We describe embeddings of $n$-dimensional Lorentzian manifolds, including Friedmann-Lemaître-Robertson-Walker spaces, in ${\mathbb{R}}^{n+2}$ such that the metrics of the submanifolds are inherited by a restriction from that of ${\mathbb{R}}^{n+2}$, and the action of the linear group $\mathrm{SO}(2,n)$ of the ambient space reduces to conformal transformations on the submanifolds.

Regular black holes (RBHs)—geometries free from curvature singularities—arise naturally in theories of nonlinear electrodynamics. Here we study the absorption and superradiant amplification of a monochromatic planar wave in a charged, massive scalar field impinging on the electrically charged Ayón-Beato-García (ABG) RBH. Comparisons are drawn with absorption and superradiance for the Reissner-Nordström (RN) black hole in linear electrodynamics. We find that, in a certain parameter regime, the ABG absorption cross section is negative, due to superradiance, and moreover it is unbounded from below as the momentum of the wave approaches zero; this phenomenon of “unbounded superradiance” is absent in the RN case. We show how the parameter space can be divided into regions, using the bounded/unbounded and absorption/amplification boundaries. After introducing a high-frequency approximation based on particle trajectories, we calculate the absorption cross section numerically, via the partial-wave expansion, as a function of wave frequency, and we present a gallery of results. The cross section of the ABG RBH is found to be larger (smaller) than in the RN case when the field charge has the same (opposite) sign as the black hole charge. We show that it is possible to find “mimics”: situations in which the cross sections of both black holes are very similar. We conclude with a discussion of unbounded superradiance and superradiant instabilities.

It has been established that black hole (BH) spacetimes obeying some general set of assumptions always possess, at least, one light ring (LR) per rotation sense [Cunha and Herdeiro, Phys. Rev. Lett. 124, 181101 (2020)]. This theorem was originally established for asymptotically flat, stationary, axial symmetric, $1+3$-dimensional circular spacetimes harboring a nonextremal and topologically spherical Killing horizon. Following the mantra that a theorem is only as strong as its assumptions, in this work we extend this theorem to nontopologically spherical (toroidal) BHs and to spacetimes harboring more than one BH. As shown previously, we here show that each BH still contributes with, at least, one LR (per rotation sense).

We examine the contribution of tensor modes, in addition to the dominant scalar ones, on the temperature anisotropies of the cosmic microwave background (CMB). To this end, we analyze in detail the temperature two-point angular correlation function $C(\theta )$ from the Planck 2018 dataset, focusing on large angles ($\theta \gtrsim 120°$) corresponding to small $\ell$ multipoles. A hierarchical set of infrared cutoffs are naturally introduced to the scalar and tensor power spectra of the CMB by invoking an extra Kaluza-Klein spatial dimension compactifying at about the grand unified theory scale between the Planck epoch and the start of inflation. We associate this set of lower scalar and tensor cutoffs with the parity of the multipole expansion of the $C(\theta )$ function. By fitting the Planck 2018 data we compute the multipole coefficients, thereby reproducing the well-known odd-parity preference in angular correlations seen by all three satellite missions: Cosmic Background Explorer, WMAP, and Planck. Our fits improve significantly once tensor modes are included in the analysis, hence providing a hint of the imprints of primordial gravitational waves on the temperature correlations observed in the CMB today. To conclude, we suggest a relationship between, on the one hand, the lack of (positive) large-angle correlations and the odd-parity dominance in the CMB and, on the other hand, the effect of primordial gravitational waves on the CMB temperature anisotropies.

Hubble constant ($H_0$) tension and tension in the matter fluctuation amplitude ($s_8$) are fascinating puzzles in cosmology nowadays. The phantom dynamical dark energy model (PDDE), also known as the little sibling of the big rip, is an abrupt event that can happen in the far future evolution of the universe. A recent analysis of the PDDE model based on cosmic microwave backround radiation data shows that the model is a potential candidate to alleviate these tension problems. In this work, we study the background evolution of the universe within the PDDE model. Analysis based on the $\mathrm{SNIa}+\mathrm{BAO}+\mathrm{OHD}$ data shows that the model successfully explains the late-phase acceleration of the universe. Also, the values of the cosmological parameters computed by the PDDE model are consistent with those obtained by the $\mathrm{\Lambda }\mathrm{CDM}$ model for the same dataset. However, most of the phantom dark energy models do not give a stable solution in the asymptotic future. In this regard, we address the PDDE model’s dynamical stability and test the validity of the generalized second law (GSL) of thermodynamics. We show that the model is dynamically unstable and violates the GSL. The model does not satisfy the convexity condition, so the universe does not behave as an ordinary macroscopic system within the PDDE model.

