The sky localization of the gravitational wave (GW) source is an important scientific objective for GW observations. A network of space-based GW detectors dramatically improves the sky localization accuracy compared with an individual detector not only in the inspiral stage but also in the ringdown stage. It is interesting to explore what plays an important role in the improvement. We find that the angle between the detector planes dominates the improvement, and the time delay is the next important factor. A detector network can dramatically improve the source localization for short signals and long signals with most contributions to the signal-to-noise ratio (SNR) coming from a small part of the signal in a short time, and the more SNR contributed by smaller parts, the better improvement by the network. We also find the effects of the arm length in the transfer function and higher harmonics are negligible for source localization with the detector network.

Time delay interferometry (TDI) is an algorithm proposed to suppress the laser frequency noise in space-borne gravitational-wave detectors. As a post-processing technique, it is implemented by constructing a virtual equal-arm interferometer through an appropriate combination of the time-shifted data streams. Such an approach is tailored to the intrinsic feature of space-based gravitational-wave detection, namely, the distances between spacecraft are governed by orbital dynamics and thus cannot be held constant. Among different implementations, geometric TDI was introduced as a method of exhaustion to evaluate the second-generation TDI combinations. The applications of the algebraic approach based on computational algebraic geometry, on the other hand, are mostly restricted to first- and modified first-generation TDI. Besides, geometric TDI furnishes an intuitive physical interpretation of the synthesis of the virtual optical paths. In this paper, geometric TDI is utilized to investigate the modified second-generation TDI combinations in conjunction with a ternary search algorithm. The distinction between second-generation and modified second-generation TDI solutions is elaborated regarding the rate of change of the arm lengths with respect to the opposite cyclic directions. For the 16-link combinations, 40 second-generation TDI solutions are recovered, among which nine are identified as the modified second-generation ones. Furthermore, we explore the properties of the modified second-generation TDI solutions, which turn out to be potentially preferable in practice. Regarding the Taylor expansion of arm lengths in time, the expressions for the leading-order optical path residuals for the relevant geometric TDI combinations are derived, which are further specified using the Keplerian orbits of the spacecraft for the LISA detector constellation. The response function, noise power spectral density, and signal-to-noise ratio of the TDI solutions are given analytically and discussed. We obtain three distinct sensitivity curves among nine 16-link modified second-generation TDI combinations, while eight sensitivity curves are encountered out of 31 second-generation ones. It is argued that the modified second-generation TDI solutions present a quantitative advantage over their second-generation counterparts. Even though the noise suppressions of both scenarios are found to be at the same level, owing to the cancellations in the response function caused by the temporal symmetry of the arm lengths, the magnitude of the gravitational-wave signals is less pronounced for the second-generation TDI solutions. Moreover, analytic analysis confirms that the alternative modified second-generation TDI solutions are desirable as they possess fewer zeros in the average response function and the noise power spectral density, in accordance with previous findings.

Recent observations have shown that pulsars are surrounded by extended regions which emit TeV-scale gamma rays through the inverse Compton scattering of very high energy electrons and positrons. Such TeV halos are responsible for a large fraction of the Milky Way’s TeV-scale gamma-ray emission. In this paper, we calculate the gamma-ray spectrum from the population of TeV halos located within the Andromeda Galaxy, predicting a signal that is expected to be detectable by the Cherenkov Telescope Array. We also calculate the contribution from TeV halos to the isotropic gamma-ray background, finding that these sources should contribute significantly to this flux at the highest measured energies, constituting up to $\sim 20\%$ of the signal observed above $\sim 0.1\mathrm{TeV}$. We also comment on the implications of our results for the origin of the diffuse neutrino flux detected by IceCube.

Pulsar-timing arrays (PTAs) are in the near future expected to detect a stochastic gravitational-wave background (SGWB) produced by a population of inspiralling supermassive black hole binaries. In this work, we consider a background that can be anisotropic and circularly polarized. We use the expansion of the intensity and the circular polarization in terms of spherical harmonics and the overlap reduction functions for each term in this expansion. We propose an unbiased real-space estimator that can separate the intensity and circular-polarization contributions of the SGWB to pulsar-timing-residual correlations and then validate the estimator on simulated data. We compute the signal-to-noise ratio of a circular-polarization component that has a dipole pattern under different assumptions about the PTA. We find that a nearly maximal circular-polarization dipole may be detectable, which can aid in determining whether or not the background is dominated by a handful of bright sources.

