This article investigates the quasinormal modes (QNMs) features of regular asymptotically flat spacetimes using Hayward and Bardeen spacetimes as examples. It is found that, using tortoise coordinates $r_{*}$, the boundary conditions for QNMs in horizonless asymptotically flat regular (HFR) spacetime are similar to those of asymptotically anti–de Sitter (AAdS) black holes. Subsequently, this work demonstrates quasinormal mode oscillations in both types of spacetime using finite difference methods and observe certain similarities in their QNM characteristics. What is more, our results by the matrix method also show the proportional relationships: $\omega \propto r_p$ for large event horizon $r_p$ in extremal AAdS black hole spacetime and $w\propto \sqrt{f_o}$ for large values of extremum of shift function $f_o$ in HFR spacetime. These similar characteristics suggests that studying the properties of extremal AAdS black holes could provide insights into HFR spacetime, as well as similar spacetimes like gravastars or compact objects. Additionally, this work also show that the potential function for QNMs in HFR spacetime with small $f_o$ may include potential well regions, implying the presence of an echo effect in its quasinormal mode oscillations.

Supermassive black holes might originate from supermassive primordial black holes (PBHs). However, the hypothesis that these PBHs formed through the enhancement of the primordial curvature perturbations has consistently faced significant challenges due to the stringent constraints imposed by $\mu$ distortion in the cosmic microwave background. In this work, we investigate the impact of non-Gaussianity on $\mu$-distortion constraint in the context of broad power spectra. Our results show that, under the assumption of non-Gaussian curvature perturbations, a broad power spectrum may lead to weaker $\mu$-distortion constraints compared to the Gaussian cases. Our findings highlight the potential of the broad power spectrum to alleviate the $\mu$-distortion constraints on supermassive PBHs under large non-Gaussianity.

We discuss the neutrino mass and Hubble tension solutions and examine their effects on the redshift-space distortion (RSD) observations. An analysis with RSD data indicates smaller amplitude of perturbation. Including RSD data results in a slightly weaker upper limit on the neutrino mass than that derived for data without RSD, which is common in other extended models too. We have evaluated the impacts of RSD observations on some extended models, including the varying electron mass model, a time-dependent dark energy model with two parameter equations of state (EOS), and a model where the number of neutrino species is free. When we estimate the cosmological parameters for data including RSD, we found that the EOS parameter for dark energy is larger than that of the cosmological constant, and the effective number of neutrino species is smaller than the standard value, which infers a smaller present Hubble parameter $H_0$. From the viewpoint of cosmological tensions, the varying electron mass model with nonzero neutrino mass option looks promising to relax the Hubble tension and the $S_8$ tension simultaneously.

Considering inflationary magnetogenesis induced by time-dependent kinetic and axial couplings of a massless Abelian vector boson field breaking the conformal invariance, we show in this article that, surprisingly, the spectral shape of the large-scale primordial magnetic field power spectrum is insensitive to the postinflationary history, namely, the barotropic parameter ($w$) and the gauge coupling functions of the postinflationary era.

Neutron stars provide an ideal theoretical framework for exploring fundamental physics when nuclear matter surpasses densities encountered within atomic nuclei. Despite their paramount importance, uncertainties in the equation of state (EOS) have shrouded their internal structure. For rotating neutron stars, the shape of their surface is contingent upon the EOS and the rotational dynamics. This work proposes new universal relations regarding the star’s surface, employing Machine Learning techniques for regression. More specifically, we developed highly accurate universal relations for a neutron star’s eccentricity, the star’s ratio of the polar to the equatorial radius, and the effective gravitational acceleration at both the pole and the equator. Furthermore, we propose an accurate theoretical formula for $(d\log R(\mu )/d\theta {)}_{\max }$. This research addresses key astronomical aspects by utilizing these global parameters as features for the training phase of a neural network. Along the way, we introduce new effective parametrizations for each star’s global surface characteristics. Our regression methodology enables accurate estimations of the star’s surface $R(\mu )$, its corresponding logarithmic derivative $d\log R(\mu )/d\theta$, and its effective acceleration due to gravity $g(\mu )$ with accuracy better than 1%. The analysis is performed for an extended sample of rotating configurations constructed using a large ensemble of 70 tabulated hadronic, hyperonic, and hybrid EOS models that obey the current multimessenger constraints and cover a wide range of stiffnesses. Above all, the suggested relations could provide an accurate framework for the star’s surface estimation using data acquired from the NICER x-ray telescope or future missions, and constrain the EOS of nuclear matter when measurements of the relevant observables become available.

