A key consequence of Lorentz-violating gravity is the emergence of modified dispersion relations implying the absence of a universal maximum propagation speed. This challenges the conventional notion of the event horizon as a causal boundary common to all degrees of freedom. However, certain solutions in these theories exhibit universal horizons—surfaces capable of trapping signals of arbitrarily high speed, thereby restoring the notion of a black hole. Previous studies have extensively characterized universal horizons in settings where Lorentz violation is encoded via a hypersurface-orthogonal æther. In this work, we explore the possibility of extending this concept to more general cases where hypersurface orthogonality is relaxed. To do so, we construct a candidate trapping surface and analyze its causal properties using a general model for Lorentz-violating matter. We find that, in addition to the standard conditions associated to universal horizons, a local vanishing of the æther’s twist is also necessary. We then provide an explicit example of such a universal horizon by suitably deforming the æther flow in a stealth Kerr solution recently found in Einstein-æther theory. Moreover, we analyze the behavior of trajectories which are not analytical at the universal horizon and discuss the implications of our findings for Hawking radiation. While our analysis is motivated by Einstein-æther gravity, our results apply to broader classes of Lorentz-violating theories, further supporting the relevance of black hole phenomenology in these frameworks.

We consider nonlinear trident, $e^{-}\to e^{-}{e}^{-}{e}^{+}$, in various electric background fields. This process has so far been studied for plane-wave backgrounds, using Volkov solutions. Here we first use WKB for trident in time-dependent electric fields, and then for fields which vary slowly in space. Then we show how to use worldline instantons for more general fields which depend on both time and space. For time-dependent fields the WKB approach is at least as simple to use as the worldline approach, but already the relatively modest step of including a slow spatial dependence makes the worldline approach much more efficient.

Motivated by the deviations observed between the experimental measurements and the Standard Model (SM) predictions of the branching ratios of ${\overline{B}}_s^0\to D_s^{+}{\pi }^{-}$ and ${\overline{B}}_d^0\to D^{+}{K}^{-}$ decays, we revisit the two-body hadronic $B_c^{-}\to J/\psi ({\eta }_c)L^{-}$ decays, with $L=\pi ,K^{(*)},\rho$, both within the SM and beyond. Since these processes are also mediated by the quark-level $b\to c\overline{u}d(s)$ transitions and hence dominated by the color-allowed tree topology, the QCD factorization is generally expected to hold in the heavy-quark limit. First, we update the SM predictions of the observables of these decays by including the nonfactorizable vertex corrections to the hadronic matrix elements of the SM four-quark operators up to the next-to-next-to-leading order (NNLO) in ${\alpha }_s$. It is found that, relative to the leading-order results, the branching ratios of these decays up to the next-to-leading-order (NLO) and NNLO corrections are always enhanced, with a relative amount given by ${\delta }_{\mathrm{NLO}}=({\mathcal{B}}^{\mathrm{NLO}}-{\mathcal{B}}^{\mathrm{LO}})/{\mathcal{B}}^{\mathrm{LO}}\approx +6\%$ and ${\delta }_{\mathrm{NNLO}}=({\mathcal{B}}^{\mathrm{NNLO}}-{\mathcal{B}}^{\mathrm{LO}})/{\mathcal{B}}^{\mathrm{LO}}\approx +9\%$, respectively. To minimize the uncertainties brought by the Cabibbo-Kobayashi-Maskawa matrix element $V_{cb}$ as well as the $B_c\to J/\psi ({\eta }_c)$ and $B_{(s)}\to D_{(s)}^{(*)}$ transition form factors, we construct ratios of the nonleptonic decay rates with respect to the corresponding differential semileptonic decay rates evaluated at $q^2=m_L^2$ (for $R_{J/\psi ({\eta }_c)L}$ and $R_{(s)L}^{(*)}$) or integrated over the whole $q^2$ range (for $R_{\pi /\mu {\nu }_{\mu }}$), which are then used to constrain the model-independent new physics (NP) Wilson coefficients. After considering the latest Belle data on ${\overline{B}}^0\to D^{(*)+}\pi (K{)}^{-}$ decays and the updated fitting results of the $B_{(s)}\to D_{(s)}^{(*)}$ transition form factors, we find that the deviations can still be explained by the NP four-quark operators with $(1+{\gamma }_5)\otimes (1-{\gamma }_5)$ and $(1+{\gamma }_5)\otimes (1+{\gamma }_5)$ structures, while the solution with ${\gamma }^{\mu }(1+{\gamma }_5)\otimes {\gamma }_{\mu }(1-{\gamma }_5)$ structure does not work anymore, under the combined constraints from the ratios $R_{(s)L}^{(*)}$ at the $2\sigma$ level. Furthermore, the ratio $R_{\pi /\mu {\nu }_{\mu }}$, once measured precisely, could provide complementary constraint on these NP Wilson coefficients allowed by $R_{(s)L}^{(*)}$. With the large $B_c$ events expected at the Large Hadron Collider, we hope to obtain more precise measurements of these observables, which can be exploited to discriminate the different NP scenarios responsible for the deviations observed in the two-body nonleptonic $B$-meson decays.