The focus on the dark matter search has expanded to include low-mass particles such as axions or axionlike particles, and novel theoretical schemes extending the phenomenological landscape, within QCD and beyond, also garnered additional interest in recent decades. Assuming dark matter is composed of axions, in the presence of a solenoidal magnetic field, they induce a displacement current that gives rise to a toroidal magnetic field. The weakly interacting slender particle detection with an $LC$ circuit (WISPLC) is a precision direct detection experiment that will search for light dark matter candidates such as axionlike particles in parts of the parameter space previously unexplored. We present two detection schemes of the signal in a pickup loop capturing the flux of this toroidal magnetic field. WISPLC operates in a broadband and a resonant scheme where an $LC$ circuit is used to enhance the signal with an expected $Q$ factor $\sim {10}^4$. Taking into account the irreducible flux noise of the detector, we estimate the sensitivity of the experiment in the axion mass range between ${10}^{-11}$ and ${10}^{-6}\mathrm{eV}$ to reach a detectable axion-photon coupling of $g_{a\gamma \gamma }\approx {10}^{-15}{\mathrm{GeV}}^{-1}$, making it possible to probe mass ranges corresponding to ultralight axions motivated by string theory. The WISPLC experiment is fully funded and currently in the construction phase.

We evaluate the potential for gravitational-wave (GW) detection in the frequency band from 10 nHz to $1\mu \mathrm{Hz}$ using extremely high-precision astrometry of a small number of stars. In particular, we argue that nonmagnetic, photometrically stable, hot white dwarfs (WD) located at $\sim \mathrm{kpc}$ distances may be optimal targets for this approach. Previous studies of astrometric GW detection have focused on the potential for less precise surveys of large numbers of stars; our work provides an alternative optimization approach to this problem. Interesting GW sources in this band are expected at characteristic strains around $h_c\sim {10}^{-17}\times (\mu \mathrm{Hz}/f_{\mathrm{GW}})$. The astrometric angular precision required to see these sources is $\mathrm{\Delta }\theta \sim h_c$ after integrating for a time $T\sim 1/f_{\mathrm{GW}}$. We show that jitter in the photometric center of WD of this type due to starspots is bounded to be small enough to permit this high-precision, small-$N$ approach. We discuss possible noise arising from stellar reflex motion induced by orbiting objects and show how it can be mitigated. The only plausible technology able to achieve the requisite astrometric precision is a space-based stellar interferometer. Such a future mission with few-meter-scale collecting dishes and baselines of $\mathcal{O}(100\mathrm{km})$ is sufficient to achieve the target precision. This collector size is broadly in line with the collectors proposed for some formation-flown, space-based astrometer or optical synthetic-aperture imaging-array concepts proposed for other science reasons. The proposed baseline is however somewhat larger than the km-scale baselines discussed for those concepts, but we see no fundamental technical obstacle to utilizing such baselines. A mission of this type thus also holds the promise of being one of the few ways to access interesting GW sources in this band.

Delayed single- and few-electron emissions plague dual-phase time projection chambers, limiting their potential to search for light-mass dark matter. This paper examines the origins of these events in the XENON1T experiment. Characterization of the intensity of delayed electron backgrounds shows that the resulting emissions are correlated, in time and position, with high-energy events and can effectively be vetoed. In this work we extend previous S2-only analyses down to a single electron. From this analysis, after removing the correlated backgrounds, we observe rates $<30\mathrm{events}/(\mathrm{electron}\times \mathrm{kg}\times \mathrm{day})$ in the region of interest spanning 1 to 5 electrons. We derive 90% confidence upper limits for dark matter-electron scattering, first direct limits on the electric dipole, magnetic dipole, and anapole interactions, and bosonic dark matter models, where we exclude new parameter space for dark photons and solar dark photons.

High-multiplicity signatures at particle colliders can arise in Standard Model processes and beyond. With such signatures, difficulties often arise from the large dimensionality of the kinematic space. For final states containing a single type of particle signature, this results in a combinatorial problem that hides underlying kinematic information. We explore using a neural network that includes a Lorentz Layer to extract high-dimensional correlations. We use the case of squark decays in $R$-Parity-violating Supersymmetry as a benchmark, comparing the performance to that of classical methods. With this approach, we demonstrate significant improvement over traditional methods.

