We present a novel approach for implementing baryogenesis via leptogenesis at low scale within the neutrino seesaw framework where a sufficient lepton asymmetry can be generated via out-of-equilibrium $CP$-violating decays of right-handed neutrinos (RHNs) even when their mass falls below the Standard Model (SM) Higgs mass. It becomes possible by keeping the sphaleron in equilibrium below its conventional decoupling temperature $T_{\mathrm{sp}}^{\mathrm{SM}}\sim 131.7\mathrm{GeV}$ in SM so as to facilitate the conversion of lepton asymmetry to baryon asymmetry at such a low scale, thanks to the flexibility of the bubble nucleation temperature in case the electroweak phase transition (EWPT) is of first order. The scenario emerges as an exciting (and perhaps unique) possibility for low scale leptogenesis, particularly if the Universe attains a reheating temperature lower than 131.7 GeV. We show that a stochastic gravitational wave, characteristic of such first order EWPT, may be detected in near future detectors, while the presence of RHNs of mass as low as 35 GeV opens up an intriguing detection possibility at current and future accelerator experiments.

A massless scalar field in two spacetime dimensions splits into two independent sectors of left- and right-moving modes on the light cone. At the quantum level, these two sectors carry a representation of the group of affine transformations of the real line, with translations corresponding to transformations generated by light-cone momenta and dilations given by light-cone Rindler momenta formed by a linear combination of generators of boosts and dilations. One-particle states for inertial observers are eigenvectors of translation generators belonging to irreducible representations of the affine group. Rindler one-particle states are related to eigenfunctions of the generator of dilations. We show that simple manipulations connecting these two representations involving the Mellin transform can be used to derive the thermal spectrum of Rindler particles observed by an accelerated observer. Beyond providing a representation-theoretic basis for vacuum thermal effects, our results suggest that analogous phenomena may arise in any quantum system admitting realizations of translation and dilation eigenstates.

We investigate the geometry of a tidally deformed, rotating black hole and timelike geodesics in its vicinity. Our framework provides a local picture of the structural evolution of a relativistic restricted three-body problem around a deformed black hole in an adiabatically evolving binary, motivated by various astrophysical settings including disk dynamics and extreme mass-ratio inspirals. As the tidal-field strength is increased, initially regular, bound geodesics undergo four stages: (i) weak chaos emerges within the bound motion; (ii) a subset of trajectories plunges into the black hole; (iii) a fraction of the remaining trajectories becomes unbound; and (iv) no bound trajectories persist. We provide semianalytic estimates for the critical tidal amplitudes associated with each transition. Our estimates, within the idealized test-particle description, indicate that, within the frequency band of ground-based gravitational-wave detectors, the matter flow around black holes may already be depleted, whereas the Laser Interferometer Space Antenna and (B-)DECIGO could probe the earlier stages. Our results suggest that an object orbiting a tidally deformed massive black hole may remain near resonances in a long term, indicating an accumulated, non-negligible impact on the gravitational-wave phase. Another finding is that tidal perturbations can modulate nonlinear couplings among epicyclic oscillations of geodesics, and could therefore, in principle, affect resonant excitation mechanism potentially relevant to quasiperiodic oscillations in x-ray light curves from accreting black holes.

We study both primordial gravitational waves (GWs) and scalar-induced gravitational waves (SIGWs) in a class of the parity-violating scalar-tensor (PVST) theory, of which the Lagrangian is the linear combination of seven ghost-free parity-violating scalar-tensor monomials dubbed the “Qi-Xiu” Lagrangians. At linear order, we obtain the quadratic action for tensor perturbations and show that parity-violating terms associated with ${\mathcal{L}}_{1,2,5,6,7}$ render the tensor propagation polarization dependent, leading to chiral primordial spectra and a nonvanishing degree of circular polarization. At second order, we derive the equation of motion for SIGWs and identify the explicit parity-violating source terms. In particular, ${\mathcal{L}}_3$ and ${\mathcal{L}}_4$ enter exclusively through the source term for SIGWs, allowing parity violation to arise even when the linear GWs’ propagation remains effectively general-relativity-like. During the radiation-dominated era, we compute the fractional energy density of SIGWs for both monochromatic and log-normal curvature power spectra. We find that, around the peak frequency, SIGWs in PVST gravity exhibit characteristic deviations from those in general relativity, resulting in a nonzero degree of circular polarization.