We present forecasts on cosmological parameters for a CMB-HD survey. For a $\mathrm{\Lambda }\mathrm{CDM}+N_{\mathrm{eff}}+\sum m_{\nu }$ model, we find $\sigma (n_{\mathrm{s}})=0.0013$ and $\sigma (N_{\mathrm{eff}})=0.014$ using CMB and CMB lensing multipoles in the range of $\ell \in [30,20000]$, after adding anticipated residual foregrounds, delensing the acoustic peaks, and adding DESI BAO data. This is about a factor of two improvement in ability to probe inflation via $n_{\mathrm{s}}$ compared to precursor CMB surveys. The $N_{\mathrm{eff}}$ constraint can rule out light thermal particles back to the end of inflation with 95% CL; for example, it can rule out the QCD axion in a model-independent way assuming the Universe’s reheating temperature was high enough that the axion thermalized. We find that delensing the acoustic peaks and adding DESI BAO tightens parameter constraints. We also find that baryonic effects can bias parameters if not marginalized over, and that uncertainties in baryonic effects can increase parameter error bars; however, the latter can be mitigated by including information about baryonic effects from kinetic and thermal Sunyaev-Zel’dovich measurements by CMB-HD. The CMB-HD likelihood and Fisher estimation codes used here are publicly available; the likelihood is integrated with Cobaya to facilitate parameter forecasting.

The pulsar timing array (PTA) collaborations have recently suggested the presence of a gravitational wave background at nano-Hertz frequencies. In this paper, we explore potential inflationary interpretation of this signal within the context of a simple and health parity-violating gravity model termed the Nieh-Yan modified teleparallel gravity. Through this model, two inflationary scenarios are evaluated, both yielding significant polarized primordial gravitational waves (PGWs) that align well with the results from PTA observations. Furthermore, the resulting PGWs can display strong circular polarization and significant anisotropies in the PTA frequency band, which are distinct features to be verified by observations of both PTA and the cosmic microwave background. The detection of such a distinctive background of PGWs is expected to provide strong evidence supporting our scenarios and insights into inflationary dynamics and gravity theory.

Accretion disks formed in binary neutron star mergers play a central role in many astrophysical processes of interest, including the launching of relativistic jets or the ejection of neutron-rich matter hosting heavy element nucleosynthesis. In this work we analyze in detail the properties of accretion disks from 44 ab initio binary neutron star merger simulations for a large set of nuclear equations of state, binary mass ratios, and remnant fates, with the aim of furnishing reliable initial conditions for disk simulations and a comprehensive characterization of their properties. We find that the disks have a significant thermal support, with an aspect ratio decreasing with the mass ratio of the binary from $\sim 0.7$ to 0.3. Even if the disk sample spans a broad range in mass and angular momentum, their ratio is independent from the equation of state and from the mass ratio. This can be traced back to the rotational profile of the disk, characterized by a constant specific angular momentum (as opposed to a Keplerian one) of $3-5\times {10}^{16}{\mathrm{cm}}^2{\mathrm{s}}^{-1}$. The profiles of the entropy per baryon and of the electron fraction depend on the mass ratio of the binary. For more symmetric binaries, they follow a sigmoidal distribution as a function of the rest mass density, for which we provide a detailed description and a fit. The disk properties discussed in this work can be used as a robust set of initial conditions for future long-term simulations of accretion disks from binary neutron star mergers, posing the basis for progress in the quantitative study of the outflow properties.

In dense astrophysical environments, notably core-collapse supernovae and neutron star mergers, neutrino-neutrino forward scattering can spawn flavor conversion on very short scales. Scattering with the background medium can impact collective flavor conversion in various ways, either damping oscillations or possibly setting off novel collisional flavor instabilities (CFIs). A key feature in this process is the slowness of collisions compared to the much faster dynamics of neutrino-neutrino refraction. Assuming spatial homogeneity, we leverage this hierarchy of scales to simplify the description accounting only for the slow dynamics driven by collisions. We illustrate our new approach both in the case of CFIs and in the case of fast instabilities damped by collisions. In both cases, our strategy provides new equations, the slow-dynamics equations, that simplify the description of flavor conversion and allow us to qualitatively understand the final state of the system after the instability, either collisional or fast, has saturated.

GW200129 is claimed to be the first-ever observation of the spin-disk orbital precession detected with gravitational waves (GWs) from an individual binary system. However, this claim warrants a cautious evaluation because the GW event coincided with a broadband noise disturbance in LIGO Livingston caused by the 45 MHz electro-optic modulator system. In this paper, we present a state-of-the-art neural network that is able to model and mitigate the broadband noise from the LIGO Livingston interferometer. We also demonstrate that our neural network mitigates the noise better than the algorithm used by the LIGO-Virgo-KAGRA Collaboration. Finally, we reanalyze GW200129 with the improved data quality compared to the data used by the LIGO-Virgo-KAGRA Collaboration and show that the evidence for precession is still observed.

