Astrophysical black holes appear well-represented by the Kerr metric, whose maximal analytic extension contains a ringlike curvature singularity. We study a phenomenological regularized modification of Kerr, the modified Kerr-Hayward metric, which removes curvature blowup at the level of the stationary metric while preserving in detail many features of polarized black hole images now testable by the Event Horizon Telescope (EHT). To establish this, we produce new general relativistic magnetohydrodynamics (GRMHD) simulations of a magnetized plasma in a Kerr-Hayward spacetime, then we extend the EHT analysis framework to perform polarized radiative transfer in this spacetime. We detail our methodology for implementing this modified spacetime into an open-source pipeline. From fluid quantities such as the magnetic flux parameter and jet efficiency, to image quantities such as the polarization pattern and the photon ring structure, our results for the Kerr-Hayward metric appear functionally indistinguishable from the Kerr metric. Our study finds that under the assumptions of a fixed-background GRMHD and polarized radiative transfer pipeline, this regularized phenomenological modification to the Kerr metric can yield observables that are effectively indistinguishable in EHT measurements.

We present the SPARK (Simulation of Pop III in arepo with Radiation and Kinetic feedback) framework, a new $\mathrm{Pop}\mathrm{III}+\text{Pop}\mathrm{II}$ subgrid framework implemented in the moving-mesh code arepo, designed to study the impact of Pop III feedback on star formation in the early Universe. The framework combines primordial nonequilibrium chemistry, metal-line cooling, initial mass function (IMF)-sampled stellar evolution with supernova feedback, and approximate Lyman-Werner (LW) and ionizing radiation transport. We run a suite of $1c\mathrm{Mpc}/h$ box simulations with different initial conditions and resolutions from $z=127$ to $z=10$. The highest gas mass and spatial resolution in the fiducial simulation reach $\sim 10M_{\odot }$ and $\sim 4\mathrm{pc}$, respectively. The model successfully reproduces the Pop II star formation rate density consistent with previous theoretical works across all initial conditions, with minor variations mainly driven by local halo interactions and LW irradiation. We find that the volume filling factor of metal-enriched gas converges to $\sim 0.5–2\%$ at $z=10$, with scatter driven by stochastic and mixing model variations. Convergence is achieved once subhalos with $M_{\text{subhalo}}\gtrsim {10}^{6.5}{M}_{\odot }$ are resolved, and the total stellar mass at $z=10$ is largely insensitive to initial conditions or the resolution considered in this work. A fiducial simulation requires $\sim {10}^4$ CPU hours, making the framework computationally tractable for larger box simulations and enabling future large parameter studies of stellar physics or environment effects such as Pop III IMF variations, x-ray radiation, or the streaming velocity at high redshift.

Matter interacts with two fundamental long-range forces: gravity and electromagnetism. While elementary building blocks of matter always contribute to the gravitational potential, the electromagnetic vector potential was expected to cancel out in large systems because of the symmetry between positive and negative charges. Yet, long range electromagnetic phenomena are present in astrophysical systems, so that the cancellation cannot be considered as perfect. The aim of this paper is to find a possible model for stationary aggregation of matter to make a “star” which consistently includes angular momentum and electromagnetic phenomena. We recast the standard polytropic stellar model as a variational problem and extend it to include the kinetic energy of (rigid) rotation and the electromagnetic energy of interaction between positively and negatively charged baryonic matter. We argue that the electromagnetic contribution to the action should be represented by the minimal energy needed to induce the stellar magnetic dipole moment. This energy consists of two parts; the pure electromagnetic energy, which can be expressed as a surface integral, and the free-energy difference between the magnetized and unmagnetized state of matter. This second part of the action is formulated by considering the magnetized quantum state of a Fermi gas of electrons in a sea of cold ions. Differential forms calculus is a useful tool to perform the mathematical analysis. Our mathematical model includes electromagnetism to stellar modeling in a way that is consistent with the linearized version of general relativity. The complete model presents a complex open boundary problem. The problem can be solved exactly under simplifying circumstances. The distribution of stellar objects in the phase diagram, based on the simplified model, exhibits a pattern which may prompt research to provide a deeper understanding of the natural balance between matter, gravitation, and electromagnetism.