Gravitational wave (GW) astronomy opens a new venue to explore the universe. Future observatories such as LISA, the Laser Interferometer Space Antenna, are expected to observe previously undetectable fundamental physics effects in signals predicted by general relativity (GR). One particularly interesting such signal is associated to the displacement memory effect, which corresponds to a permanent deformation of spacetime due to the passage of gravitational radiation. In this work, we explore the ability of LISA to observe and characterize this effect. In order to do this, we use state-of-the-art simulations of the LISA instrument, and we perform a Bayesian analysis to assess the detectability and establish general conditions to claim detection of the displacement memory effect from individual massive black hole binary merger events in LISA. We perform parameter estimation both to explore the impact of the displacement memory effect and to reconstruct its amplitude. We discuss the precision at which such a reconstruction can be obtained thus opening the way to tests of GR and alternative theories. To provide astrophysical context, we apply our analysis to black hole binary population models and estimate the rates at which the displacement memory effect could be observed within the LISA planned lifetime.

Gravitational memory is a zero-frequency effect associated with a permanent change in the asymptotic spacetime metric induced by radiation. While its universal manifestation is a net change in the proper distances between freely falling test masses, gravitational wave detectors are intrinsically insensitive to the final offset and can only probe the associated transition. A central challenge for any claim of detection, therefore, lies in defining a physically meaningful and operationally robust model of the corresponding time-dependent signal that is uniquely attributable to gravitational memory and clearly distinguishable from purely oscillatory radiation. We show that while the Bondi–van der Burg–Metzner–Sachs balance laws rigorously establish the total memory offset, a robust definition of the observable memory rise requires an additional physical input: a separation of scales between high-frequency gravitational waves and the lower-frequency buildup of memory. We formulate this separation using the Isaacson description of gravitational wave energy momentum. Motivated by this observation, we develop a theoretical framework for defining and modeling the time-dependent memory rise, building on a self-contained review of the theory of gravitational memory and focusing in particular on compact binary coalescences. Specializing to space-based detectors, we analyze the response of LISA to gravitational radiation including a memory contribution, with emphasis on mergers of supermassive black hole binaries, which offer the most promising prospects for a first single-event detection. The framework developed here provides the theoretical foundation for statistically well-defined hypothesis testing between memory-free and memory-full radiation and enables quantitative assessments of detection prospects. These results establish a principled pathway toward a future observational claim of gravitational memory.

We show that quasinormal modes (QNMs) of a massive scalar field in Kerr–de Sitter and Myers-Perry black holes exhibit an exceptional line (EL), which is a continuous set of exceptional points (EPs) in parameter space, at which two QNM frequencies and their associated solutions coincide. We find that the EL appears in the parameter space spanned by the scalar mass and the black hole spin parameter, and also in the Nariai limit, i.e., $r_{\mathrm{c}}-r_{\mathrm{h}}\to 0$, where $r_{\mathrm{c}}$ and $r_{\mathrm{h}}$ denote the radii of the cosmological and black hole horizons, respectively. We analytically study the amplitudes or excitation factors of QNMs near the EL. Such an analytic treatment becomes possible since, in the Nariai limit, the perturbation equation reduces to a wave equation with the Pöschl-Teller potential. We discuss the destructive excitation of QNMs and the stability of the ringdown near and at the EL. The transient linear growth of QNMs—a characteristic excitation pattern near an EP or EL—together with the conditions under which this linear growth dominates the early ringdown, is also studied analytically. Our conditions apply to a broad class of systems that involve the excitation of (nearly) double-pole QNMs.

Numerical relativity (NR) enables the study of physics in strong and dynamical gravitational fields and provides predictions for the gravitational wave signals produced by merging black holes. Despite the impressive accuracy of modern codes, the resulting waveforms inevitably contain numerical uncertainties. Quantifying these uncertainties is important, especially for studies probing subdominant or nonlinear effects around the merger and ringdown. This paper describes a flexible Gaussian process model for the numerical uncertainties in all the spherical-harmonic waveform modes across a state-of-the-art catalog of NR waveforms and a highly efficient procedure for sampling the posteriors of quasinormal mode models without the need for expensive Markov chain Monte Carlo. The Gaussian process model is used to define a likelihood function which allows many Bayesian data analysis techniques—already widely used in the analysis of experimental gravitational wave data—to be applied to NR waveforms as well. The efficacy of this approach is demonstrated by applying it to the analysis of quasinormal modes in Cauchy characteristic evolved waveforms.