We study the impact of machine-learning algorithms on LHC searches for leptoquarks in final states with hadronically decaying tau leptons, multiple $b$-jets, and large missing transverse momentum. Pair production of scalar leptoquarks with decays only into third-generation leptons and quarks is assumed. Thanks to the use of supervised learning tools with unbinned methods to handle the high-dimensional final states, we consider simple selection cuts which would possibly translate into an improvement in the exclusion limits at the 95% confidence level for leptoquark masses with different values of their branching fraction into charged leptons. In particular, for intermediate branching fractions, we expect that the exclusion limits for leptoquark masses extend to $\sim 1.3\mathrm{TeV}$. As a novelty in the implemented unbinned analysis, we include a simplified estimation of some systematic uncertainties with the aim of studying their possible impact on the stability of the results. Finally, we also present the projected sensitivity within this framework at 14 TeV for $300{\mathrm{fb}}^{-1}$ and $3000{\mathrm{fb}}^{-1}$ that extends the upper limits to $\sim 1.6\mathrm{TeV}$ and $\sim 1.8\mathrm{TeV}$, respectively.

Embedding inflation within a grand unified theory is an intriguing idea towards a more fundamental understanding of particle physics and cosmology. However, within the framework of supersymmetric $SU(5)$ grand unified theory inflation, models typically feature light color octets and weak triplets that ruin the successful gauge coupling unification of the mimimal supersymmetric Standard Model. We present a new possibility for naturally splitting the electronlike states from the colored ones within vectorlike 10-plets by utilizing their couplings with 40-plets that lack electronlike states. Together with the extra light weak doublets from the double missing partner mechanism, the appearance of the light electronlike states from our proposed generalized missing partner mechanism restores gauge coupling unification. Remarkably, the same vectorlike 10-plets cure the bad mass relations between the down quarks and the charged leptons. Discovering the light relic states at future colliders would provide a “smoking gun” signal of the scenario.

Despite being remarkably predictive, the Standard Model leaves unanswered several important issues, which motivate an ongoing search for its extensions. One fashionable possibility are the so-called 331 models, where the electroweak gauge group is extended to ${\mathrm{SU}}_L(3)\times \mathrm{U}(1)$. We focus on a minimal extension which includes vectorlike quarks (VLQs) and new gauge bosons, performing a consistent analysis of the production at the LHC of a pair of doubly charged bileptons. We include for the first time all the relevant processes where VLQs contribute and, in particular, the associate production VLQ bilepton. Finally, we extract the bound on the bilepton mass, $m_Y>1300\mathrm{GeV}$, from a reinterpretation of a recent ATLAS search for doubly charged Higgs bosons in multilepton final states.

The neutrinoless double beta decay experimental effort continues to make tremendous progress with hopes of covering the inverted neutrino mass hierarchy in coming years and pushing from the quasidegenerate hierarchy into the normal hierarchy. As neutrino oscillation data are starting to suggest that the mass ordering may be normal, we may well be faced with staring down the funnel of death: a region of parameter space in the normal ordering where—for a particular cancellation among the absolute neutrino mass scale, the Majorana phases, and the oscillation parameters—the neutrinoless double beta decay rate may be vanishingly small. To answer the question of whether this region of parameter space is theoretically preferred, we survey five broad categories of flavor model structures which make various different predictions for parameters relevant for neutrinoless double beta decay to determine how likely it is that the rate may be in this funnel region. We find that a non-negligible fraction of predictions surveyed are at least partially in the funnel region. Our results can guide model builders and experimentalists alike in focusing their efforts on theoretically motivated regions of parameter space.

The reconstruction of spectral functions from Euclidean correlation functions is a well-known, yet ill-conditioned inverse problem in the fields of many-body and high-energy physics. In this paper, we present a comprehensive investigation of two recently developed analytic continuation methods, namely stochastic pole expansion and Nevanlinna analytic continuation, for extracting spectral functions from mock lattice QCD data. We examine a range of Euclidean correlation functions generated by representative models, including the Breit-Wigner model, the Gaussian mixture model, the resonance-continuum model, and the bottomonium model. Then, we apply the two methods to reconstruct spectral functions from charmonium correlation functions computed using lattice QCD simulations at a finite temperature. Our findings demonstrate that the stochastic pole expansion method, when combined with the constrained sampling algorithm and the self-adaptive sampling algorithm, successfully recovers the essential features of the spectral functions and exhibits excellent resilience to noise of input data. In contrast, the Nevanlinna analytic continuation method suffers from numerical instability, often resulting in the emergence of spurious peaks and significant oscillations in the high-energy regions of the spectral functions, even with the application of the Hardy basis function optimization algorithm.