We report new constraints and sensitivities to heavy neutral leptons (HNLs) with transition magnetic moments, also known as dipole-portal HNLs. This is accomplished using data from the T2K ND280 near detector in addition to the projected three-year dataset of the upgraded ND280 detector. Dipole-portal HNLs have been extensively studied in the literature and offer a potential explanation for the $4.8\sigma$ MiniBooNE anomaly. To perform our analysis, we simulate HNL decays to $e^{+}{e}^{-}$ pairs in the gaseous time projection chambers of the ND280 detector and its upgrade. Recasting an ND280 search for mass-mixed HNLs, we find that ND280 data places world-leading constraints on dipole-portal HNLs in the 390–743 MeV mass range, disfavoring the region of parameter space favored by the MiniBooNE anomaly. The addition of three years of ND280 upgrade data will be able to disfavor the MiniBooNE solution at the $5\sigma$ confidence level and extend the world-leading constraints to dipole-portal HNLs in the 148–860 MeV mass range. Our analysis suggests that ND280 data excludes dipole-portal HNLs as a solution to the MiniBooNE excess, motivating a dedicated search within the T2K collaboration and potentially highlighting the need for alternative explanations for the MiniBooNE anomaly.

We present an update of HPQCD’s lattice quantum chromodynamics (QCD) determination of the $B_c\to J/\psi$ vector and axial-vector form factors, and provide new results for the tensor form factors. We use the highly improved staggered quark (HISQ) action for all valence quarks, together with the second generation MILC $n_f=2+1+1$ HISQ gluon field configurations. This calculation includes two additional ensembles, one with physically light up and down quarks and $a\approx 0.06\mathrm{fm}$ and one with $a\approx 0.03\mathrm{fm}$ on which we are able to reach the physical bottom quark mass. Our calculation uses nonperturbatively renormalized current operators and covers the full kinematical range of the decay. We use our recent results for the heavy-charm susceptibilities, as a function of $u=m_c/m_h$, in order to employ the full dispersive parametrization for $B_c\to J/\psi$ in our physical-continuum extrapolation. We give updated Standard Model (SM) predictions $R(J/\psi )=0.2597(27)$, $A_{{\lambda }_{\tau }}=0.5093(42)$, $F_L^{J/\psi }=0.4421(55)$, and ${\mathcal{A}}_{\mathrm{FB}}=-0.0567(61)$, reducing uncertainties by 29%, 45%, 40%, and 50% respectively. Since our lattice form factors cover the full kinematic range we can use them to test extrapolations using data in a truncated range, at low-recoil. We investigate different physical continuum parametrization schemes, with lattice results in the first $1/3$ of the kinematic range near $q_{\max }^2$. We find that unexpectedly large systematic uncertainties near $q^2=0$ can emerge when extrapolating synthetic data in the high-$q^2$ region if higher order kinematical terms are omitted from the physical continuum extrapolation. This suggests a potentially underestimated systematic uncertainty entering extrapolations of synthetic lattice QCD data for the related $B\to D^{*}\ell \overline{\nu }$ decay from the high-$q^2$ region into the low-$q^2$ region.