We construct shadows of superspinars described by the revisited Kerr–de Sitter (rKdS) naked singularity (NS) spacetimes and compare them with those of the standard KdSNS spacetimes. For all the classes of the rKdSNS spacetimes, we determine local escape cones related to a variety of fundamental frames: locally nonrotating frames (LNRFs), radially escaping frames, and circular geodesic frames related to marginally stable obits of the rKdSNS spacetimes. The local escape cones (and their complementary cones) are then applied to construct the shadow of the KdS superspinars related to the distant static observers represented by the LNRFs, located near the so-called static radius, where the spacetime is close to the asymptotically flat region of the Kerr spacetimes, or the superspinars radially approaching, due to the Universe’s expansion, the cosmic horizon of the spacetime. Differences of the shadows in the rKdSNS and standard KdSNS spacetimes are established and demonstrated for sufficiently large values of the dimensionless cosmological constant. For the observationally given cosmological constant and masses of the largest objects in the Universe, the shadow differences are not observable using recent observational instruments.

Classical critical collapse yields naked singularities from smooth initial data, challenging cosmic censorship, and shaping the spectrum of primordial black holes. We show that one-loop vacuum polarization near the threshold qualitatively changes this outcome by dressing the singularity with a horizon within a controlled semiclassical regime. In analytically tractable Einstein-scalar critical spacetimes, a one-loop $s$-wave treatment linearized around self-similar backgrounds shows that regularity uniquely selects an asymptotically Minkowskian, vacuum polarization state. Its renormalized stress tensor carries a universal quantum growing mode that competes with the classical unstable mode, shifts the critical point, and generates a trapped surface along with a finite mass gap at the new threshold, thereby enforcing horizon formation even under arbitrary fine-tuning. In primordial collapse, the threshold shift enters exponentially into the formation fraction, while the mass gap truncates the low-mass tail, suggesting potentially important consequences for the predicted mass spectrum. These results provide a self-consistent semiclassical treatment of critical collapse and yield sharp predictions within the one-loop, near-critical, linearized regime.

We study a self-gravitating ultralight dark matter condensate (a boson star) hosting a central black hole, in the nonrelativistic limit, which we refer to as a boson star black hole (BS-BH) system. We numerically solve the equations of hydrostatic equilibrium, consistently incorporating the gravitational potential of the black hole, to obtain all possible configurations of this BS-BH system for different boson star masses, interaction types, and black hole masses. We also propose an analytic expression for the density profile and compare it with the numerical results, finding good agreement for attractive interactions and for a finite range of mass ratios between the black hole and boson star. Finally, considering the inspiral of this BS-BH system with a second, smaller black hole, we study the dephasing of gravitational waves due to the presence of the dark matter environment. A Fisher matrix analysis reveals the regions of parameter space of the dark matter mass and self-coupling that future gravitational wave observatories such as LISA can probe.

Constraints on local primordial non-Gaussianity (LPnG) obtained from galaxy power spectra are limited by the perfect degeneracy between the LPnG parameter $f_{\mathrm{NL}}$ and the bias parameter $b_{\varphi }$, which encodes the response of galaxy clustering to a change in the amplitude of primordial curvature fluctuations. For galaxies observed by galaxy surveys, the relation between $b_{\varphi }$ and the galaxy bias $b_g$ is poorly understood and differs significantly from the universal mass function ansatz. In this paper, we investigate this nonuniversality in the context of dark-matter halos using the Separate Universe framework, focussing on dark-matter halos selected by mass and/or concentration as a test case—because halo concentration is the simplest example of a halo property correlated with nonuniversality. We show that the Separate Universe framework provides a natural explanation of the observed universality in the bias of dark-matter halos selected purely by their mass in terms of the response of the small-scale mass variance, ${\sigma }_8$, to ambient long-wavelength matter overdensity and to the amplitude of the primordial curvature fluctuations. Notably, the Separate Universe explanation for universality in this case is conceptually independent of the spherical collapse picture of halo formation. Furthermore, our Separate Universe arguments also show that halo clustering is in general nonuniversal whenever the halo abundance depends on properties of the small-scale matter density contrast whose responses to primordial curvature fluctuations and to a background long-wavelength matter overdensity differ from those of ${\sigma }_8$. Applying this insight from Separate Universe arguments to clustering of halos selected by mass and concentration, we propose an explanation for the observed nonuniversality in such halos and corroborate it with $N$-body simulations in scale-free (Einstein–de Sitter) and $\mathrm{\Lambda }$ cold dark matter cosmologies. We subsequently show that the clustering of concentration-selected halos in matter-dominated cosmologies tends toward universality at the highest halo masses because such halos gravitationally dominate their environment throughout their evolution. We also argue that concentration-selected halos of lower masses exhibit nonuniversality due to their mass accretion being significantly affected by the gravitational influence of neighboring, more massive halos. Our results also suggest that any nonuniversality in high redshift ($z\gtrsim 3$), high-bias objects observed by realistic galaxy surveys is likely just an artifact of the associated selection function.

