Solving the classical equations of motion in general relativity recursively, we consider the metric of a spatially localized and stationary source of matter. Having in mind a star of general composition, we characterize it by means of its infinite set of mass and current multipoles. Specializing to de Donder gauge we set up the recursive equations that produce the metric outside the star to any desired order in perturbation theory, expanded both in Newton’s constant and in the order of multipoles. Up to second post-Minkowskian order we express the result to any order in the multipole expansion in terms of generalized (tensor) bubble integrals in momentum space and a corresponding simple expansion in inverse distances. In a special corner of the space of multipoles we recover the Kerr black hole solution to the given order. By tweaking just slightly the multipoles away from the Kerr limit the metric will describe stars that are Kerr-like and yet are not black holes. A subtlety with respect to the gauge ambiguity of de Donder gauge is also pointed out.

The spins of merging binary black holes (BBHs) inferred from gravitational-wave (GW) observations provide key insights into their formation channels. However, spin parameters are typically weakly constrained from data, and their inferred values are often strongly influenced by the assumed prior in Bayesian analyses. A commonly used prior, uniform in spin magnitudes and isotropic in spin directions, assigns vanishing probability density to spin-orbit-aligned configurations, potentially biasing inferences for BBH parameters. The prior choice can also affect population-level analyses by degrading the convergence of Monte Carlo integrations used to evaluate the likelihood in hierarchical Bayesian inference. In this work, we propose a novel spin prior that is uniform in the effective spin parameters ${\chi }_{\mathrm{eff}}$ and ${\chi }_{\mathrm{p}}$, two spin combinations that can be relatively well measured from GW data, conditioned on the mass ratio. Using simulated BBH populations, we show that the inferred spin population can depend on the choice of prior, and that the proposed prior more accurately recovers the underlying spin population, particularly when the true distribution favors aligned-spin configurations. Because mass and spin measurements are correlated, our prior also enables a more accurate recovery of the underlying mass distribution.

Determining the active screening mechanism from a general scalar-tensor Lagrangian remains a challenging problem. As a diagnostic tool, we present a systematic study of nonlinear cosmological perturbations in luminal Horndeski theories. Working in the $\alpha$ basis on a flat Friedmann-Lemaître-Robertson-Walker background, we derive and organize the full set of unapproximated second-order perturbation equations and systematically apply the quasistatic and weak-field limits. We find that second-order effects modify only the scalar-field equation. We derive, for static and spherically symmetric configurations, a master screening equation that classifies the nonlinear operators driving screening, recovering the Vainshtein mechanism and the onset of chameleon screening. We also identify a novel candidate regime, which we term Phaedrus screening, characterized by a screening radius that scales linearly with the source mass. For each mechanism, we derive analytical and numerical solutions and clarify the conditions under which they activate. Two new publicly available software packages are introduced: (i) xAlpha, a Mathematica package to compute and organize perturbation equations in scalar-tensor theories, and (ii) escut, a python module to solve the nonlinear scalar equation. In many cases, these tools enable the identification of the active screening type directly from a luminal Horndeski Lagrangian.

We derive analytical approximations to describe the ultrahigh-frequency secondary peak of the stochastic gravitational wave background generated by cosmic strings that is sourced by loops created in the friction-dominated era. We show that these approximations provide a very good description of the contribution of the friction-era loops over the relevant frequency range and for a broad range of cosmic string parameters, thus enabling a fast and accurate characterization of this signature. We also use these approximations to uncover the full parameter range in which this ultrahigh-frequency peak should be distinguishable on the stochastic gravitational wave background spectrum and show that it should be present in a broader range of high-energy physics scenarios than originally reported in [Phys. Rev. D 110, 063516 (2024)].

Several papers have recently calculated the contribution to pulsar timing array overlap reduction functions (ORFs) induced by our peculiar velocity with respect to the rest frame of the stochastic gravitational-wave background. Here we show that a harmonic-space calculation confirms the most recent result. We note that, with the harmonic-space calculation, the ORFs for spin-1 GWs and the correlations with astrometry measurements are also easily obtained.