We measure the $CP$ asymmetry in $D^{+}\to {\pi }^{+}{\pi }^0$ decays reconstructed in $e^{+}{e}^{-}$ collisions at the Belle II experiment using a dataset corresponding to an integrated luminosity of $428{\mathrm{fb}}^{-1}$. A control sample of $D^{+}\to {\pi }^{+}{K}_S^0$ decays is used to correct for detection and production asymmetries. The result $A_{CP}(D^{+}\to {\pi }^{+}{\pi }^0)=(-1.8\pm 0.9\pm 0.1)\%$, where the first uncertainty is statistical and the second systematic, is the most precise determination to date. It agrees with the prediction of $CP$ symmetry from the standard model and with results of previous measurements.

Kaluza-Klein reductions of 11-dimensional supergravity lead to exceptional global symmetries in lower dimensions. Certain nongeometric elements of these symmetries, parametrized by a trivector $\gamma$, are not inherited from the higher-dimensional local symmetries, but represent instead a symmetry enhancement produced by the isometries of the background. Here, we demonstrate how to realize this enhancement in 11 dimensions, as a symmetry principle with constrained parameters. We show that $\gamma$ transformations exchange the equations of motion of the metric and the three-form with their Bianchi identities, in a closed form, structuring them into trivector multiplets. Implementing this principle as an off shell symmetry of the theory requires the introduction of a hierarchy of dual fields, including a six-form and a dual graviton in the initial levels.

We explore, within the framework of linearized quantum gravity, the Casimir-Polder (CP)–like quantum gravitational interaction between an excited nonpointlike object and a gravitational Dirichlet boundary based on perturbation theory. Our result shows that, the gravitational Casimir-Polder interaction potential for an excited object decreases as $z^{-5}$ in the near regime where the distance between the object and the boundary is smaller than the characteristic transition wavelength of the object, while it oscillates as $z^{-1}$ in the far regime where the distance is larger than the characteristic transition wavelength, with $z$ denoting the object-boundary distance. Compared to the gravitational CP force between a ground-state object and a gravitational Dirichlet boundary, which is repulsive in both the near and far regimes, the present interaction can be either attractive or repulsive in the far zone, depending on the object-boundary distance and the transition wavelength of the object. Moreover, the far-region behavior of an excited object decays as $z^{-1}$, which is significantly slower than the $z^{-6}$ dependence observed for a ground-state object. This suggests that the gravitational CP interaction can be enhanced or modulated by altering the initial state of the nonpointlike object.

We study the quasinormal modes (QNMs) of a family of generalized black bounces interpolating between regular black holes and traversable wormhole solutions according to a single extra parameter $a$. Firstly, working with a generic spherically symmetric spacetime with arbitrary radial function and an anisotropic fluid matter source, the general equations for the gravitational waves are obtained. Then, we focus on such particular spacetime metric and use the time domain method to find the evolution of the QNMs with respect to the parameter $a$, finding larger frequencies and damped modes as $a$ grows. Furthermore, we find that, for a gap in the values of $a$ for which no horizon is present but several photon spheres are, echoes are produced. Such echoes, which come from trapped modes in the potential well that are eventually leaked off for higher frequencies, appear as repetitions of the original wave but with modulated amplitude and decreased frequencies, and we study their evolution with $a$. In addition, in the light of the correspondence recently discussed in the literature between QNMs and black hole imaging, we discuss the relation of the features of such echoes with those features (photon rings and shadows) of optical images from thin accretion disks. Despite working with simplified models and settings, our analysis provides useful insights on the usefulness of the correspondence for both gravitational waves and shadows.