A small fraction of gravitational-wave (GW) signals from binary black holes (BBHs) will be gravitationally lensed by intervening galaxies and galaxy clusters. Strong lensing will produce multiple identical copies of the GW signal arriving at different times. Jana et al. [Phys. Rev. Lett. 130, 261401 (2023)] recently proposed a method to constrain cosmological parameters using strongly lensed GW events detected by next-generation (XG) detectors. The idea is that the number of strongly lensed GW events and the distribution of their lensing time delays encode imprints of the cosmological parameters. From the observed number of lensed GW events (tens of thousands) and their time delay distribution, this method can provide a new probe of cosmology, obtaining information at intermediate redshifts. In this work, we explore the possibility of doing lensing cosmography using upcoming observations of the upgraded LIGO-Virgo-KAGRA (LVK) network. This requires incorporating the detector network selection effects in the analysis, which was neglected earlier. We expect dozens of lensed GW events to be detected by upgraded LVK detectors, potentially enabling modest constraints on cosmological parameters. Even with relatively modest numbers of lensed detections, we demonstrate the potential of lensing cosmography. For XG detectors, our revised forecasts are consistent with the earlier forecasts that neglected the selection effects.

Diffusive shock acceleration (DSA) and diffusion propagation are essential components of the standard cosmic ray model. These theories are based on extensive observations of high-energy solar processes, providing substantial direct evidence in the MeV energy range. Although the model is widely and successfully used to explain high-energy cosmic phenomena, direct validation has been elusive. The multiwavelength spectra and angular profile measurements of the Geminga pulsar wind nebula and its pulsar halo, particularly the precise spectral observations by HAWC and LHAASO-KM2A in recent years, offer a rare opportunity to test these theories with cosmic rays energies between 1 TeV and several hundred TeV. These observations are expected to elevate the direct testing of theoretical models from multi-MeV to sub-PeV energies. In this work, a method is developed to test the diffusive shock acceleration and diffusion propagation model between one and several hundred TeV energies through the latest spectral and morphological data of the Geminga region from HAWC and Fermi-LAT. Our results show that the theories of diffusive shock acceleration and diffusion propagation are consistent with experimental observations. However, the published morphological data adopted rather wide energy bins and currently do not allow a high precision test of the inferred energy dependent diffusion coefficient by observed energy spectra with DSA theory. It is anticipated that future HAWC and LHAASO-KM2A observations will yield higher-precision results, and the confirmation of a rapidly increasing diffusion coefficient above 100 TeV would serve as important evidence supporting the diffusive shock acceleration and diffusion propagation theory. We emphasize that such a confirmation specifically tests this acceleration and propagation theory, without confirming or excluding other models. Similar tests would be both important and valuable for other models.

We study quantum electrodynamics (QED) corrections to operator matrix elements involving heavy composite particles (e.g., heavy-mesons, nuclei, and atoms). We define a new notion of reducible and irreducible graphs which is useful for systems with many discrete excited states. The equivalence of the Lehman-Symanzik-Zimmerman (LSZ) reduction formula and old fashioned perturbation theory is explicitly demonstrated. The self-energy and vertex corrections are defined (to all orders), and the one-loop corrections are reduced to operator matrix elements which may be evaluated by hadronic, nuclear, or atomic theorists. The gauge dependence of the various pieces are studied in detail at one loop, and cancellation of spurious contributions are demonstrated in a class of covariant gauges; Coulomb gauge is also discussed. The formalism is applied to superallowed beta decay where the one-loop structure is connected to existing literature based on current algebra techniques. We further identify the well known $O(Z^2{\alpha }^2)$ isospin breaking correction from the intranuclear Coulomb field as arising from two-loop diagrams. We comment on future applications of our results to the radiative corrections necessary in extractions of $|V_{ud}|$, in particular for corrections that required beyond one-loop order.