Assuming that $D_{s0}^{*}(2317)$ is a $DK$ molecular state with a binding energy of 45 MeV, we investigate the existence of four-hadron molecules, $DDKK$ and $D\overline{D}K\overline{K}$, with the Gaussian expansion method. Their binding energies are 138–155 MeV and 123–163 MeV below the mass thresholds of $DDKK$ and $D\overline{D}K\overline{K}$. The $D\overline{D}K\overline{K}$ state has a decay width of 74–107 MeV due to the complex $K\overline{K}$ interaction. Further theoretical studies of and experimental searches for such four-hadron molecules can help deepen the understanding of the nonperturbative strong interaction in a nontrivial way.

We examine the internal structure of the $\mathrm{\Lambda }(1670)$ through an analysis of lattice QCD simulations and experimental data within Hamiltonian effective field theory. Two scenarios are presented. The first describes the $\mathrm{\Lambda }(1670)$ as a bare three-quark basis state, which mixes with the $\pi \mathrm{\Sigma }$, $\overline{K}N$, $\eta \mathrm{\Lambda }$ and $K\mathrm{\Xi }$ meson-baryon channels. In the second scenario, the $\mathrm{\Lambda }(1670)$ is dynamically generated from these isospin-0 coupled channels. The $K^{-}p$ scattering data and the pole structures of the $\mathrm{\Lambda }(1405)$ and the $\mathrm{\Lambda }(1670)$ can be simultaneously described well in both scenarios. However, a comparison of the finite-volume spectra to lattice QCD calculations reveals significant differences between these scenarios, with a clear preference for the first case. Thus the lattice QCD results play a crucial role in allowing us to distinguish between these two scenarios for the internal structure of the $\mathrm{\Lambda }(1670)$.

We study the properties of the charged kaon in symmetric nuclear matter using a Bethe-Salpeter amplitude to model the quark-antiquark bound state, which is well constrained by previous studies of its vacuum properties. The electromagnetic form factor, charge radius, decay constant, and the light-front valence component probability are investigated in symmetric nuclear matter. In order to describe the constituent up and antistrange quarks in nuclear matter, we adopt the “quark-meson coupling (QMC) model,” which has been widely applied to various hadronic and nuclear phenomena in the nuclear medium.

Using a data sample of $4.481\times {10}^8\psi (3686)$ events collected with the BESIII detector, we report the first observation of the four-lepton-decays $J/\psi \to e^{+}{e}^{-}{e}^{+}{e}^{-}$ and $J/\psi \to e^{+}{e}^{-}{\mu }^{+}{\mu }^{-}$ utilizing the process $\psi (3686)\to {\pi }^{+}{\pi }^{-}J/\psi$. The branching fractions are determined to be $[5.48\pm 0.31(\mathrm{stat})\pm 0.45(\mathrm{syst})]\times {10}^{-5}$ and $[3.53\pm 0.22(\mathrm{stat})\pm 0.13(\mathrm{syst})]\times {10}^{-5}$, respectively. The results are consistent with theoretical predictions. No significant signal is observed for $J/\psi \to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$, and an upper limit on the branching fraction is set at $1.6\times {10}^{-6}$ at the 90% confidence level. A $CP$ asymmetry observable is constructed for the first two channels, which is measured to be $(-0.012\pm 0.054\pm 0.010)$ and $(0.062\pm 0.059\pm 0.006)$, respectively. No evidence for $CP$ violation is observed in this process.

In this paper, we have conducted a thorough investigation into the shadow characteristics exhibited by the axially symmetric Finslerian extension of a Schwarzschild black hole and supplementary numerically simulated the motion behavior of photons near the black hole with respect to varying Finsler parameter $\epsilon$ and observation inclinations ${\theta }_o$. In this spacetime, the apparent shape and size of the black hole shadow remain the same as the one in the Schwarzschild black hole, regardless of changes in the Finsler parameter $\epsilon$. However, the apparent position of shadow in the observer’s sky plane appears to shift along the horizontal axis, and the distance moved is positively correlated with the value of the Finsler parameter $\epsilon$. Interestingly, with a fixed value of $\epsilon$, the displacement of the visual position of shadow is rectified as the observation angle decreases. In addition, it is demonstrated that the motion behavior of photons around the axially symmetric Finslerian extension of a Schwarzschild black hole has a greater degree of deflection than a Schwarzschild black hole, although their shadows are in some cases consistent in shape, size, and apparent position. These results can serve as a valuable tool in distinguishing the axially symmetric Finslerian extension of a Schwarzschild black hole from its counterparts in general relativity.