Extremely large-mass-ratio inspirals (XMRIs), consisting of a brown dwarf orbiting a supermassive black hole, emit long-lived and nearly monochromatic gravitational waves in the millihertz band and constitute a promising probe of strong-field gravity and black-hole properties. However, dedicated data-analysis pipelines for XMRI signals have not yet been established. In this work, we develop, for the first time, a hierarchical semicoherent search pipeline for XMRIs tailored to space-based gravitational wave detectors, with a particular focus on the TianQin mission. The pipeline combines a semicoherent multiharmonic $\mathcal{F}$ statistic with particle swarm optimization and incorporates a novel eccentricity estimation method based on the relative power distribution among harmonics. We validate the performance of the pipeline using simulated TianQin data for a Galactic Center XMRI composed of a brown dwarf and Sgr A*. For a three-month observation, the pipeline successfully recovers the signal and achieves high-precision parameter estimation, including fractional uncertainties of $2.0\times {10}^{-6}$ in the orbital frequency, $2.9\times {10}^{-4}$ in the eccentricity, $2.5\times {10}^{-5}$ in the black-hole mass, and $5.6\times {10}^{-4}$ in the black-hole spin. Our framework establishes a practical foundation for future XMRI searches with space-based detectors and highlights the potential of XMRIs as precision probes of stellar dynamics and strong-field gravity in the vicinity of supermassive black holes.

Dark matter admixed neutron stars (DANSs) serve as a specific astrophysical laboratory for probing the features of dark matter (DM) and have emerged as a promising candidate for interpreting recent astrophysical observations (e.g., by NICER and LIGO/Virgo). Accurately constraining the internal DM content of DANSs is therefore of critical importance. In this work, we construct the equations of state (EOS) for DANS matter by employing twelve nuclear matter (NM) models within the covariant density functional (CDF) theory and a self-interacting fermionic model for DM. Using these EOSs as input, we solve the two-fluid Tolman-Oppenheimer-Volkoff (TOV) equations to systematically investigate the influence of DM on the global properties of neutron stars (NSs). By incorporating recent observational constraints on NS properties, the maximum DM mass fraction $f_{\chi }^{\max }$ in DANSs is determined for each NM EOS model. Our analysis reveals a strong linear correlation between $f_{\chi }^{\max }$ and the maximum mass of a pure NS, $M_{\mathrm{NS}}^{\max }$. Leveraging this correlation and the observed NS maximum mass distribution, $P(M_{\mathrm{NS}}^{\max }|\mathrm{EM})$, we derive the probability distribution function (PDF) for the maximum DM mass, $P(M_{\chi }^{\max }|\mathrm{EM})$, in DANSs. We find that at the 68% confidence level, $M_{\chi }^{\max }=0.15_{-0.05}^{+0.07}{M}_{\odot }$, which is confirmed to hold for a wide selection of dark matter EOS models. This quantitative constraint on the DM mass provides a critical prior for interpreting potential observational signatures of DANSs, such as anomalous tidal deformabilities and distinctive gravitational-wave signals.

In recent years, astrophysical observations have placed tight constraints on key properties of the nuclear equation of state (EoS). Using 93 two-dimensional simulations for three different EoS compatible with the current tight constraints, we show that the EoS remains a major uncertainty for the outcome of core-collapse supernovae. Whereas explosions are obtained in most cases for the SFHo and SFHx EoS, for the CMF EoS, which includes a crossover from nucleonic matter to a quark phase, explosions occur only for two out of 15 progenitors. Less favorable conditions for neutrino-driven explosions arise for the CMF EoS due to lower neutrino luminosities and mean energies and slightly weaker contraction of the warm protoneutron star. Our results suggest that the explodability of massive stars cannot yet be predicted based on first principles without better knowledge of the nuclear EoS. Conversely, observational constraints on stellar explodability may help further constrain the EoS.

Multifrequency Proca stars are excited self-gravitating solutions of the $s=1$ Schrödinger-Poisson system that generalize the conventional stationary states of a massive vector field. Unlike stationary states, which are characterized by a single oscillation frequency, multifrequency configurations exhibit a quasiperiodic dynamics involving two or three distinct frequencies. In this paper, we present a systematic study of the spectrum of spherical multifrequency Proca stars and show that, at fixed particle number, they form continuous families interpolating between discrete stationary states of constant linear polarization. Furthermore, we analyze their stability and demonstrate that a subset of these multifrequency configurations are linearly stable against general perturbations. In particular, we show that a necessary, although not sufficient, condition for stability is the presence of a non-negligible nodeless component, and that radial stability alone is not sufficient to guarantee full linear stability. Finally, we briefly discuss the potential implications of multifrequency states for proving the particle spin in ultralight dark matter models.

