In linear perturbation theory for Kerr black holes, there are two equivalent formalisms, namely, the Teukolsky and the Sasaki-Nakamura (SN) formalism. Typically, one defaults to the Teukolsky formalism, especially when calculating extreme mass ratio inspiral waveforms, and uses the SN formalism when dealing with extended sources, as it offers superior convergence when employing the Green’s function method for calculating the inhomogeneous solution. In this work, we present a new scheme for solving the inhomogeneous SN equation, based on integration by parts, that eliminates the extra radial integration step required in the standard formulation to construct the source term for convolution with the SN variable. We derive also a SN source term that is valid for point particles on arbitrary motions around Kerr black holes. Our approach enables efficient computations of gravitational waveforms within the SN formalism in all cases, from compact to extended sources. We validate our scheme and code implementation against the literature and find excellent agreement, achieving comparable performance without employing any special optimization techniques.

We study the asymptotic behavior of parametrized black hole quasinormal modes (QNMs) in the high-overtone limit. To gain insights into their analytical structure, we apply the exact WKB method, which was recently developed by the same authors. Our theoretical predictions are confirmed in good agreement with the numerical results based on Leaver’s method. For specific values of parametrization parameters that characterize deviations from general relativity, we find that the real part of asymptotic QNM frequencies diverges in the high-overtone limit, in sharp contrast to the case of a Schwarzschild black hole. This demonstrates that the convergence of the real parts of high-overtone QNMs is a distinctive feature of general relativity, while parametrized corrections generically lead to divergent spectral behaviors.

We derive static spherically symmetric regular black holes as vacuum solutions to purely gravitational theories in four dimensions. To that end, we construct four-dimensional nonpolynomial gravities starting from subclasses of two-dimensional Horndeski actions. By construction, these theories possess second-order equations of motion on spherically symmetric backgrounds. We show that a subset of these nonpolynomial gravities, referred to as nonpolynomial quasitopological gravities, admit single-function static spherically symmetric solutions whereby the metric function is determined by an algebraic equation. Solutions to these theories include the Hayward regular black-hole spacetime, for which a corresponding gravitational action is stated explicitly.

Astrometric missions like Gaia provide exceptionally precise measurements of stellar positions, parallaxes, and proper motions. Gravitational waves traveling between the observer and distant stars can induce small, correlated shifts in their apparent positions, a phenomenon known as astrometric deflection. The precision and scale of astrometric datasets make them well suited for searching for a stochastic gravitational-wave background, whose signature appears in the two-point correlation function of the deflection field across the sky. In space-based astrometry, the ultimate sensitivity of such measurements is reduced by systematic uncertainties in the satellite’s absolute attitude reconstruction, which constrain the accuracy of absolute astrometry. These orientation errors can be mitigated by focusing on relative angular separation between pairs of stars, which effectively cancel out common-mode orientation noise. In this work, we compute the astrometric response and the overlap reduction functions for this differential approach, correcting previous expressions presented in the literature. We use a Fisher matrix analysis to compare the sensitivity of differential astrometry to that of conventional absolute astrometry. Our analysis shows that while the differential method is theoretically sound, its sensitivity is limited for closely spaced star pairs. Pairs with large angular separations provide competitive sensitivity, but, when considered in connection with Gaia, it is unclear whether the differential strategy would be effective, since instrumental systematics are not expected to remain correlated on such scales. Finally, we demonstrate that combining astrometric data with observations from pulsar-timing arrays leads to slight improvements in sensitivity at frequencies $\gtrsim {10}^{-7}\mathrm{Hz}$.

For axions present during inflation, it has been shown that a nonminimal coupling ${\xi }_{\sigma }$ of the inflaton to gravity worsens isocurvature bounds [arXiv:2504.02952], while a nonminimal coupling ${\xi }_{\rho }$ of the radial Peccei-Quinn field can alleviate them [Revisiting isocurvature bounds on the minimal QCD axion, J. High Energy Phys. 12 (2025) 028.]. We analyze the simultaneous presence of both couplings and determine when one effect dominates the other, in both the metric and Palatini formulations of gravity. The two tendencies interpolate smoothly, but introducing a nonminimal inflaton coupling reduces the viable interval of ${\xi }_{\rho }$ in which isocurvature bounds can be alleviated while avoiding backreaction on the inflationary dynamics. We illustrate our findings in Palatini Higgs inflation and Starobinsky inflation.