Dark matter search strategies have started advancing towards the neutrino fog. In this regard, compact objects such as neutron stars have already demonstrated their ability in probing such low dark matter-nucleon cross sections from dark matter induced effects. In the optically thin limit, effect of dark matter self-interaction becomes relevant and may assist the capture and thermalization of dark matter inside stars, imparting observable changes on neutron star temperatures. The resulting radiation, although weak, can be potentially detected by the James Webb Space Telescope and upcoming Thirty Meter Telescope and the European Extremely Large Telescope. Observation of cold neutron stars accompanied by advancements in direct detection probes would provide stringent constraints or a smoking-gun signature for dark matter self-interactions. The potential detection of a neutron star with surface temperatures $\sim (1000–1200)\mathrm{K}$ in the optically thin limit can push the bounds on asymmetric dark matter self-interaction cross section to approximately 2 orders of magnitude more stringent than the bullet cluster.

The nature of dark matter (DM) is one of the greatest mysteries of modern astrophysics. While the standard cold DM (CDM) model successfully explains observations on most astrophysical scales, DM particles have not yet been detected, leaving room for a plethora of different models. In order to identify their observable signatures, we use the AIDA-TNG cosmological simulation suite to predict the distributions of gas and neutral hydrogen (HI) in the CDM, Self-interacting DM (SIDM), velocity-dependent SIDM (vSIDM), and warm DM (WDM) models. We find that the DM models investigated have very limited impact on the median gas and HI profiles of haloes. Nevertheless, for the most massive haloes ($M_{\mathrm{vir}}\sim {10}^{14}{M}_{\odot }$), we find that DM self-interactions can reduce the central potential, thereby lowering gravitational heating and the central gas temperature. We find that, in all models, the halo-to-halo variation in the HI profiles is explained by AGN feedback, and that the specific characteristics of the DM model are largely subdominant. Nevertheless, we detect some systematic difference in the case of SIDM, with more HI surviving close to the centre with respect to other models. We provide fitting functions for the gas and HI profiles. We investigate the galaxy-$\mathrm{Ly}\alpha$ cross-correlation function ($\mathrm{GaL}\alpha \mathrm{CC}$) for different halo masses, redshifts and observational strategies. We find that at $z=0$ vSIDM can be distinguished from CDM in haloes with ${10}^{12}\lesssim M_{\mathrm{vir}}\lesssim {10}^{13}{M}_{\odot }$, while SIDM1 can be distinguished from CDM in haloes with $M_{\mathrm{vir}}\gtrsim {10}^{13}{M}_{\odot }$. We estimate that statistically robust detection requires sampling $\sim 160$ haloes with $\sim 20$ sightlines each, a task that can be achieved with current and future facilities like WEAVE, 4MOST, PFS, ELT and WST.

We report time-resolved Neutron star Interior Composition Explorer and Swift x-ray spectroscopy of a bright flare from the black hole x-ray binary GRS $1915+105$ during its obscured state, which is characterized by heavy line-of-sight absorption by dense material with complex geometry. In April 2023, an unexpected flare was detected, with the observed x-ray flux increasing by nearly an order of magnitude relative to the typical obscured-state level. The spectra show pronounced variability, including significant evolution of the Fe K emission features. Time-resolved spectral modeling indicates that the main flare is associated with a combination of enhanced intrinsic emission and reduced obscuration. We further find that neutral and ionized reflection components are subject to distinct absorbers, whose evolving visibility implies a stratified absorber-reflector geometry. These properties are consistent with a reillumination phase following a failed disk wind. A delayed radio flare detected about 2.5 days later suggests a coupling between accretion and jet activity.