What flavor structure of $t$-channel thermal dark matter remains compatible with current flavor physics and direct detection bounds? We broadly chart the space of hypotheses using the framework of flavor symmetries and their breaking patterns. We then focus on scenarios in which the fermionic dark matter and its scalar mediator are flavor singlets, falling into the class of rank-1 flavor violation. For two representative benchmarks, quarkphilic ($q_L$) and leptophilic ($e_R$), we perform a comprehensive phenomenological analysis, fitting the relic abundance and examining the interplay among flavor observables, direct detection, and collider searches. Our results quantify the allowed deviations from flavor-symmetric limits and assess the discovery prospects in future flavor and direct detection experiments.

We introduce an optimization technique to discriminate signal and background in any phenomenological study based on the cut and count-based method. The core ideas behind this technique are the introduction of a ranking scheme that can quantitatively assess the relative importance of various observables involved in a new physics process, and a more methodical way of choosing what cuts to impose. The technique is an iterative process that works with the help of the madanalysis5 interface. Working in the context of a beyond the Standard Model scenario where we carry out a signal search of singly charged Higgs in the context of the two Higgs doublet model, we demonstrate how automating the cut and count process in this specific way results in an enhanced discovery potential compared with the more traditional way of imposing cuts.

We present a search for the pair production of Higgsinos in final states with large missing transverse momentum and either two reconstructed muons or a reconstructed lepton (muon or electron) and an isolated track. The analyzed data correspond to proton-proton collisions with an integrated luminosity of $137{\mathrm{fb}}^{-1}$, collected by the CMS experiment at $\sqrt{s}=13\mathrm{TeV}$ in 2016, 2017, and 2018. The signal scenario assumes four nearly mass degenerate Higgsino mass eigenstates: two neutralino states ${\tilde{\chi }}_2^0$ and ${\tilde{\chi }}_1^0$ with a small mass difference in the range 1–10 GeV and two chargino states ${\tilde{\chi }}_1^{\pm }$ with an intermediate mass. The analysis focuses on the decay of the heavier neutralino into the lighter one and a virtual $Z$ boson, which decays into two same-flavor leptons. The leptons have small transverse momentum and/or a small opening angle between the identified muons. An isolated track is used to recover events in which only one of the two leptons is identified. Multivariate discriminants are used to enhance the sensitivity by efficiently rejecting backgrounds from SM processes or misreconstructed tracks and/or leptons. The search explores a unique phase space and probes a previously unexplored region of the signal model parameter space. Mass differences between the two neutralinos are probed down to 1.5 GeV, assuming a Higgsino mass of 100 GeV. The maximum excluded Higgsino mass is 115 GeV.

Using $(2259.3\pm 11.1)\times {10}^6$ $\psi (2S)$ events acquired with the BESIII detector, the branching fraction of $\psi (2S)\to {\tau }^{+}{\tau }^{-}$ is measured with improved precision to be ${\mathcal{B}}_{\psi (2S)\to {\tau }^{+}{\tau }^{-}}=(3.240\pm 0.023\pm 0.081)\times {10}^{-3}$, where the first and second uncertainties are statistical and systematic, respectively, which is consistent with the world average value within 1 standard deviation. This value, along with those for the branching fractions of the $\psi (2S)$ decaying into $e^{+}{e}^{-}$ and ${\mu }^{+}{\mu }^{-}$, is in good agreement with the relation predicted by the sequential lepton hypothesis. Combining the branching fraction values with the leptonic width of the $\psi (2S)$, the total width of the $\psi (2S)$ is determined to be $(287\pm 9)\mathrm{keV}$.

We study the production of heavy, $\mu \gtrsim 1\mathrm{TeV}$, bosonic spin $s=0$, 1 dark matter (DM) via the simultaneous processes of Hawking evaporation and superradiance (SR) from an initial population of small, $\lesssim {10}^6\mathrm{kg}$, primordial black holes (PBHs). Even for small initial PBH spins, the SR process can produce extremely dense gravitationally bound DM Bose or Proca soliton “stars” of radius $\lesssim \mathrm{pm}$ and mass $\sim 10^{\mathrm{few}}\mathrm{kg}$ that can survive to today, well after PBH decay. These solitons can constitute a significant fraction of the DM density, rising to $\gtrsim 50\%$ in the vector DM case.