We propose a new model of black hole spontaneous scalarization that combines a scalar-Gauss-Bonnet interaction with a nonminimal coupling to a U(1) gauge field (a dark photon or an electromagnetic field). This construction generalizes earlier single-coupling setups and allows both curvature-induced and matter-induced scalarization within one framework, which allows us to overcome the limitations of each mechanism alone. We focus on charged, spherically symmetric black holes and demonstrate that our model substantially expands the range of black hole masses and charges that permit scalar hair. Negative Gauss-Bonnet couplings, previously associated only with near-extremal charges or rapidly spinning black holes, now trigger scalarization for much broader charge intervals. We develop a numerical procedure to solve the field equations, and investigate the various properties of these black holes. This results in new branches emerging at distinct mass thresholds, a behavior not seen in the pure Einstein-scalar-Gauss-Bonnet or Einstein-scalar-Gauss-Bonnet-Ricci models. The scalar charge depends sensitively on the coupling parameters and on the $U(1)$ charge. Our analysis also shows that these black holes have larger entropy than their Reissner-Nordström counterparts and can become overcharged, surpassing the usual extremal limit of charge-to-mass ratio. Analyzing the scalar charge behavior suggests that adding matter coupling appears to stabilize solutions that were previously prone to higher-order instabilities in pure Gauss-Bonnet models with quadratic coupling and broadens the range of possible configurations, making this model a promising candidate for further studies in strong gravity.

We study the dynamics of a binary system orbiting a rotating supermassive black hole (SMBH). Using Fermi-Walker transport, we construct a local inertial reference frame in the Kerr spacetime and set up a Newtonian binary system. Assuming the binary moves on a spherical orbit with constant radius around the black hole, we derive the equations of motion governing its dynamics. We focus on von Zeipel-Lidov-Kozai (vZLK) oscillations, which arise when the binary is compact and its initial inclination exceeds a critical angle. The libration of the binary’s orbit in the latitudinal direction affects the vZLK oscillations: as the libration angle increases, the oscillation period shortens, and the maximum eccentricity grows, particularly when the oscillations become chaotic. Notably, when the binary is sufficiently soft yet remains stable, the oscillation period is reduced to the dynamical timescale rather than the secular timescale. This effect arises due to the interaction between the SMBH spin and the binary’s angular momentum. As the Kerr rotation parameter increases or the radius of the spherical orbit decreases, both the enhancement of maximum eccentricity and the reduction in oscillation period become more pronounced.

Matched-filtering searches for gravitational-wave signals from compact binary mergers employ template banks which are a collection of modeled waveforms described by unique intrinsic parameters. We present two banks designed for low-latency and archive subsolar mass (SSM) searches in data from the fourth observing run of LIGO-Virgo-KAGRA, and demonstrate the efficacy of the banks via simulated signals. Further, we introduce several new methodological developments to the geometric placement algorithm [C. Hanna et al., Phys. Rev. D 108, 042003 (2023); C. Hanna, manifold (2024)], including a more stable metric-estimation technique and a boundary-padding procedure. Together, these enhancements enable the method to work for exceedingly low component masses necessary for SSM bank production, representing the first successful application of the manifold-method to lower-mass regions of the compact binary parameter space. The archive search bank contains a total of 3,452,006 templates, and covers a mass parameter space of 0.2 to $10M_{\odot }$ in the larger component and 0.2 to $1.0M_{\odot }$ in the smaller component, the spin parameter space of $-0.9$ to 0.9 for masses above $0.5M_{\odot }$ and $-0.1$ to 0.1 for masses below $0.5M_{\odot }$, and the mass ratio parameter space of 1 to 10. The power spectral density used was from a week of the first half of the fourth observing run of Advanced LIGO, Advanced Virgo, and KAGRA, and the low frequency cutoff was set to 45 Hz with a maximum waveform duration of 128 secs. The bank simulations performed using sbank have shown that the banks presented in this paper have sufficient efficacy for use in their respective searches.