Neutrino rest mass enables left-handed states to “flip” to right-handed states and vice versa. In-medium effects can enhance the probability for such spin flip. We demonstrate that a full many-body calculation of this process in neutrino-dense environments can lead to spin-flip probabilities that exceed by orders of magnitude those calculated with mean-field treatments. We study simple configurations with a few neutrinos in well-defined momentum states, for which we show that the helicity conversion enhancement is connected to many-body momentum exchange. Such an effect would therefore be missed in a calculation that considers only forward processes. We speculate on the potential astrophysical implications of these results and the range of applicability of our calculation and its limitations.

We consider the problem of estimating cross-spectral quantities in the low-frequency regime, where long observation times limit averaging over large ensembles of periodograms, thereby preventing the use of approximate Gaussian statistics. This case is relevant for precision low-frequency gravitational experiments such as LISA and LISA Pathfinder. We present a Bayesian method for estimating spectral quantities in multivariate Gaussian time series. The approach, based on periodograms and Wishart statistics, yields closed-form expressions at any given frequency for the marginal posterior distributions of the individual power spectral densities, the pairwise coherence, and the multiple coherence, as well as for the joint posterior distribution of the full cross-spectral density matrix. In the context of noise projection—where one series is modeled as a linear combination of filtered versions of the others, plus a background component—the method also provides closed-form posteriors for both the susceptibilities, i.e., the filter transfer functions, and the power spectral density of the background. We apply the method to data from the LISA Pathfinder mission, showing effective decorrelation of temperature-induced acceleration noise and reliable estimation of its coupling coefficient.

In this work, I consider scalar field theory with negative quartic self-interaction, corresponding to an upside-down classical potential. Despite not possessing a classically stable ground state, such potentials are known to behave properly when treated quantum mechanically, leading to stable and unitary time evolution. Using two different saddle-point expansions for the same theory, I discuss the phase diagram in terms of bare parameters in Euclidean dimensions one to four, as well as the generalization to finite temperature. Comparing to other methods where available, I find that negative coupling field theory is a promising candidate for an interacting scalar field theory in the continuum. In particular, in four dimensions it exploits a loophole in mathematical proofs of quantum triviality, suggesting that negative coupling scalar field theory could offer a UV-complete and interacting description of the Higgs.

A kinetically mixed dark photon produced in $D^0\to \gamma A^{\prime }$ probes the timelike electromagnetic current at $q^2=m_{A^{\prime }}^2$, where resolved vector-meson structure can render a pointlike continuation from the real photon point inadequate. The magnitudes of the ${\rho }^0$ and $\varphi$ pole residues are normalized to measured radiative charm data and electronic vector-meson widths, while the $\omega$ contribution is bounded using the experimental upper limit on $D^0\to \gamma \omega$. Treating the parity conserving and parity violating amplitudes as a complex vector with two components, norm inequalities yield a lower envelope valid for arbitrary relative phases and orientations in this amplitude space once an upper bound on the smooth remainder is specified. At the $\varphi$ pole, the combined ceiling of the explicit non-$\varphi$ poles amounts to only 1.93% of the $\varphi$ pole amplitude norm for the central inputs. Consequently, complete cancellation at resonance requires the smooth remainder to reach at least 98.1% of the peak $\varphi$ pole amplitude norm within the adopted pole decomposition. For the representative $B_3$ benchmark, which bounds the norm of the smooth remainder by half the peak $\varphi$ pole amplitude, the conditional floor of the reduced branching fraction at resonance is ${\widehat{\mathcal{B}}}_{\mathrm{low}}^{(B_3)}(m_{\varphi }^2)=(1.89_{-0.14}^{+0.13})\times {10}^{-4}$. The corresponding lower envelope remains positive within approximately $\pm 3.5\mathrm{MeV}$ of the $\varphi$ pole. At resonance, this $B_3$ benchmark floor exceeds the pointlike comparison baseline, normalized to the experimental upper limit on $D^0\to \gamma \gamma$, by a factor of $(3.23\pm 0.23)\times {10}^2$. These results demonstrate that resolved hadronic structure can substantially affect dark photon production near the $\varphi$ resonance.