The pulsar timing technique, which compares the observed arrival times of electromagnetic radiation from a pulsar with the predicted arrival times derived from a theoretical model of the pulsar system, is used in pulsar astronomy to infer a multitude of physical information and to constrain possible corrections to general relativity (GR). The propagation delay is usually computed using formulas based on a post-Newtonian approach, for both the light trajectory and the orbital motion. However, evidence has recently emerged that this approximation may no longer be sufficient when the companion object is a supermassive black hole; deviations from a full GR computation of the propagation delay can reach a few seconds. In this paper, we analyze the case of binary pulsars with a stellar or intermediate black hole companion, whose discovery and timing are key goals of SKA. With a numerical algorithm, we have found that in this case, the full GR value depends only on the semimajor axis of the relative orbit and on the mass of the black hole companion. If the mass of the latter is sufficiently large ($100M_{\odot }$), the maximum difference between the two approaches is significant ($\sim {10}^{-7}\mathrm{s}$) even for large binaries ($\sim {10}^{16}\mathrm{cm}$), and increases up to $\sim {10}^{-4}\mathrm{s}$ when the mass is ${10}^5{M}_{\odot }$. We also consider relativistic corrections to the orbital motion, and discover that they can strongly affect the value of the propagation delay. We conclude that in the future, post-Newtonian formulas should be replaced with a more accurate approach in these systems, especially in view of future discoveries made by new large telescopes such as SKA.

Neutrino quantum kinetics in dense astrophysical environments is investigated relying on the mean-field approximation. In this paper, we heuristically explore whether beyond-mean-field effects due to neutrino degeneracy could hinder flavor instabilities that are otherwise foreseen. Our results show that these corrections shift the stability regions for a suite of (anti)neutrino ensembles; the flavor conversion of previously unstable distributions can be damped, but angular distributions that are stable in the mean-field case can also become unstable. Our work should serve as a motivation to further investigate the limitations of the mean-field treatment.

Core-collapse supernovae (SNe) provide a unique environment to study feebly interacting particles (FIPs) such as axionlike particles (ALPs), sterile neutrinos, and dark photons (DPs). This paper focuses on heavy FIPs produced in SNe, whose decay produces electrons and positrons, generating observable secondary signals during their propagation and annihilation. We focus on the in-flight annihilation of positrons, which emerge as the most significant contribution to the resulting $\gamma$-ray spectrum. Using data from COMPTEL and EGRET, we derive the most stringent bounds on the FIP-electron couplings for heavy ALPs, sterile neutrinos, and DPs. These results strengthen existing bounds by one to two orders of magnitude, depending on the FIP model.

We study the long-term evolution of selected hierarchical triple systems in Newtonian gravity. We employ analytic equations derived in Paper II for the evolution of orbit-averaged orbital elements for both inner and outer orbits, which include two classes of contributions. One class consists of linear-order contributions, including quadrupole, octupole, hexadecapole and dotriacontapole orders, the latter scaling as ${\epsilon }^6$, where $\epsilon =a/A$, the ratio of the semimajor axes of the inner and outer orbits. The second class consists of contributions at second order in the fundamental perturbation parameter; they contribute at orders ${\epsilon }^{9/2}$, ${\epsilon }^5$, ${\epsilon }^{11/2}$ and ${\epsilon }^6$. For well studied triples such as star-planet systems perturbed by a low-mass third body (“hot Jupiters”), second-order and dotriacontapole (SOD) effects induce only small corrections. For stellar-mass binaries orbiting supermassive black holes, SOD corrections can suppress orbital flips that are generated by purely first-order effects. Planets orbiting binary star systems are susceptible to significant variations in the planetary semimajor axis, an effect that does not occur at first perturbative order. SOD effects in triple black hole systems can induce migrations of the eccentricity to significantly larger values than predicted by first-order perturbations, with implications for the gravitational-wave induced inspiral of the inner binary. We also show that in most cases, evolutions using our SOD equations are in better agreement with those from direct integration of the N-body equations of motion than those from first-order perturbations through hexadecapole order.

The planned space-based gravitational wave detector, the Laser Interferometer Space Antenna, will provide a fundamentally new means of studying the orbital alignment of close white dwarf binaries. However, due to the inherent symmetry of their gravitational wave signals, a fourfold degeneracy arises in the transverse projections of their angular momentum vectors. In this paper, we demonstrate that by incorporating timing information from electromagnetic observations, such as radial velocity modulations and light curves, this degeneracy can be reduced to twofold.

We propose a novel $R_{\xi }$ gauge in the five-vector (5V) framework within the Abelian Higgs model. In the Cartesian basis of the complex Higgs field, the 5V $R_{\xi }$ gauge ensures nondivergent tree-level amplitudes for each Feynman diagram in the high-energy limit. This framework pinpoints the origin of high-energy divergences in tree-level amplitudes for each diagram, providing a criterion for quantifying the degree of divergences from other gauges. The 5V description necessitates treating the Goldstone field as the fifth gauge-field component, offering deeper insight into the dynamics of massive gauge bosons, particularly its longitudinal mode. The impact of this framework is demonstrated by rigorously comparing tree-level amplitudes from the 5V $R_{\xi }$ gauge with those from the conventional 4V $R_{\xi }$ gauge and the Feynman diagram gauge, the latter of which exhibits no gauge cancellation, similar to the 5V $R_{\xi }$ gauge.

