At high densities in compact astrophysical sources, the coherent forward scattering of neutrinos onto each other is responsible for making the flavor evolution nonlinear. Under the assumption of spherical symmetry, we present the first simulations tracking flavor transformation in the presence of neutrino-neutrino forward scattering, neutral and charged current collisions with the matter background, as well as neutrino advection. We find that, although flavor equipartition could be one of the solutions, it is not a generic outcome, as often postulated in the literature. Intriguingly, the strong interplay between flavor conversion, collisions, and advection leads to a spread of flavor conversion across the neutrino angular distributions and neighboring spatial regions. Our simulations show that slow and fast flavor transformation can occur simultaneously. In the light of this, looking for crossings in the electron neutrino lepton number as a diagnostic tool of the occurrence of flavor transformation in the high-density regime is a limiting method.

The LHCb Collaboration has recently released a new study of $B^{+}\to K^{+}{\ell }^{+}{\ell }^{-}$ and $B\to K^{*0}{\ell }^{+}{\ell }^{-}$ ($\ell =e$, $\mu$) decays, testing lepton universality with unprecedented accuracy using the whole Run 1 and 2 dataset. In addition, the CMS Collaboration has reported an improved analysis of the branching ratios $B_{(d,s)}\to {\mu }^{+}{\mu }^{-}$. While these measurements offer, per se, a powerful probe of new physics, global analyses of $b\to s{\ell }^{+}{\ell }^{-}$ transitions also rely on the assumptions about nonperturbative contributions to the decay matrix elements. In this work, we perform a global Bayesian analysis of new physics in (semi)leptonic rare $B$ decays, paying attention to the role of charming penguins which are difficult to evaluate from first principles. We find data to be consistent with the Standard Model once rescattering from intermediate hadronic states is included. Consequently, we derive stringent bounds on lepton universality violation in $|\mathrm{\Delta }B|=|\mathrm{\Delta }S|=1$ (semi)leptonic processes.

We study the causality and stability of relativistic hydrodynamics with the inclusion of the spin degree of freedom as a hydrodynamic field. We consider two specific models of spin hydrodynamics for this purpose. A linear mode analysis for static background shows that a first-order dissipative spin hydrodynamics remains acausal and admits instabilities. In addition, it is found that the inclusion of the spin field in hydrodynamics leads to new kinds of linear modes in the system. These new modes also exhibit instability and acausal behavior. The second model of the spin hydrodynamics that we have considered here is equivalent to a particular second-order conventional hydrodynamics with no dissipative effects. For a static background, it is found that the linear modes of this model support the sound waves only. However, when the background has constant vorticity, then the model admits instability and acausality in certain situations. It is found that the spin dynamics have an effect on the hydrodynamic response of the fluid. These findings point toward the need for a causal and stable theory with spin as a hydrodynamic field to describe the spin-polarized fluid.

This article presents a model-independent search for an additional, mostly sterile, heavy neutral lepton (HNL), that is capable of mixing with the Standard Model $\tau$ neutrino with a mixing strength of $|U_{\tau 4}{|}^2$, corresponding to the absolute square of the extended Pontecorvo-Maki-Nakagawa-Sakata matrix element. Data from the BABAR experiment, with a total integrated luminosity of $424{\mathrm{fb}}^{-1}$, are analyzed using a kinematic approach that makes no assumptions on the model behind the origins of the HNL, its lifetime or decay modes. No significant signal is found. Upper limits on $|U_{\tau 4}{|}^2$ at the 95% confidence level, depend on the HNL mass hypothesis and vary from $2.31\times {10}^{-2}$ to $5.04\times {10}^{-6}$ (with all uncertainties considered), across the mass range $100<m_4<1300\mathrm{MeV}/c^2$; the more stringent limits being placed at higher masses.