In this article, we study loop-induced Higgs decays into gauge bosons within the framework of radiative natural supersymmetry. We reproduce the one-loop MSSM calculations for the Higgs partial decay widths into a Z boson-photon pair, two photons, and two gluons, providing the corresponding analytical expressions for the scattering amplitudes. We focus on the region of parameter space that maximizes the rare decay width of the process $h\to Z\gamma$, and analyze the correlated predictions for the remaining Higgs decay channels. In the selected region of parameter space, the $h\to Z\gamma$ decay width is enhanced, reaching a maximum value of $\simeq 7.5\mathrm{keV}$, while remaining compatible with the current combined ATLAS measurement. At the same time, this region satisfies current Higgs constraints from the $h\to \gamma \gamma$ and $h\to gg$ channels. The diphoton mode remains close to the Standard Model expectation, with deviations at the level of $\lesssim 5\%$. The gluon channel exhibits a stronger sensitivity to the considered region of parameter space, leading to a moderate suppression of about 10% in the corresponding partial width.

We investigate constraints on large extra dimensions (LED) using the latest results from reactor antineutrino experiments. Specifically, we analyze the full data sets from Daya Bay, RENO, KamLAND, NEOS, and STEREO to derive updated bounds. For the case of one extra dimension, we find constrains on its radius $a$ of $a\lesssim 0.58\mathrm{\mu }\mathrm{m}$ ($a\lesssim 0.12\mathrm{\mu }\mathrm{m}$) at the 99% confidence level for normal (inverted) ordering, an improvement of approximately $\sim 20\%$ ($\sim 25\%$) compared to previous bounds from reactor experiments, assuming a massless lightest active neutrino. Furthermore, we present new limits on $4+d$ LED scenarios, with $d=2$, 3, 4 denoting the number of extra dimensions, based on the same reactor data and assuming equal radii for all extra dimensions. We find that the constraints become increasingly stringent with a larger number of extra dimensions. In particular, $d=4$ with a massless lightest active neutrino, we obtain limits of $a\lesssim 0.112\mathrm{\mu }\mathrm{m}$ for normal and $a\lesssim 0.018\mathrm{\mu }\mathrm{m}$ for inverted orderings at the 99% confidence level.

This paper explores a simplified extension of the standard model featuring a neutral fermion quintuplet and a scalar quadruplet, which together generate neutrino masses through tree and loop level mechanisms. The quintuplet fermions decay into standard model gauge bosons via the scalars, producing unique collider signatures at the LHC characterized by multilepton and multijet final states. The study focuses on the pair production of quintuplet fermions in the 700–1200 GeV mass range, where their decays produce highly boosted $W$ and $Z$ bosons identifiable as fat-jets. Emphasis is placed on the production and decay of doubly charged fermions due to their higher cross section. Advanced jet substructure and kinematic techniques are applied to enhance sensitivity by reducing standard model backgrounds. A detailed analysis of signal significance is performed in the two lepton, two fat-jet ($2\mathrm{L}2{\mathrm{F}}_{\mathrm{j}}$) and three lepton, one fat-jet ($3\mathrm{L}1{\mathrm{F}}_{\mathrm{j}}$) channels for different masses of the fermion and the scalars, optimizing selection cuts to maximize signal efficiency over standard model backgrounds. The study found that both channels exhibit excellent performance, with significance exceeding $5\sigma$ under realistic conditions including a 50% background uncertainty at integrated luminosities up to $3000{\mathrm{fb}}^{-1}$.

We present a UV completion of the Standard Model in which quarks and leptons are unified under color SU(4). A single fermionic representation, the real antisymmetric, provides the building blocks to address the strong $CP$ problem via the Nelson-Barr mechanism, while simultaneously correcting the charged-lepton and down-quark mass relations predicted by Pati-Salam theories for the two heaviest generations. We show that the characteristic scales of the theory are strongly constrained by its phenomenology. The interplay between the quality of the Nelson-Barr mechanism and the nonobservation of baryon-number-violating processes determines the scale of spontaneous $CP$ violation and the mass of the new vectorlike down quark. The theory predicts a distinctive baryon-number-violating decay mode of the neutron, $n\to K^{+}{\ell }^{-}$ (with $\ell =e$, $\mu$), which lies within the projected sensitivity of upcoming nucleon-decay experiments such as Hyper-Kamiokande, Jiangmen Underground Neutrino Observatory, and the Deep Underground Neutrino Experiment.