Observing gravitational waves from sub-solar-mass, inspiraling compact binaries would provide almost smoking-gun evidence for primordial black holes. Here, we develop a method to search for ultracompact binaries with chirp masses ranging from $\mathcal{M}\in [{10}^{-2},{10}^{-1}]M_{\odot }$. This mass range represents a previously unexplored gap in gravitational-wave searches for compact binaries: it was thought that the signals were too long for matched-filtering analyses but too short for time-frequency pattern-recognition techniques. Despite this, we show that a pattern-recognition technique, the generalized frequency-Hough transform, can be employed with particular modifications that allow us to handle rapidly spinning-up binaries and to increase the statistical robustness of our method; we call this improved method BinaryGFH-v2. We then design a hypothetical search for binaries in this mass regime, compare the empirical and theoretical sensitivities of this method, and project constraints on formation rate densities and the fraction of dark matter that primordial black holes could compose in both current- and future-generation gravitational-wave detectors. Our results show that our method can be used to search for sub-solar-mass, ultracompact objects in a mass regime that remains to-date unconstrained with gravitational waves.

The angular distribution of gravitational-wave power from persistent sources may exhibit anisotropies arising from the large-scale structure of the Universe. This motivates directional searches for astrophysical and cosmological gravitational-wave backgrounds, as well as continuous-wave emitters. We present results of such a search using data from the first observing run through the first portion of the fourth observing run of the LIGO-Virgo-KAGRA Collaborations. We apply gravitational-wave radiometer techniques to generate skymaps and search for both narrowband and broadband persistent gravitational-wave sources. Additionally, we use spherical harmonic decomposition to probe spatially extended sources. No evidence of persistent gravitational-wave signals is found, and we set the most stringent constraints to date on such emissions. For narrowband point sources, our sensitivity estimate to effective strain amplitude lies in the range $(0.03–8.4)\times {10}^{-24}$ across all-sky and frequency range ($20-160$) Hz. For targeted sources—Scorpius X-1, SN 1987A, the Galactic Center, Terzan 5, and NGC 6397—we constrain the strain amplitude with best limits ranging from $\sim 1.1\times {10}^{-25}$ to $6.5\times {10}^{-24}$. For persistent broadband sources, we constrain the gravitational-wave flux $F_{\alpha ,\widehat{n}}^{95\%,\mathrm{UL}}(25\mathrm{Hz})<(0.008–5.5)\times {10}^{-8}\mathrm{erg}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{\mathrm{Hz}}^{-1}$, depending on the sky direction $\widehat{n}$ and spectral index $\alpha =0,2/3$, 3. Finally, for extended sources, we place upper limits on the angular power spectrum $C_{\ell }^{1/2}<(0.63–17)\times {10}^{-10}{\mathrm{sr}}^{-1}$.

We present a phenomenological study of partonic spin correlations and $\mathcal{C}\mathcal{P}$ properties in $H\to gg$ decay channel at future lepton colliders. We investigate two classes of observables: Lund observable defined based on subjets and four-point energy-energy correlator (E4C) between particles inside two jets. Our results show that the E4C with energy weighted to the power of $n=4$ achieves the strongest sensitivity to the spin correlations of gluons from Higgs boson decay. Under the assumption of ideal identification of different gluon splitting modes, we estimate that future lepton colliders operating at $\sqrt{s}=240\mathrm{GeV}$ with an integrated luminosities of $5.6{\mathrm{ab}}^{-1}$ can successfully probe gluon spin correlations, while $20{\mathrm{ab}}^{-1}$ of data can probe the $\mathcal{C}\mathcal{P}$-mixing angle in the $Hgg$ coupling to $\lesssim 0.03\pi$ using E4C. We outline strategies for extending this framework to realistic detector-level analyses, which can provide a new pathway for the precision test of Standard Model and searches for new physics.

We investigate the perturbative and nonperturbative aspects of heavy quark transport in a thermal SU(3) gluon plasma. Based on the soft-hard factorized model, we extend the original perturbative framework to the near-critical temperature region, where nonperturbative effects become significant. The transition behavior of the semi-quark-gluon-plasma (semi-QGP) is described via a temperature-dependent background field incorporated in the background field effective theory. By implementing this approach, we quantitatively evaluate the collisional energy loss and momentum diffusion coefficients of charm and bottom quarks as functions of the incoming energy and medium temperature. Our results show a distinct suppression of both the energy loss and the diffusion coefficients relative to conventional perturbative estimates, especially near the critical temperature. This suppression originates from the emergence of a temperature-dependent color background field, which effectively reduces the color charge screening of the medium. These findings provide important theoretical insight into the phenomenology of heavy-flavor probes, offering a unified theoretical framework applicable across both high- and low-momentum regimes.