In an endeavor to explain the light neutrino masses and dark matter (DM) simultaneously, we study a gauged $U(1{)}_{\mathrm{B}-\mathrm{L}}$ extension of the standard model (SM). The neutrino masses are generated through a variant of type-II seesaw mechanism in which one of the scalar triplets has a mass in a scale that is accessible at the present generation colliders. Three SM singlet right chiral fermions ${\chi }_{iR}$ ($i=e$, $\mu$, $\tau$) with $\mathrm{B}-\mathrm{L}$ charges $-4$, $-4$, $+5$ are invoked to cancel the $\mathrm{B}-\mathrm{L}$ gauge anomalies and the lightest one among these three fermions becomes a viable DM candidate as their stability is guaranteed by a remnant ${\mathcal{Z}}_2$ symmetry to which $U(1{)}_{\mathrm{B}-\mathrm{L}}$ gauge symmetry gets spontaneously broken. Interestingly in this scenario, the neutrino mass and the coannihilation of DM are interlinked through the breaking of $U(1{)}_{\mathrm{B}-\mathrm{L}}$ symmetry. Apart from giving rise to the observed neutrino mass and dark matter abundance, the model also predicts exciting signals at the colliders. Especially we see a significant enhancement in the production cross section of the TeV scale doubly charged scalar in presence of the $Z_{\mathrm{BL}}$ gauge boson. We discuss all the relevant constraints on model parameters from observed DM abundance and null detection of DM at direct and indirect search experiments as well as the constraints on the $\mathrm{B}-\mathrm{L}$ gauge boson from recent colliders.

We consider the $Sp(4)$ gauge theory coupled to $N_f=2$ fundamental and $n_f=3$ antisymmetric flavors of Dirac fermions in four dimensions. This theory serves as the microscopic origin for composite Higgs models with $SU(4)/Sp(4)$ coset, supplemented by partial top compositeness. We study numerically its lattice realization, and couple the fundamental plaquette action to Wilson-Dirac fermions in mixed representations, by adopting a (rational) hybrid Monte Carlo method, to perform nontrivial tests of the properties of the resulting lattice theory. We find evidence of a surface (with boundaries) of first-order bulk phase transitions in the three-dimensional space of bare parameters (one coupling and two masses). Explicit evaluation of the Dirac eigenvalues confirms the expected patterns of global symmetry breaking. After investigating finite-volume effects in the weak-coupling phase of the theory, for the largest available lattice we study the mass spectra of the lightest spin-0 and spin-1 flavored mesons composed of fermions in each representation, and of the lightest half-integer spin composite particle made of fermions in different representations—the chimera baryon. This work sets the stage for future systematical studies of the nonperturbative dynamics in phenomenologically relevant regions of parameter space.

In this work the $k_{\perp }$-factorization formalism is applied to compute the exclusive $Z^0$ boson photoproduction in $ep$ and $eA$ collisions. The study is also extended to $pp$ and $AA$ processes. The nuclear effects are investigated considering heavy and light ions. Analytical models for the unintegrated gluon distribution are taken into account and the corresponding theoretical uncertainty is quantified. The analysis is done for electron-ion collisions at the Large Hadron-Electron Collider, its high-energy upgrade and at the Future Circular Collider in lepton-hadron mode. Additionally, ultra-peripheral heavy ion collisions at future runs of the Large Hadron Collider and at the Future Circular Collider (hadron-hadron mode) are also considered.

This paper describes the operation of the Coherent CAPTAIN-Mills (CCM) detector located at the Los Alamos Neutron Science Center at Los Alamos National Laboratory. CCM is a 10-ton liquid argon detector located 20 meters from a high flux neutron/neutrino source and is designed to search for sterile neutrinos (${\nu }_s$’s) and light dark matter (LDM). An engineering run was performed in fall 2019 to study the characteristics of the CCM120 detector by searching for coherent scattering signals consistent with ${\nu }_s$’s and LDM resulting from the production and decays of ${\pi }^{+}$ and ${\pi }^0$ in the tungsten target. New parameter space in a leptophobic dark matter (DM) model was excluded for DM masses between $\sim 2.0$ and 30 MeV. The lessons learned from this run have guided the development and construction of the new CCM200 detector that will begin operations in 2021 and significantly improve on these searches.

In this paper we construct the 5D uplift of 4D gravitational waves in de Sitter cosmology for the brane world scenario based on a nucleated bubble in ${\mathrm{AdS}}_5$. This makes it possible to generalize the connection between the dark bubbles and Vilenkin’s quantum cosmology to include gravitational perturbations. We also use the uplift to explain the interpretation of the apparently negative energy contributions in the 4D Einstein equations, which distinguish the dark bubble scenario from Randall-Sundrum.

Numerical relativity simulations are the only way to calculate exact gravitational waveforms from binary neutron star mergers and to design templates for gravitational-wave astronomy. The accuracy of these numerical calculations is critical in quantifying tidal effects near merger that are currently one of the main sources of uncertainty in merger waveforms. In this work, we explore the use of an entropy-based flux-limiting scheme for high-order, convergent simulations of neutron star spacetimes. The scheme effectively tracks the stellar surface and physical shocks using the residual of the entropy equation thus allowing the use of unlimited central flux schemes in regions of smooth flow. We perform the first neutron star merger simulations with such a method and demonstrate up to fourth-order convergence in the gravitational waveform phase. The scheme reduces the phase error up to a factor 5 when compared to state-of-the-art high-order characteristic schemes and can be employed for producing faithful tidal waveforms for gravitational-wave modeling.