We investigate the impact of a nonstandard electron mass $m_e$ on early-Universe thermal history, focusing on neutrino decoupling and big bang nucleosynthesis (BBN). In the standard cosmology, neutrino-electron interactions keep neutrinos in thermal contact with the electromagnetic plasma until shortly before $e^{\pm }$ annihilation. Varying $m_e$ shifts the decoupling epoch and the entropy transfer from $e^{\pm }$ annihilation, thereby modifying the neutrino energy density and the inferred effective number of relativistic species, $N_{\mathrm{eff}}$. Independently, during BBN, the rates of charged-current weak processes, and hence the neutron-to-proton ratio, depend on $m_e$. By confronting BBN predictions for the primordial light-element abundances with observations and imposing cosmological constraints on $N_{\mathrm{eff}}$, we obtain the following $1\sigma$ bounds on $m_e$ in the early Universe: $m_e=0.505_{-0.007}^{+0.006}\mathrm{MeV}$ (for the NACRE II nuclear reaction network) or $m_e=0.509_{-0.004}^{+0.005}\mathrm{MeV}$ (for the PRIMAT nuclear reaction network). These bounds have been derived by adopting the recent determination of the primordial helium-4 abundance by the Large Binocular Telescope observations of 54 metal-poor H ii regions. If instead we adopt the Particle Data Book helium-4 abundance, the bounds are $m_e=0.503_{-0.015}^{+0.011}\mathrm{MeV}$ (NACRE II) or $m_e=0.521_{-0.007}^{+0.009}\mathrm{MeV}$ (PRIMAT) The obtained allowed ranges are close to the present laboratory value at the level of $\sim 0.4\%–2\%$, depending on the dataset and nuclear network, thus supporting the constancy of the electron mass over cosmological timescales.

Fast radio bursts (FRBs) are a $\sim$ millisecond-long transient phenomenon that propagate across extragalactic distances and are effectively a point source. Radio wave propagation through inhomogeneous distributions of plasma can act as a lens, generating multiple images of the emitted electric field. A lens can produce images of a point source where the phase of the electric field between images remains coherent when observed by a radio telescope. FRB 20220413B shows a complicated pulse structure with time separated components that may be image copies of the main components due to plasma lensing. We perform several analyses to determine if FRB 20220413B is consistent with expectations of a plasma lensed FRB. We analyze and fit the morphology of the burst to a plasma lens model and find consistency in the spectro-temporal profile but not the observed flux. Using the complex-valued channelized voltage data from the CHIME telescope, we perform a time-lag correlation analysis and report correlation signatures present in the electric field of FRB 20220413B. We find no evidence for a signal with a nonzero expectation in the phase correlation, $⟨C(f,\widehat{t})⟩$, but we do find an excess correlation signature is present in the absolute power of the phase correlation, ${|⟨C(f,\widehat{t})⟩|}^2$. We perform a frequency-lag correlation analysis on the spectra of all subcomponents of the burst and find a consistent scintillation bandwidth across all components. We find the scintillation bandwidth is consistent with expectations of scattering due to the Milky Way. We interpret this as all burst components propagating through the same scintillation screen located in the Milky Way, which would generate the excess variance signature observed, even in the absence of phase coherence between burst components. We find that while the burst morphology can be modeled by a plasma lens, the coherent signature present in the time-lag correlation is consistent with the expectations of a common scattering screen, but not coherent plasma lensing.

Galactic binaries are expected to be the most numerous LISA sources and to produce a stochastic gravitational-wave foreground whose spectral shape encodes information about the underlying population. Extracting this information with standard hierarchical methods is challenging due to the high dimensionality of the problem and the computational cost of global-fit analyses. We present a simulation-based inference framework to measure the population properties of Galactic binaries directly from the reconstructed foreground. Adopting an astrophysically agnostic parametrization in the observable space—defined by signal amplitude, frequency, and frequency derivative—we generate synthetic catalogs and foreground spectra using a global-fit-inspired subtraction algorithm. We then train a neural posterior estimator to map spectra to population parameters. We validate our method on simulated data and recover population parameters with good accuracy, including the total number of binaries. As a byproduct, we present a GPU-accelerated version of the subtraction algorithm, which delivers a $\sim {10}^2\times \mathrm{speed}\text{−}\mathrm{up}$ compared to previous implementations in the literature. Our results demonstrate that LISA’s stochastic foreground alone carries significant information about the Galactic binary population and provide a practical step toward joint inference from resolved and unresolved sources.