We analyze the charged particle multiplicity distributions in $p-p$ collisions and discuss the violation of the Koba-Nielsen-Olesen (KNO) scaling. We extract the deviations from the leading exponential behavior of the KNO scaled probability and identify a reciprocal symmetry $z\leftrightarrow 1/z$ in the KNO violating corrections observed in the ATLAS and CMS data at $\sqrt{s}=7,8,13\mathrm{TeV}$. The symmetry imposes a local constraint on the multiplicity distribution at $n=⟨n⟩$, namely $P^{\prime }(⟨n⟩)=-P(⟨n⟩)/⟨n⟩$, which we verify directly in the data. We use this constraint to extract the entanglement entropy from the well-measured region $n\simeq ⟨n⟩$, avoiding the large uncertainties associated with the distribution tail.

In the present work, we analyze the properties of the transition form factors (TFFs) for the $\gamma {\gamma }^{*}\to {\eta }^{(\prime )}$ process, employing the ${\eta }^{(\prime )}$-meson light-cone distribution amplitude (LCDA) derived within the light-cone sum rule framework. To this end, we adopt the quark-flavor mixing scheme for the ${\eta }^{(\prime )}$ meson, and compute the TFFs by systematically incorporating transverse-momentum corrections and contributions beyond the leading Fock state. We utilize light-cone harmonic oscillator models to parametrize the longitudinal and transverse behavior of the leading-twist light-cone wave function, for which the corresponding LCDA exhibits a unimodal profile. We further examine the potential contributions of intrinsic charm components to the scaled TFFs $Q^2{F}_{\eta \gamma }(Q^2)$ and $Q^2{F}_{{\eta }^{\prime }\gamma }(Q^2)$. Leveraging a range of values for the decay constant $f_{{\eta }_{c_0}}$ and implementing the $\eta –{\eta }^{\prime }–{\eta }_c$ and $\eta –{\eta }^{\prime }–G\text{−}{\eta }_c$ mixing mechanisms accordingly, together with the recently updated mixing angles, we investigate the impact of the intrinsic $c\overline{c}$ and gluonic component on these observables. In the high-$Q^2$ regime, $Q^2{F}_{{\eta }^{\prime }\gamma }(Q^2)$ exhibits a marked increase in sensitivity to the charm quark component, whereas $Q^2{F}_{\eta \gamma }(Q^2)$ becomes notably stabilized. A detailed discussion of ${\chi }^2/\mathrm{d}.\mathrm{o}.\mathrm{f}.$ (chi-squared per degree of freedom) and $p$ values indicates that the intrinsic charm quark component is important and yields a substantial, non-negligible contribution across the entire $Q^2$ range.

The method of simulation based inference is shown to lead to a more accurate resonance parameter estimation than traditional ${\chi }^2$ minimization in certain cases of model misspecification in a case-study of $\pi \pi$ scattering and the $\rho (770)$-resonance. Models fit to certain datasets using ${\chi }^2$ minimization can make inaccurate predictions for the pole position of the $\rho (770)$. Simulation based inference (SBI) is shown to make a more robust predictions for the pole positions. This is significant, both as a proof of concept that the SBI method can be used in cases of model misspecification, and because models of $\pi \pi$ scattering are a crucial part to many physical systems of contemporary interest [$a_1(1260)$, $\omega (782)$ etc.].

We conduct a mode analysis of a general $U(1)$-charged first-order relativistic hydrodynamics within the framework of effective field theory for dissipative fluids in flat Minkowski spacetime. We derive the most general quadratic action for hydrodynamic modes, including stochastic noise, and analyze the corresponding dispersion relations in a consistent gradient expansion. We argue that spontaneous breaking of spacetime symmetry arises in the presence of a local thermal state specified by a local timelike four-vector. We demonstrate that hydrodynamical perturbations can be identified as Nambu-Goldstone modes, analogous to their embedding in global $U(1)$-invariant theories. We find that frame-invariant combinations of hydrodynamic transport coefficients determine the first-order dispersion relations in the low-energy limit, making the mode analysis manifestly independent of the choice of hydrodynamic frame. Assuming local Kubo-Martin-Schwinger symmetry and unitarity of the underlying UV theory, we show that first-order hydrodynamics is stable if the enthalpy density is positive.

Millicharged particles are a well-motivated target for far-forward searches at the Large Hadron Collider. We identify and quantify a significant new source of these particles: secondary production in hadronic and electromagnetic showers initiated by energetic neutral particles striking the target neutral beam absorber. By combining Monte Carlo simulations with geant4-based modeling, we show that these secondary cascades yield a substantial millicharged particle flux that complements the primary production from the interaction point. For the proposed FORMOSA detector, this contribution can enhance the expected signal yield by approximately 50% for $m_{\chi }\lesssim 0.1\mathrm{GeV}$. Our results demonstrate that secondary production in downstream infrastructure is an essential ingredient for realistic sensitivity projections and new-physics searches at the High-Luminosity LHC. The simulated secondary spectra are made publicly available to facilitate future forward physics studies.

