced $3/2^{-}$ hyperon as a pentaquark candidate, e.g., an $S$-wave $K\mathrm{\Xi }(1530)$ hadronic molecule. However, since the polarization data suffer from rather large uncertainties, more data inputs are needed in future experiments, for example, J-PARC, HIAF, and JLab.

We perform coupled channel analysis for $D{\overline{D}}^{*}$, $J/\psi \pi$, and related meson pairs for the $Z_c(3900)$ channel in an effective model of hadrons and quarks. The model incorporates meson exchange potential such as one pion and $D^{(*)}$ meson exchanges, and quark exchanges. It turns out that the meson exchange potential is small, while the off-diagonal interactions by the quark exchanges at short distances, particularly for transitions between $D{\overline{D}}^{*}-J/\psi \pi$ are strong, which plays a main role for the scattering amplitudes for the $Z_c(3900)$ channel, in consistent with the results of the lattice simulations of the HALQCD group.

We present a systematic study of prompt $B_c$ meson photoproduction at electron-proton colliders within the framework of nonrelativistic QCD factorization. In addition to the dominant direct channel $\gamma +g\to B_c+X$, we include resolved contributions initiated by $g+g$ and $q+\overline{q}$ ($q=u$, $d$, $s$) subprocesses. The production of the $1S$, $2S$, and $1P$ color-singlet $B_c$ states is calculated, and the feeddown contributions from the excited states are included in the prompt-$B_c$ yield. Total cross sections and transverse-momentum distributions are presented for several collider configurations, including HERA, LHeC, FCC-$ep$, and EIC. The numerical results show that the direct $\gamma +g$ channel provides the leading contribution over the entire kinematic range. The resolved $g+g$ channel gives a non-negligible correction, reaching the level of $\mathcal{O}(10\%)$ in the low-$p_T$ region where most events are produced, and it becomes increasingly important at higher collision energies.

We analyze the fine structure and “fall-apart” decay patterns of hidden-strangeness tetraquarks within the dynamical diquark model. Several well-established negative-parity resonances listed by the Particle Data Group (PDG) [$\varphi (2170)$, $\eta (2225)$, $\eta (2370)$] are examined as potential $q\overline{q}s\overline{s}$ tetraquark candidates using a Hamiltonian that incorporates (iso)spin-dependent, spin-orbit, and tensor interactions. We further show that the isovector resonances $\rho (2150)$ and ${\rho }_3(2250)$ observed by BESIII in $\psi (2S)\to K^{+}{K}^{-}\eta$ are compatible with a tetraquark assignment. Predictions for 28 states, including those with exotic quantum numbers, are also presented. Comparison of the model spectrum with the PDG’s unverified “Further States” highlights additional promising candidates worth future experimental investigation. The predicted fall-apart decay channels are easily reconstructed by experiment and may stimulate ongoing searches for hidden-strangeness tetraquarks at GlueX and BESIII. While current data do not yet provide an unambiguous interpretation for any state in this range, the observation of the fall-apart modes, in addition to the distinct multiplet structure for tetraquark states from that of hybrids or glueballs, provide the most incisive tests for discriminating state content.

We perform the first simultaneous global QCD analysis of pion and kaon parton distribution functions (PDFs) constrained by pion- and kaon-induced Drell-Yan (DY) and leading neutron electroproduction data together with lattice QCD data on pion and kaon PDF moments. The analysis indicates a softer valence $\overline{u}$ distribution in the $K^{-}$ than in the ${\pi }^{-}$ and a significantly more peaked valence $s$-quark density in $K^{-}$ compared with the $\overline{u}$. The effective exponent governing the high-$x$ behavior of the PDF is found to be larger for $\overline{u}$ in the kaon, ${\beta }_{\overline{u}}^{K^{-}}=1.6(2)$, than in the pion, ${\beta }_{\overline{u}}^{{\pi }^{-}}=1.16(4)$, in the range $0.7\le x\le 0.95$. From the gluon momentum fractions, we find the pion’s gluon content accounts for $\approx 1/3$ of the mass budget of the pion at $\mu =2\mathrm{GeV}$, but only $\approx 1/4$ for the kaon.

We have studied the higher-twist distributions of the proton, including T-even and T-odd transverse momentum-dependent parton distributions (TMDs). Under the umbrella of the light-front framework, we have chosen two distinctive approaches of quark-spectator systems for comparison, one inspired by the soft-wall AdS/QCD and another with a dipolar form factor at the nucleon-quark-diquark vertex. The comprehensive picture at higher twist provided by both T-even and T-odd TMDs not only aids deeper insights into the internal structure of the proton in the quark sector but also provides an interpretation of different components of the energy-momentum tensor in quantum chromodynamics. Hence, using these standard parton distribution functions, further predictions regarding the physical insights of gravitational TMDs in momentum space are also provided.

The light-front wave functions (LFWFs) of a hadron are nonperturbative objects that encode information on the configurations of the constituent partons. We show how to extract the LFWFs of baryons, such as the proton, from equal-time correlators suitable for lattice QCD simulations. Using an operator product expansion, we prove the factorization of the relevant correlator in the three-quark color-singlet LFWF, a residual lattice factor, and a soft factor that systematically subtracts the additional divergences arising from the factorization. We verify up to next-to-leading order the independent renormalizability of the LFWF, and we derive the evolution equations that govern its scale dependence.