In this study, we investigate the influence of an admixed fermionic dark matter (DM) component on the equilibrium structure of white dwarfs (WDs). We adopt a single-fluid description in which dark matter couples to baryonic matter through the Higgs portal, and the baryonic and DM contributions to the total energy density and pressure are treated within a unified framework in the locally equilibrated regime. We examine how variations in DM particle mass ($m_{\chi }$) and DM fraction ($f_{\chi }$) modify the equation of state (EoS), the mass-radius relationship, and the internal mass and pressure distributions of WDs. Our results show that the presence of DM softens the EoS, with lighter DM particles providing stronger pressure support and leading to more extended stellar structures. Increasing the DM fraction leads to a more compact configuration, reducing both the WD’s radius and maximum mass. We further demonstrate that heavier DM particles enhance stellar compactness and can eventually drive the star toward gravitational instability. Moreover, the analysis of the mass-radius relationship reveals that while small fractions of DM are consistent with observed WD masses, the radii predicted by our models are smaller than observations, suggesting additional influences such as rotation or magnetic fields. Global dynamical stability is analyzed by computing the squared frequency ${\omega }^2$ of the fundamental radial mode within the single-fluid framework. We find that ${\omega }^2>0$ throughout the stable branch of solutions in the explored parameter range, indicating that moderate DM admixtures do not induce dynamical instability. Overall, our results demonstrate that the single-fluid formalism, previously applied to other compact stars, can be consistently extended to white dwarfs and provides a controlled framework for studying the structural impact of dark matter admixtures.

Motivated by horizon-scale polarization observations of $\mathrm{M}{87}^{*}$, we investigate polarized emission from rotating black holes (BHs) immersed in a perfect fluid dark matter (PFDM) background. Fully relativistic ray tracing is utilized to examine how the polarization structure jointly depends on magnetic-field geometry, higher-order imaging, and the PFDM intensity parameter $k$. We show that, for a given $k$ value, magnetic configurations with a polar component naturally produce a continuous spiral pattern in the electric vector position angle (EVPA), while the large-scale EVPA morphology remains primarily controlled by the magnetic-field topology. Variations in $k$ affect light propagation and polarization transport near the horizon, leading to corresponding changes in EVPA deflection and polarized intensity around the photon ring. A decomposition of the total polarized image further reveals that higher-order images provide localized yet non-negligible corrections near the ring. Compared with the polarization characteristics of $\mathrm{M}{87}^{*}$ inferred by the EHT, image-domain quantities such as $|m{|}_{\mathrm{net}}$ and $\arg {\beta }_2$ indicate that PFDM induces systematic shifts relative to the pure Kerr BH case, lowering the net linear polarization fraction and modifying the large-scale polarization phase. Overall, PFDM acts as an additional strong-gravity ingredient that systematically reorganizes the horizon-scale polarization structure, suggesting that surrounding dark matter can leave observable imprints on black hole polarization signatures.

We investigate the axial $w$-mode oscillations of anisotropic neutron stars. Equilibrium stellar configurations are constructed using two realistic equations of state, BSk21 and SLy4, together with two prescriptions for pressure anisotropy, namely the Horvat Ansatz and the Bowers-Liang Ansatz. Only neutron-star models satisfying the stability and physical acceptability criteria are considered. On these backgrounds, the complex axial $w$-mode frequencies are computed by solving the linearized perturbation equations using a continued-fraction method. We find that, for each fixed anisotropy strength, the axial $w$-mode frequency decreases monotonically with increasing stellar mass along the stable branch, with its magnitude depending on both the equation of state and the nature of the anisotropy. At relatively low stellar masses, configurations with dominant radial pressure ($p_r>p_t$) exhibit higher frequencies than those with dominant tangential pressure, whereas toward the upper end of the stable branch this ordering is reversed, and configurations with $p_t>p_r$ attain higher frequencies at the same mass. When expressed as a function of compactness, the axial $w$-mode frequency displays an approximately linear dependence on $M/R$, with anisotropy modifying both the slope and the intercept of the relation. The Bowers-Liang prescription produces a wider spread in the frequency values compared to the Horvat Ansatz. We also analyze the damping times associated with the axial $w$-modes and find that they increase with stellar mass, with a rapid rise toward the upper end of the stable branch. At a fixed mass, increasing the tangential pressure relative to the radial pressure leads to shorter damping times, while configurations with dominant radial pressure exhibit longer damping times. The sensitivity of the damping time to anisotropy is more pronounced for more compact stars, and the Bowers-Liang prescription yields systematically larger damping times than the Horvat Ansatz. Finally, motivated by the numerical results, we provide empirical expressions for the axial $w$-mode frequency and damping time as functions of stellar compactness and anisotropy strength. These relations accurately capture the behavior of the axial $w$-mode spectrum within the physically allowed parameter space considered in this study.