Neutrino emission offers a direct probe into the hydrodynamics and energy transport processes within a supernova. Fast-time variations in the neutrino luminosity and mean energy can provide insights into phenomena like turbulence, convection, and shock revival. In this paper, we explore the detection capabilities of large-volume neutrino telescopes such as the IceCube Neutrino Observatory and the planned IceCube-Gen2 detector in identifying generic fast-time features in the neutrino lightcurve. We also investigate the potential enhancement in detection sensitivity using wavelength shifters, which can improve light collection efficiency. By employing a short-time Fourier transform analysis, we quantify the excess power in the frequency spectrum arising from fast-time modulations and compute the detection horizon for a range of generic models. We find that with IceCube we can already see the strongest modulation models ($>50\%$ amplitude) for progenitors located anywhere in the Milky Way. Sensitivity to weaker modulations ($>20\%$ amplitude) is possible in future detectors like IceCube-Gen2, in particular with the use of wavelength shifters. For all detector configurations, the frequency and central time of the fast-time feature at the $5\sigma$ detection horizon can be measured with a resolution of $7.0\mathrm{Hz}$ and 17 ms, respectively.

The Laser Interferometer Space Antenna (LISA) will detect gravitational waves from the population of merging massive black holes binaries (MBHBs) throughout the Universe. The LISA data stream will feature many superposed signals from different astrophysical sources, requiring a global fit procedure. Most of the MBHB signals will be loud enough to be detected days or even weeks before the merger, and for those sources LISA will be able to predict the time of the merger well in advance of the coalescence, as well as an approximate position in the sky. In this paper, we present a fast detection and signal reconstruction scheme for massive black hole binaries in the LISA observation band. We propose: (i) a detection scheme for MBHB mergers allowing a first subtraction of these signals for the purpose of a global fit, and (ii) an efficient early detection scheme providing a time-of-merger estimate for a premerger signal, that will allow one to trigger a protection period, placing LISA in “do not disturb” mode and enabling more detailed analysis that will facilitate multimessenger observations. We highlight the effect of confusion of several overlapping in time MBHB signals in the premerger detection.

Accurate modeling of how high-energy proton-proton collisions produce gamma rays through the decays of pions and other secondaries is needed to correctly interpret astrophysical observations with the Fermi-LAT telescope. In the existing literature on cosmic-ray collisions with gas, the focus is on the gamma-ray yield spectrum, $d{N}_{\gamma }/dE$. However, in some situations, the joint energy and angular distribution can be observed, so one needs instead $d^2{N}_{\gamma }/dEd\mathrm{\Omega }$. We provide calculations of this distribution over the energy range from the pion production threshold to 100 GeV, basing our results on fluka simulations. We provide the results in tabular form and provide a python tool on github to aid in utilization. We also provide an approximate analytic formula that illuminates the underlying physics. We discuss simplified examples where this angular dependence can be observed to illustrate the necessity of taking the joint distribution into account.

We present a comprehensive study of axion condensed neutron stars that arise in models of an exceptionally light axion that couples to quantum chromodynamics (QCD). These axions solve the strong-charge-parity ($CP$) problem, but have a mass-squared lighter than that due to QCD by a factor of $ϵ<1$. Inside dense matter, the axion potential is altered, and much of the matter in neutron stars resides in the axion condensed phase where the strong-$CP$ parameter $\theta =\pi$ and $CP$ remains a good symmetry. In these regions, masses and interactions of nucleons are modified, in turn changing the equation of state (EOS), structure, and phenomenology of the neutron stars. We take the first steps toward the study of the EOS of neutron star matter at $\theta =\pi$ within chiral effective field theory and use relativistic mean field theory to deduce the resulting changes to nuclear matter and the neutron star low-density EOS. We derive constraints on the exceptionally light axion parameter space based on observations of the thermal relaxation of accreting neutron stars, isolated neutron star cooling, and pulsar glitches, excluding the region up to $5\times {10}^{-7}\lesssim ϵ\lesssim 0.2$ for $m_a\gtrsim 2\times {10}^{-9}\mathrm{eV}$. We comment on potential changes to the neutron star mass-radius relationship, and discuss the possibility of novel, nuclear-density compact objects with $\theta =\pi$ that are stabilized not by gravity but by the axion potential.