We construct a neural network to learn the Reissner-Nordström-anti–de Sitter black hole metric based on the data of optical conductivity by holography. The linear perturbative equation for the Maxwell field is rewritten in terms of the optical conductivity such that the neural network is constructed based on the discretization of this differential equation. In contrast to all previous models in anti–de Sitter/deep learning duality, the derivative of the metric function appears in the equation of motion and we propose distinct finite difference methods to discretize this function. The notion of the reduced conductivity is also proposed to avoid the divergence of the optical conductivity near the horizon. The dependence of the training outcomes on the location of the cutoff, the temperature as well as the frequency range is investigated in detail. This work provides a concrete example for the reconstruction of the bulk geometry with the given data on the boundary by deep learning.

The Fabbri–Russo model is a generalized model of a two-dimensional dilaton gravity theory with various parameters “$n$” describing various specific gravities. Particularly, the Russo–Susskind–Thorlacius gravity model fits the case $n=1$. In the Fabbri–Russo model, we investigate Page curves and the entanglement island. Islands are considered in eternal and evaporating black holes. Surprisingly, in any black hole, the emergence of islands causes the rise of the entanglement entropy of the radiation to decelerate after the Page time, satisfying the principle of unitarity. For eternal black holes, the fine-grained entropy reaches a saturation value that is twice the Bekenstein–Hawking entropy. For evaporating black holes, the fine-grained entropy finally reaches zero. At late times and large distance limit, the impact of the parameter “$n$” is a subleading term and is exponentially suppressed. As a result, the shape of Page curves is “$n$” independent in the leading order, which also indicates the universality of the Page curve in generalized two-dimensional models. Furthermore, we discuss the relationship between islands and firewalls. We show that the island is a better candidate than firewalls for encountering the quantum entanglement-monogamy problem. Finally, we briefly review the gravity/ensemble duality as a possible resolution to the state paradox resulting from the island formula.

A nonuniformly accelerated quantum field in a cavity undergoes the coordinate transformation of annihilation and creation operators, known as the Bogoliubov transformation. This study considers the entropy production of a quantum field in a cavity induced by the Bogoliubov transformation. By classifying the modes in the cavity into the system and environment, we obtain the lower bound of the entropy production, defined as the sum of the von Neumann entropy in the system and the heat dissipated to the environment. This lower bound represents the refined second law of thermodynamics for a quantum field in a cavity and can be interpreted as the Landauer principle, which yields the thermodynamic cost of changing information contained within the system. Moreover, it provides an upper bound for the quantum mutual information to quantify the extent of the information scrambling in the cavity due to acceleration.

The geodesics around compact objects are one of the best tools to understand the geometrical properties of the central gravity source. In this paper, we study timelike and lightlike geodesics of a quantum black hole proposed in [E. Binetti et al., Phys. Rev. D 106, 046006 (2022).], which does not require committing to a specific model of quantum gravity. We analyze the quantum correction on the related obsrevables: periapsis precession of bounded orbits and angular shadow size. We find that, compared to the classical case, the quantum correction will enhance the periapsis precession as well as the black hole shadow. This effect is more significant for a smaller quantum black hole.

We compare numerical relativity (NR) data on the scattering of equal-mass, nonspinning binary black holes to various analytical predictions based on post-Minkowskian (PM) results. While the usual sequence of PM-expanded scattering angles shows a rather poor convergence towards NR data, we find that a reformulation of PM information in terms of effective-one-body radial potentials leads to remarkable agreement with NR data, especially when using the radiation-reacted 4PM information. Using Firsov’s inversion formula we directly extract, for the first time, from NR simulations a (radiation-reacted) gravitational potential describing the scattering of equal-mass, nonspinning binary black holes. We urge the NR community to compute more sequences of scattering simulations, so as to extend this knowledge to a wider region of parameter space.