A small body orbiting around an accreting massive object and periodically crossing its accretion disk is a common configuration in astrophysics. In this work, we study the secular evolution of extreme mass-ratio inspirals (EMRIs), where a stellar-mass object (SMO)—such as a star or a stellar-mass black hole (sBH)—collides with the accretion disk surrounding a central supermassive black hole (SMBH). Using a perturbation method applied to EMRI geodesics, we find the following: (1) the disk tends to align the SMO regardless of its initial inclination $\iota$ relative to the disk, (2) the final orbital eccentricity of an SMO captured by the disk is low, although the eccentricity can temporarily increase when the initial inclination $\iota$ is large and the SMO is an sBH, (3) through collisions with the accretion disk alone, only a small fraction of sBHs that initially lie close to both the SMBH and the disk can be captured within the typical disk lifetime of active galactic nuclei. Two-body scatterings among SMOs in the nuclear stellar cluster play an essential role, randomly kicking sBHs toward the disk and significantly boosting the capture rate.

We develop a generative framework based on denoising diffusion for the model-independent reconstruction of hadronic form factors from sparse and noisy data. The generative prior is built from a large ensemble of synthetic curves drawn from ten distinct functional classes rooted in different theoretical approaches to hadron structure. Applied to the proton gravitational form factors $A(t)$, $J(t)$, and $D(t)$, the framework yields nonparametric reconstructions consistent with lattice QCD across the full kinematic range $0\le -t\le 2{\mathrm{GeV}}^2$, remaining robust even when only one or two conditioning points are retained. The densely sampled output enables a direct extraction of the chiral low-energy constants $c_8=-4.6\pm 0.8{\mathrm{GeV}}^{-1}$ and $c_9=-0.61\pm 0.19{\mathrm{GeV}}^{-1}$. Using these values at the physical pion mass, we obtain $D(0)=-4.3\pm 0.8$ for the nucleon $D$-term.

We propose a novel framework for low-scale leptogenesis within an extension of the Standard Model that includes three $SU(2)$ singlet right-handed neutrinos, a singlet vectorlike neutral fermion, and a real scalar field. In this setup, the $CP$ asymmetry arises through a rich interplay of mechanisms, including two-body decays of the lightest right-handed neutrino into leptons and Higgs boson or into dark-sector particles, as well as multiple $2\to 2$ scattering processes involving visible and dark sector particles. Crucially, the $CP$-violating phases originate not only from conventional vertex and self-energy corrections of the decay of the right-handed neutrinos but also from novel interference effects in the scattering processes mediated by the dark sector, which significantly enrich the sources of asymmetry. A distinctive feature of our framework is the direct connection between the dark sector and leptogenesis, providing a unified explanation for both the matter–antimatter asymmetry and Dark Matter abundance. Moreover, the viable parameter range including the mass spectrum is within the range that Large Hadron Collider can explore.

We present an interesting study of Feynman integral reduction that does not employ integration-by-parts identities. Our approach proceeds by studying the equivalence relations of integral contours in the Feynman parametrization. We find that the integration contour can take a more general form than that given by the Cheng-Wu theorem. We apply this idea to one-loop integrals and derive universal reduction formulas that can be used to efficiently reduce any one-loop integral. We expect that this approach can be useful in the reduction of multiloop integrals as well.

Hybrid quarkonia—exotic hadrons with explicit gluonic degrees of freedom—have gained increasing attention in hadron spectroscopy, particularly with the ongoing discovery of new XYZ mesons. In this work, we update the spectrum of heavy hybrid mesons in the charmonium and bottomonium sectors using the Born-Oppenheimer effective field theory framework, by incorporating the latest lattice QCD results for hybrid static potentials. We refine earlier calculations and analyze allowed transitions from hybrids to conventional quarkonia, including both spin-conserved and spin-flip decays. We carry out a comprehensive error analysis and discuss the reliability of our results. We compare them to experimental data of the Particle Data Group, which allows us to identify hybrid candidates among the observed XYZ states. We provide hybrid or quarkonium interpretations for nearly all heavy isospin-zero mesons observed and incorporate new hybrid candidates.