The interface between quantum mechanics and gravity remains an unresolved issue. Recent advances in precision measurement suggest that detecting gravity-induced entanglement in oscillator systems could provide key evidence for the quantum nature of gravity. However, thermal decoherence imposes strict constraints on system parameters. For entanglement to occur, mechanical frequency ${\omega }_m$, dissipation rate ${\gamma }_m$, environmental temperature $T$, oscillator density $\rho$, and the form factor $\mathrm{\Lambda }$—determined by the geometry and arrangement of oscillators—must satisfy a specific constraint. This constraint, intrinsic to the noise model, is considered universal and cannot be improved by quantum control. Given the difficulty in further optimizing ${\omega }_m$, ${\gamma }_m$, $\rho$, and $T$, optimizing $\mathrm{\Lambda }$ can relax the constraints on these parameters. In this work, we prove that the form factor has a supremum of $2\pi$, revealing a fundamental limit of the oscillator system. We propose designs that approach this supremum, nearly an order of magnitude higher than typical spherical oscillators. This optimization could ease experimental constraints and bring quantum gravity validation based on gravity-induced entanglement closer to realization.

The nature of the $X(3872)$ and other exotic hadrons has been a subject of extensive investigation since the first observation of the $X(3872)$ in 2003. While various theoretical models have been proposed, including hadronic molecular and compact tetraquark interpretations, some experimental evidence suggests that the $X(3872)$ may be a mixture state of a hadronic molecule and a $c\overline{c}$ core. In this work, we perform a systematic study of the hidden-charm tetraquark candidates $X(3860)$, $X(3872)$, and $Z(3930)$ using a coupled-channel model that incorporates both $c\overline{c}$ states and $D^{(*)}{\overline{D}}^{(*)}$ hadronic molecular components. The $c\overline{c}$ sector is described based on the constituent quark model predictions for the ${\chi }_{cJ}(2P)$ ($J=0$, 1, 2) states, while the meson-meson interactions are modeled using pseudoscalar and vector-meson exchange potentials. The model parameters are fixed to reproduce the masses of the $X(3872)$ and $Z(3930)$, and the resulting framework is used to predict the mass and structure of the $J^{PC}=0^{++}$ state associated with the $X(3860)$. Our results support the mixture interpretation of these exotic hadrons, exhibiting strong attractions from the transition potential between $c\overline{c}$ and $D^{(*)}{\overline{D}}^{(*)}$ components. The molecular component is found to dominate in the $X(3872)$, while the $c\overline{c}$ component plays a more prominent role in the $X(3860)$ and $Z(3930)$.

We revisit the exclusion constraints on the parameters of a narrow dark photon set by direct experimental searches. We investigate how a dark photon with a larger decay width impacts these limits, in particular, in the case where the dark photon also decays into light dark matter. As an example, taking the upper limits on the mixing parameter, $\epsilon$, reported by the CMS Collaboration, we find that they could be significantly relaxed. Indeed, even a very modest coupling of the dark photon to dark matter can lead to an increase in the bound on the mixing parameter by an order of magnitude.

Motivated by a recent Belle II measurement that suggests an excess in the rare decay $B\to K{E}_{\mathrm{miss}}$, and building upon our recent differential decay rate likelihood analysis of the existing experimental information, we investigate possible new physics (NP) scenarios in which light invisible states participate in flavor-changing $b\to s$ transitions. In particular, we consider the total and differential $B\to K^{*}{E}_{\mathrm{miss}}$ decay rates and $K^{*}$ polarization effects in each NP scenario preferred by the $B\to K{E}_{\mathrm{miss}}$ measurement. We show that future measurements of these $B\to K^{*}{E}_{\mathrm{miss}}$ observables will offer decisive discrimination among the different NP explanations. Our results highlight the strong complementarity of the rare semi-invisible $b$-hadron decay observables, and underline the importance of analysing their momentum transfer spectra when probing extensions of the Standard Model that feature new light degrees of freedom.