The construction of quasitopological gravities in three-dimensions requires coupling a scalar field to the metric. As shown in [arXiv:2104.10172], the resulting “electromagnetic” quasitopological (EQT) theories admit charged black hole solutions characterized by a single-function for the metric, $-g_{tt}=g_{rr}^{-1}\equiv f(r)$, and a simple azimuthal form for the scalar. Such black holes, whose metric can be determined fully analytically, generalize the Bañados-Teiteilboim-Zanelli black hole (BTZ) solution in various ways, including singularity-free black holes without any fine-tuning of couplings or parameters. In this paper we extend the family of EQT theories to general curvature orders. We show that, beyond linear order, $f(r)$ satisfies a second-order differential equation rather than an algebraic one, making the corresponding theories belong to the electromagnetic generalized quasitopological (EGQT) class. We prove that at each curvature order, the most general EGQT density is given by a single term which contributes nontrivially to the equation of $f(r)$ plus densities which do not contribute at all to such equation. The proof relies on the counting of the exact number of independent order-$n$ densities of the form $\mathcal{L}(R_{ab},{\partial }_a\varphi )$, which we carry out. We study some general aspects of the new families of EGQT black-hole solutions, including their thermodynamic properties and the fulfillment of the first law, and explicitly construct a few of them numerically.

Pulsar timing constraints on scalar-tensor theories with conformal and disformal couplings to matter are discussed. Reducing the dynamics to the motion in the center-of-mass frame and using the mean anomaly parametrization, we find the first post-Newtonian corrections induced by the conformal and disformal interactions in the form of a generalized quasi-Keplerian solution. We also derive the radiation reaction force due to scalar radiation and the corresponding post-Keplerian parameters (PKP). We use different pulsar time of arrival (TOA) datasets to probe the scalar corrections to the PKP. In particular, we focus on systems with large orbital frequencies as the contributions to the PKP terms induced by the disformal coupling are sensitive to higher frequencies. We find that the most constraining pulsar timings are PSR $\mathrm{B}1913+16$ and the double pulsar PSR J0737-3039A/B, being of the order of the Cassini bound on the conformal coupling obtained from the Shapiro effect in the solar system. The combined constraints using other pulsar timings give an upper bound on the conformal coupling ${\beta }^2<2.33\times {10}^{-5}$ and a lower bound on the disformal coupling scale of $\mathrm{\Lambda }\ge 1.12\mathrm{MeV}$ which is comparable to the Cassini bound and to the GW-170817 constraints respectively. Future measurements for pulsar timing with black hole companions are also discussed.

The improved sensitivity of third generation gravitational wave detectors opens the possibility of detecting the primordial cosmological stochastic gravitational wave background (SGWB). Detection of the cosmological SGWB is facing a novel challenge: it will likely be masked by the foreground generated by a large number of coalescences of compact binary systems consisting of black holes and/or neutron stars. In this paper, we investigate the possibility of reducing this foreground by removing (notching) the individually resolved compact binary signals in time-frequency space. We establish that such an approach could be used to reach the SGWB sensitivity floor defined by the unresolved part of the compact binaries foreground, which we find to be between ${\mathrm{\Omega }}_{\mathrm{GW}}\sim (9.1\times {10}^{-12}–8.6\times {10}^{-11})$ for a frequency independent energy density spectrum and depending on the rate of coalescing binary neutron star systems. Since third-generation gravitational wave detectors will not be able to resolve all compact binaries, the unresolvable component of the compact binaries foreground may limit the SGWB searches with these detectors.