Hadronic electromagnetic form factors and radiative decay properties offer a crucial window into the nonperturbative dynamics of quantum chromodynamics (QCD). In this work, we employ the light-cone sum rules (LCSR) method to systematically investigate the M1 radiative decay of vector mesons. Our study covers processes including $K^{*-}\to K^{-}\gamma$, $D^{*}\to D\gamma$, $B^{*}\to B\gamma$, $D_s^{*+}\to D_s^{+}\gamma$, and $B_s^{*}\to B_s\gamma$, and further extends to the excited charmonium state $\psi (2S)$. Our calculations yield decay widths for $K^{*}$ and $\psi (2S)$ that are in excellent agreement with experimental data. For the charm and bottom meson decays, where precise measurements are lacking, we provide theoretical predictions and compare them with other theoretical approaches. Most notably, our analysis reveals a universal linear dependence of the decay width on a function A(x) in the logarithmic coordinate system, which originates from the two-body decay dynamics and the ratio of the initial and final state decay constants. This relationship holds for the ground state $V\to P\gamma$ processes here and suggests a broader applicability to radiative decays of ground-state vector mesons.

We generalize the nature of the so-called beam-dump “ceiling” beyond which the improvement on the sensitivity reach in the search for fast-decaying mediators dramatically slows down, and we point out its experimental implications that motivate tabletop-sized beam-dump experiments for the search. Light (bosonic) mediators are well-motivated new-physics particles, as they can appear in dark-sector portal scenarios and models to explain various laboratory-based anomalies. Due to their low mass and feebly interacting nature, beam-dump-type experiments, utilizing high-intensity particle beams, can play a crucial role in probing the parameter space of such visibly decaying mediators—in particular, the “prompt decay” region, where the mediators feature relatively large coupling and mass. We present a general and semianalytic proof that the ceiling effectively arises in the prompt-decay region of an experiment and show its insensitivity to data statistics, background estimates, and systematic uncertainties, considering a concrete example, the search for axion-like particles interacting with ordinary photons at three benchmark beam facilities: PIP-II at FNAL, and SPS and LHC-dump at CERN. We then identify optimal criteria to perform a cost-effective and short-term experiment to reach the ceiling, demonstrating that very short-baseline compact experiments enable access to the parameter space unreachable thus far.

Rare decays of $B$ mesons play a crucial role in understanding and exploring the intricacies of physics beyond the standard model. Various experimental results as well as theoretical investigations involving rare $B$ meson decays have motivated us to investigate the rare decay, $B\to \omega {\mu }^{+}{\mu }^{-}$, proceeding through the quark transition $b\to d{\ell }^{+}{\ell }^{-}$ in standard model and nonuniversal $Z^{\prime }$ model. We have incorporated the new physics (NP) couplings of the $Z^{\prime }$ boson with quarks from $B_d-\overline{B_d}$ mixing. We have estimated the NP leptonic couplings from the experimental values of branching fractions of other $b\to d{\ell }^{+}{\ell }^{-}$ decays. With these NP couplings, we have estimated the branching ratio, forward-backward asymmetry, and lepton polarization asymmetries. We have observed significant deviation from standard model predictions, especially for the branching fraction and forward-backward asymmetry.

Although the sources of astrophysical neutrinos are still unknown, they are believed to be produced by a population of sources in the distant Universe. Measurements of the diffuse, all-sky astrophysical flux can thus be sensitive to flavor- and energy-dependent propagation effects, such as very long–baseline oscillations. These oscillations are present in certain neutrino mass models, such as when neutrinos are quasi-Dirac. Assuming generic models for the source flux, we find that these oscillations can still be resolved even when integrated over wide distributions in source redshift. We use two sets of IceCube all-sky flux measurements, made with muon and all-flavor neutrino samples, to set constraints at the $3\sigma$ level on quasi-Dirac mass splittings between $(5\times {10}^{-19},8\times {10}^{-19}){\mathrm{eV}}^2$. Additionally, we find that when the mass-square splittings are different between generations, the quasi-Dirac scenario can explain tensions between the all-flavor and muon-flavor measurements. We consider systematic uncertainties on the source population and find that our results are robust under alternate spectral hypotheses or physical redshift distributions. Our analysis shows that spectral features in the all-sky neutrino measurements provide strong constraints on massive neutrino scenarios and are sensitive to uncharted parameter space.