Atmospheric neutrinos provide a unique avenue to probe theories beyond the Standard Model (BSM) over a wide range of energies and path lengths. The theory of nonstandard interactions (NSI) of neutrinos is one of the important BSM scenarios, which can modify flavor oscillations of atmospheric neutrinos traveling through the Earth. In this work, we use a high-purity ${\nu }_{\mu }$ charged-current (CC) sample of atmospheric neutrinos from IceCube DeepCore with a livetime of 7.5 years to search for the NSI parameters ${ϵ}_{e\mu }$, ${ϵ}_{e\tau }$, and ${ϵ}_{ee}-{ϵ}_{\mu \mu }$. The ${\nu }_{\mu }$ CC events mainly come from the ${\nu }_{\mu }$ survival channel having no significant dependence on ${\delta }_{CP}$. Therefore, the constraints on ${ϵ}_{e\mu }$ and ${ϵ}_{e\tau }$ obtained using this ${\nu }_{\mu }$ CC sample are expected to be free from the ${\delta }_{CP}$-degeneracy. The data sample is found to be in agreement with the standard neutrino interactions. Therefore, we place bounds on these NSI parameters that are consistent with and comparable to existing experimental constraints. These ${\delta }_{CP}$-free constraints from IceCube DeepCore are complementary to those from the long-baseline neutrino oscillation experiments, where the appearance channel depends on ${\delta }_{CP}$.

We study a minimally extended version of the Standard Model where baryon number is gauged with a $U(1{)}_B$ symmetry. This model can be made anomaly-free by adding a set of additional fermions. The lightest component of these fermions behaves as a viable dark matter candidate. We show that the spontaneous breaking of $U(1{)}_B$ symmetry can produce gravitational waves via bubble dynamics resulting from a first-order phase transition, which can be detected in future gravitational wave experiments like LISA and ET. Such gravitational wave signatures can be used as a probe to constrain the model in future observations and complement dark matter and collider searches. We perform a random numerical scan of the parameter space and derive the viable region consistent with theoretical bounds from running of the coupling constants, current experimental bounds from dark matter experiments such as LUX-ZEPLIN and XENONnT and sensitive to future gravitational wave experiments. We find that dark matter with mass of 8–12 TeV is the most interesting to test in future gravitational wave as well as laboratory experiments. In the viable parameter space, the mass of the $Z^{\prime }$ gauge boson associated with the $U(1{)}_B$ lies in the 16–24 TeV range, and the mass of the scalar associated with the symmetry breaking lies around 1–2.5 TeV scale. Recent results from LUX-ZEPLIN rules out mass scales below TeV in this model, while dark matter with mass larger than 12 TeV will not be sensitive to future GW experiments. Hence, the dark matter and mediator mass scales of interest are marginally accessible at current collider energies.

Analysis of data from particle physics experiments traditionally sacrifices some sensitivity to new particles for the sake of practical computability, effectively ignoring some potentially striking signatures. However, recent advances in machine learning (ML)–based tracking allow for new inroads into previously inaccessible territory, such as reconstruction of tracks that do not follow helical trajectories. This paper presents a demonstration of the capacity of ML-based tracking to reconstruct the oscillating trajectories of quirks, particles charged under an unbroken QCD-like gauge symmetry. The technique used is not specific to quirks, and opens the door to a program of searching for many kinds of nonstandard tracks.

We study neutrino nonstandard interactions (NSI) induced by the trilinear $L_i{L}_j{E}_k^c$ (LLE) couplings in $R$-parity-violating supersymmetry in a split-family spectrum where third-generation sleptons can be comparatively light. We systematically classify the relevant coupling patterns, derive the corresponding effective interactions and NSI parameters, and confront the resulting parameter space with a broad set of experimental constraints. We find that percent-level NSI can be realized in selected channels, with the largest magnitudes reaching ${ϵ}_{ee}\simeq 8\%$ and ${ϵ}_{\mu \mu }\simeq 3.8\%$, while other entries are typically restricted to the subpercent level (including ${ϵ}_{\mu \tau }\simeq -0.3\%$ in the viable off-diagonal case). We also highlight the structural origin of the different constraint patterns between ${\tilde{\tau }}_R$- and ${\tilde{\tau }}_L$-mediated scenarios and comment on the relation to the Zee model.