The Hubble horizon at matter-radiation equality ($k_{\mathrm{eq}}^{-1}$) and the sound horizon at the last scattering surface [$r_s(z_{*})$] provides an interesting consistency check for the standard $\mathrm{\Lambda }$ Cold Dark Matter ($\mathrm{\Lambda }\mathrm{CDM}$) model and its extensions. It is well known that the reduction of $r_s$ can be compensated by the increase of $H_0$, while the same is true for the standard rulers $k_{\mathrm{eq}}$. Adding extra radiational component to the early Universe can reduce $k_{\mathrm{eq}}$. The addition of early dark energy (EDE), however, tends to increase $k_{\mathrm{eq}}$. We perform $k_{\mathrm{eq}}$- and $r_s$-based analyses in both the EDE model and the Wess-Zumino dark radiation (WZDR) model. In the latter case, we find $\mathrm{\Delta }{H}_0=0.4$ between the $r_s$- and $k_{\mathrm{eq}}$-based datasets, while in the former case, we find $\mathrm{\Delta }{H}_0=1.2$. This result suggests that the dark radiation scenario is more consistent in the fit of the two standard rulers ($k_{\mathrm{eq}}$ and $r_s$). As a forecast analyses, we fit the two models with a mock $k_{\mathrm{eq}}$ prior derived from Planck best-fit $\mathrm{\Lambda }\mathrm{CDM}$ model. Compared with the best-fit $H_0$ in baseline $\mathrm{\Lambda }\mathrm{CDM}$ model, we find $\mathrm{\Delta }{H}_0=1.1$ for the WZDR model and $\mathrm{\Delta }{H}_0=-2.4$ for EDE model.

Compact stars (CS) are stellar remnants of massive stars. Inside CSs the density is so high that matter is in subatomic form composed of nucleons. With an increase of density of matter toward the center of the objects, other degrees of freedom like hyperons, heavier nonstrange baryons, meson condensates may appear. Not only that, at higher densities the nucleons may get decomposed into quarks and form deconfined strange quark matter (SQM). If it is so then CSs may contain SQM in the core surrounded by nucleonic matter forming hybrid stars (HSs). However, the nature and composition of matter inside CSs can only be inferred from the astrophysical observations of these CSs. Recent astrophysical observations in terms of CS mass-radius (M-R) relation and gravitational wave (GW) observation indicate that the matter should be soft in the intermediate density range and stiff enough at higher density range to attain the maximum possible mass above $2M_{\odot }$ which is not compatible with pure hadronic equations of states (EOSs). Consequently, we study the HS properties with different models of SQM and find that within vector bag model considering density dependent bag parameter, the model goes well with the astrophysical observations so far.

Strong gravitational lensing of gravitational waves (GWs) has been forecasted to become detectable in the upcoming LIGO/Virgo/KAGRA observing runs. However, definitively distinguishing pairs of lensed sources from random associations is a challenging problem. We investigate the degree to which unlensed events mimic lensed ones because of the overlap of parameters due to a combination of random coincidence and errors in parameter estimation. Lensed events are expected to have consistent masses and sky locations, and constrained relative phases, but may have differing apparent distances due to lensing magnification. We construct a mock catalog of lensed and unlensed events. We find that the probability of a false alarm based on coincidental overlaps of the chirp mass, sky location, and coalescence phase are approximately 9%, 1%, and 10% per pair, respectively. Combining all three parameters, we arrive at an overall false alarm probability per pair of $\sim {10}^{-4}$. We validate our results against the GWTC-2 data, finding that the catalog data is consistent with our simulations. As the number of events, $N$, in the GW catalogs increases, the number of random pairs of events increases as $\sim N^2$. Meanwhile, the number of lensed events will increase linearly with $N$, implying that, for sufficiently high $N$, the false alarms will always dominate over the true lensing events. This issue can be compensated for by placing higher thresholds on the lensing candidates (e.g., selecting a higher signal-to-noise ratio (SNR) threshold, ${\rho }_{\mathrm{thr}}$), which will lead to better parameter estimation and, thus, lower false alarm probabilities per pair—at the cost of dramatically decreasing the size of the lensing sample ($\propto {\rho }_{\mathrm{thr}}^{-3}$). We show that with our simple overlap criteria for current detectors at design sensitivity, the false alarms will dominate for realistic lensing rates ($\lesssim {10}^{-3}$) even when selecting the highest SNR pairs. These results highlight the necessity to design alternative identification criteria beyond simple waveform and sky location overlap for conclusive detection of strong lensing. Future GW detectors such as Cosmic Explorer and Einstein Telescope may provide sufficient improvement in parameter estimation and a commensurate decrease in the incidence of coincidental overlap of parameters, allowing for the conclusive detection of strong lensing of GWs even without additional detection criteria.