The observation of $X_0(2900)$ in the $B^{+}\to D^{+}{D}^{-}{K}^{+}$ decay process indicates the existence of open flavor tetraquark states. We investigate the production and decay of the $X_0(2900)$ state with the final state interaction mechanism, where we calculate the strong vertices such as $g_{\overline{D}K{X}_0}$, $g_{{\overline{D}}^{*}{K}^{*}{X}_0}$, $g_{D_s^{*}\overline{D}K}$, and $g_{D_{s1}\overline{D}{K}^{*}}$ in the framework of the QCD sum rule method. We find that for the interpretation of the ${\overline{D}}^{*}{K}^{*}$ molecule of $X_0(2900)$, the branching fraction of the production process and the decay width are consistent with the experimental results, indicating that the observed $X_0(2900)$ could be interpreted as a ${\overline{D}}^{*}{K}^{*}$ molecule. However, we cannot exclude the possibility of a compact tetraquark interpretation within the uncertainty. More experimental and theoretical efforts are needed to fully understand the nature of the $X_0(2900)$ state.

The collider phenomenology of leptoquarks (LQs) and right-handed neutrinos (RHNs) has been studied extensively in the literature. Due to their gauge singlet nature, the production of RHNs at the LHC is typically suppressed by the tiny light-heavy neutrino mixing angles. In this study, we explore a promising scenario where the presence of a LQ mediator significantly enhances the RHN production: the LQ essentially couples only to first-generation fermions (quarks and leptons, including the RHN), and the coupling with the first-generation RHN dominates. We consider all possible scalar and vector LQs that could lead to such a scenario. The prospects are better for the first-generation scenario than the other generations because of the enhanced parton distribution functions (PDFs) of first-generation quarks. The enhanced PDFs boost the production cross sections of LQs, particularly their single and indirect productions. Incorporating all production modes of LQs that result in a pair of RHNs, we estimate the discovery prospects by analyzing the monoelectron and dielectron channels arising from the decay of the RHN pair. We find that the indirect production of LQs is crucial in determining the discovery reach at the HL-LHC for the first-generation scenario.

Heavy neutral leptons (HNLs) provide a compelling extension to the Standard Model, addressing the neutrino masses, baryogenesis, and dark matter problems. We perform a model-independent collider study, decoupling the active-sterile mixing angle ($V$) from the Yukawa coupling ($y$), and explore sensitivities at the HL-LHC for prompt and displaced decays. We also consider the possibility of HNLs being long-lived particles decaying in far detectors as FASER. In addition, we study the expected reach at FCC-ee for the prompt and displaced cases. For zero mixing, FCC-ee and HL-LHC sensitivities to $y$ are comparable, with Higgs width measurements imposing the strongest constraints. With nonzero mixing, sensitivities are dominated by $V$, significantly constraining parameter space. This work highlights the importance of precision Higgs studies and displaced searches in probing HNLs at current and future colliders.

We revisit the phenomenology of dark matter (DM) scenarios within radius-stabilized Randall-Sundrum models. Specifically, we consider models where the dark matter candidates are Standard Model (SM) singlets confined to the TeV-brane and interact with the SM via spin-2 and spin-0 gravitational Kaluza-Klein (KK) modes. We compute the thermal relic density of DM particles in these models by applying recent work showing that scattering amplitudes of massive spin-2 KK states involve an intricate cancellation between various diagrams. Considering the resulting DM abundance, collider searches, and the absence of a signal in direct DM detection experiments, we show that spin-2 KK portal DM models are highly constrained. In particular, we confirm that within the usual thermal freeze-out scenario, scalar dark matter models are essentially ruled out. In contrast, we show that fermion and vector dark matter models are viable in a region of parameter space in which dark matter annihilation through a KK graviton is resonant. Specifically, vector models are viable for dark matter masses ranging from 1.1 to 5.5 TeV for theories in which the scale of couplings of the KK modes is of order 40 TeV or lower. Fermion dark matter models are viable for a similar mass region, but only for KK coupling scales of order 20 TeV. In this work, we provide a complete description of the calculations needed to arrive at these results and, provide a discussion of new KK-graviton couplings needed for the computations, which have not previously been discussed in the literature. Here, we focus on models in which the radion is light, and the backreaction of the radion stabilization dynamics on the gravitational background can be neglected. The phenomenology of a model with a heavy radion and the consideration of the effects of the radion stabilization dynamics on the DM abundance will be addressed in forthcoming work.