We explore the discovery potential of heavy neutral leptons (HNLs), motivated by models addressing the origin of neutrino masses, at the proposed high-energy same-sign muon collider known as $\mu \mathrm{TRISTAN}$. The study focuses on two complementary HNL-mediated signatures: (i) the lepton-flavor-violating (LFV) channel ${\mu }^{+}{\mu }^{+}\to W^{+}{\tau }^{+}{\overline{\nu }}_{\mu }$ and (ii) the lepton-number-violating (LNV) channel ${\mu }^{+}{\mu }^{+}\to W^{+}{W}^{+}$. The LNV process is the muon analogue of inverse neutrinoless double beta decay and, if observed, would provide strong evidence for Majorana neutrinos, while the LFV process offers a novel probe of flavor-changing neutral currents in the lepton sector. At the $\mu \mathrm{TRISTAN}$ collider with $\sqrt{s}\sim \mathcal{O}(10)\mathrm{TeV}$, the resulting sensitivity to the HNL mixing with muon and tau neutrinos, as a function of mass, can surpass current bounds from the measurements of electroweak precision observables over a broad mass range. In particular, for the mixing with muon neutrinos, the collider bound can improve by an order of magnitude for 5–10 TeV HNLs.

We investigate the lineshape of the $e^{+}{e}^{-}\to J/\psi {\pi }^{+}{\pi }^{-}$ cross section in the vicinity of the $D^{*}\overline{D}+\mathit{c}.\mathit{c}.$ threshold, where the “so-called” $G(3900)$ is observed in the $e^{+}{e}^{-}\to D\overline{D}$ channel. To take into account the possible $D^{*}\overline{D}+\mathit{c}.\mathit{c}.$ open channel effects or possible contributions from $G(3900)$, we include the intermediate meson loop transitions in $e^{+}{e}^{-}\to J/\psi {\pi }^{+}{\pi }^{-}$. As a consequence, a triangle singularity (TS) is fulfilled which can produce nontrivial structures in the $J/\psi \pi$ invariant-mass spectrum. Moreover, the TS transition also allows access to exotic quantum number of $(I,J^{P(C)})=(1,1^{-(-)})$ in the $J/\psi \pi$ invariant-mass spectrum. We present predictions for the $J/\psi \pi$ invariant-mass spectrum and our results clarify the different manifestations of the kinematic effects and genuine resonances. In particular, we show that resonance structures arising from the $P$-wave $D\overline{D}$ scatterings or hidden charm tetraquark state with $(I,J^{P(C)})=(1,1^{-(-)})$ can be identified by the $J/\psi \pi$ invariant mass spectrum. It can provide a theoretical guidance for future experimental searches for these exotic candidates.

We consider the energy loss of a high-energy parton in the early anisotropic plasma in heavy-ion collisions. Working in the harmonic approximation, we compute the change in the mean energy of an emitted gluon in the presence of an anisotropic background, characterized by anisotropic jet quenching parameters ${\widehat{q}}_x\ne {\widehat{q}}_y$. Evaluating the resulting integrals numerically, we compare with isotropic media, and obtain a simple pocket formula to estimate the impact of anisotropy on the mean emitted gluon energy, which is generally small. We then combine our results with the values of the jet quenching parameter extracted from QCD kinetic theory simulations and show that the medium length dependence of this mean energy loss exhibits the characteristics of limiting attractors, which can be obtained by extrapolating to zero and infinite coupling. Our study thus relates energy loss of jet partons to universal dynamics of anisotropic plasmas.