The next-generation (3G/XG) ground-based gravitational-wave (GW) detectors such as Einstein Telescope (ET) and Cosmic Explorer (CE) will begin observing in the next decade. Due to the extremely high sensitivity of these detectors, the majority of stellar-mass compact-binary mergers in the entire Universe will be observed. It is also expected that 3G detectors will have significant sensitivity down to 2–7 Hz; the observed duration of binary neutron star signals could increase to several hours or days. The abundance and duration of signals will cause them to overlap in time, which may form a confusion noise that could affect the detection of individual GW sources when using naive matched filtering; matched filtering is only optimal for stationary Gaussian noise. We create mock data for CE and ET using the latest population models informed by the GWTC-3 catalog and investigate the performance loss of matched filtering due to overlapping signals. We find the performance loss mainly comes from a deviation in the noise’s measured amplitude spectral density. The redshift reach of CE (ET) can be reduced by 15%–38% (8%–21%) depending on the merger rate estimate. The direct contribution of confusion noise to the total signal-to-noise ratio (SNR) is generally negligible compared to the contribution from instrumental noise. We also find that correlated confusion noise has a negligible effect on the quadrature summation rule of network SNR for ET, but might reduce the network SNR of high detector-frame mass signals for detector networks including CE if no mitigation is applied. For ET, the null stream can mitigate the astrophysical foreground. For CE, we demonstrate that a computationally efficient, straightforward single-detector signal subtraction method suppresses the total noise to almost the instrument noise level; this will allow for near-optimal searches.

A comprehensive summary of the measurements made to characterize test-mass charging due to the space environment during the LISA Pathfinder mission is presented. Measurements of the residual charge of the test mass after release by the grabbing and positioning mechanism show that the initial charge of the test masses was negative after all releases, leaving the test mass with a potential in the range from $-12$ to $-512$. Variations in the neutral test-mass charging rate between 21.7 and $30.7e{\mathrm{s}}^{-1}$ were observed over the course of the 17-month science operations produced by cosmic ray flux changes including a Forbush decrease associated with a small solar energetic particle event. A dependence of the cosmic ray charging rate on the test-mass potential between $-30.2$ and $-40.3e{\mathrm{s}}^{-1}{\mathrm{V}}^{-1}$ was observed resulting in an equilibrium test-mass potential between 670 and 960 mV, and this is attributed to a contribution to charging from low-energy electrons emitted from the gold surfaces of the gravitational reference sensor. Data from the onboard particle detector show a reliable correlation with the charging rate and with other environmental monitors of the cosmic ray flux. This correlation is exploited to extrapolate test-mass charging rates to a 20-year period giving useful insight into the expected range of charging rate that may be observed in the LISA mission.

We investigate whether the seven-loop beta function of the $\lambda {\varphi }_4^4$ theory exhibits evidence for an ultraviolet zero. In addition to a direct analysis of the beta function, we calculate and study Padé approximants and discuss effects of scheme transformations on the results. Confirming and extending our earlier studies of the five-loop and six-loop beta functions, we find that in the range of $\lambda$ where the perturbative calculation of the seven-loop beta function is reliable, the theory does not exhibit evidence for an ultraviolet zero.

We study two novel aspects of democratic three-Higgs-doublet models (3HDMs)—the custodial limit and the possibility of wrong-sign Yukawa couplings. In the custodial limit, the democratic 3HDMs can easily negotiate the constraints from the electroweak $T$-parameter. We also uncover the possibility of having wrong-sign Yukawa couplings in democratic 3HDMs, as in the case of 2HDMs. We show that a democratic 3HDM encompasses all the wrong-sign possibilities entertained by 2HDMs, and has considerably more leeway in the wrong-sign limit as compared to the 2HDM case. Our study underscores the importance of reporting analysis in the kappa-formalism without any implicit assumptions on the signs of the kappas.