In light of the recent branching fraction measurement of the $B^{+}\to K^{+}\nu \overline{\nu }$ decay by Belle II and its poor agreement with the Standard Model expectation, we analyze the effects of an axionlike particle (ALP) in $B$ meson decays. We assume a long-lived ALP with a mass of the order of the pion mass that decays to two photons. We focus on a scenario where the ALP decay length is of the order of meters such that the ALP has a non-negligible probability to decay outside the detector volume of Belle II, mimicking the $B^{+}\to K^{+}\nu \overline{\nu }$ signal. Remarkably, such an arrangement is also relevant for the long-standing $B\to \pi K$ puzzle by noting that the measured $B^0\to {\pi }^0{K}^0$ and $B^{+}\to {\pi }^0{K}^{+}$ decays could have a $B^0\to a{K}^0$ and $B^{+}\to a{K}^{+}$ component, respectively. We also argue based on our results that the required ALP-photon effective coupling belongs to a region of parameter space that can be extensively probed in future beam dump experiments like SHiP.

We present a method to directly detect the axion dark matter using nitrogen vacancy centers in diamonds. In particular, we use metrology leveraging the nuclear spin of nitrogen to detect axion-nucleus couplings. This is achieved through protocols designed for dark matter searches, which introduce a novel approach of quantum sensing techniques based on the nitrogen vacancy center. Although the coupling strength of the magnetic fields with nuclear spins is three orders of magnitude smaller than that with electron spins for conventional magnetometry, the axion interaction strength with nuclear spins is the same order of magnitude as that with electron spins. Furthermore, we can take advantage of the long coherence time by using the nuclear spins for the axion dark matter detection. Our method has the potential to be sensitive to a broad frequency range $\lesssim 100\mathrm{Hz}$ corresponding to the axion mass $m_a\lesssim 4\times {10}^{-13}\mathrm{eV}$. We present the detection limit of our method for both the axion-neutron and the axion-proton couplings and discuss its significance in comparison with other proposed ideas. We also show that the sensitivities of the NV center sensor to various spin species will open up new directions for constructing protocols that can mitigate magnetic noise effects.

We investigate the $S$-wave triply heavy tetraquark systems, including the $Q{Q}^{(\prime )}{\overline{Q}}^{(\prime )}\overline{q}$ and $Q{Q}^{(\prime )}{\overline{Q}}^{(\prime )}\overline{s}$ configurations ($Q^{(\prime )}=c$, $b$, $q=u$, $d$) with spin-parity $J^P=0^{+}$, $1^{+}$, and $2^{+}$, within the framework of the constituent quark model. We construct the wave functions for these systems, incorporating complete color-spin wave functions and spatial wave functions with three different Jacobi coordinate configurations. We use complex scaling and the Gaussian expansion method to solve the complex-scaled four-body Schrödinger equation and obtain possible exotic states. To analyze the spatial properties of the tetraquark states, we compute the root-mean-square (rms) radii, which help distinguish between meson-molecular and compact tetraquark states. We find that there do not exist any bound states in the $Q{Q}^{(\prime )}{\overline{Q}}^{(\prime )}\overline{q}$ and $Q{Q}^{(\prime )}{\overline{Q}}^{(\prime )}\overline{s}$ systems. However, we identify a possible molecular resonant state $T_{3c,2}(5819)$ in the $cc\overline{c}\overline{q}$ system with spin-parity $J^P=2^{+}$, which lies slightly above the $J/\psi D^{*}(2S)$ threshold and has a large rms radius around 2.2 fm. Furthermore, we obtain a series of compact resonant states, some of which exhibit spatial structure similar to the tritium atom in QED, where the light quark circles around the cluster of three heavy quarks. The lowest resonant state in the triply heavy systems has a mass of 5634 MeV, a width of 16 MeV, and spin-parity $J^P=1^{+}$. We suggest searching for this state in the $J/\psi D$, ${\eta }_c{D}^{*}$ and $J/\psi D^{*}$ decay channels.

We present a Monte Carlo simulation-based partial wave analysis of the decay ${\mathrm{\Xi }}_c^{+}\to {\mathrm{\Xi }}^{-}{\pi }^{+}{\pi }^{+}$ by using the Feynman-Diagram-Calculation framework. The consistency of the input and output parameters implies the reliability of the partial wave analysis method. A robust method of the spin-parity determination of the ${\mathrm{\Xi }}^{*0}$ resonance is introduced. The rejection significance of the alternative spin-parity hypothesis over the favored hypothesis is strong for different spins, but weak for different parities based on the current integrated luminosity of the Belle experiment. Further data collection is needed to improve the sensitivity of parity determination in the future, e.g., Belle II experiment.