The terminal wall velocity of a first-order phase transition bubble in the early Universe can be calculated from a set of fluid equations describing the scalar fields and the plasma’s state. We rederive these equations from the energy-momentum tensor conservation and the Boltzmann equation, without linearizing in the background temperature and fluid velocity. The resulting equations have a finite solution for any wall velocity. We propose a spectral method to integrate the Boltzmann equation, which is simple, efficient, and accurate. As an example, we apply this new methodology to the singlet scalar extension of the standard model. We find that all solutions are naturally categorized as deflagrations ($v_w\sim c_s$) or ultrarelativistic detonations (${\gamma }_w\gtrsim 10$). Furthermore, the contributions from out-of-equilibrium effects are, most of the time, subdominant. Finally, we use these results to propose several approximation schemes with increasing levels of complexity and accuracy. They can be used to considerably simplify the methodology, while correctly describing the qualitative behavior of the bubble wall.

Two important parameters inferred from the gravitational wave signals of binaries of precessing black holes are the spin tilt angles, i.e., the angles at which the black holes’ spin axes are inclined with respect to the binary’s orbital angular momentum. The LIGO-Virgo parameter estimation analyses provide spin tilts at a fiducial reference frequency, often the lowest frequency used in the data analysis. However, the most astrophysically interesting quantities are the spin tilts when the binary was formed, which can be significantly different from those at the reference frequency for strongly precessing binaries. The spin tilts at formally infinite separation are a good approximation to the tilts at formation in many formation channels and can be computed efficiently for binary black holes using precession-averaged evolution. Here, we present a new code for computing the tilts at infinity that combines the precession-averaged evolution with orbit-averaged evolution at high frequencies and illustrate its application to GW190521 and other binary black hole detections from O3. We have empirically determined the transition frequency between the orbit-averaged and precession-averaged evolution to produce tilts at infinity with a given accuracy and find that using only the precession-averaged evolution can lead to errors in the cosines of the tilts at infinity of $>0.8$ for certain binary configurations. However, the precession-averaged evolution alone is sufficient for good accuracy when obtaining the posterior distributions of the tilts at infinity for current detections. We also have regularized the precession-averaged equations in order to obtain good accuracy for the very close-to-equal-mass binary parameters encountered in practice. This additionally allows us to investigate the singular equal-mass limit of the precession-averaged expressions, where we find that to a good approximation the results only depend on the orbital angular momentum $L$ through the combination $(1-q)L$, where $q$ is the mass ratio.

We present an implementation of $t\overline{t}{\ell }^{+}{\ell }^{-}$ ($\ell =e$, $\mu$) hadronic production at next-to-leading order in QCD matched to parton-shower event generators in the powheg box framework. The program we developed includes all leading-order contributions of order ${\alpha }_s^2{\alpha }^2$ for the specified final state, as well as the corresponding first-order QCD corrections. Decays of the top quarks have been simulated retaining spin-correlations in all tree-level matrix elements. We consider the case of the Large Hadron Collider at $\sqrt{s}=13\mathrm{TeV}$ and compare results for $t\overline{t}{\ell }^{+}{\ell }^{-}$ production in the fiducial volume where the invariant mass of the lepton pairs is centered around the $Z$-boson mass to corresponding predictions for $t\overline{t}Z$ on-shell production with $Z\to {\ell }^{+}{\ell }^{-}$. We find that off-shell effects in $t\overline{t}{\ell }^{+}{\ell }^{-}$ are in general small at the level of the total cross section, but can decrease the tail of the leptons’ transverse momentum distributions by 10%–20% and, in these regions, they are visible beyond the scale uncertainty due to renormalization and factorization scale variation. Moreover, we find that accounting for top-quark decays in the narrow-width approximation with tree-level spin correlations also gives origin to 10%–20% effects in specific regions of the kinematic distributions of the $t\overline{t}{\ell }^{+}{\ell }^{-}$ decayed final state.

Using the ambitwistor string, we complete the list of celestial operator product expansion (OPE) coefficients for supersymmetric theories. This uses the ambitwistor string worldsheet conformal field theory to dynamically generate the OPE coefficients for maximally supersymmetric gauge theory, as well as gravity and Einstein-Yang-Mills theories, including all helicity and orientation configurations. This extends previous purely bosonic results [T. Adamo et al., arXiv:2111.02279] to include supersymmetry and provides explicit formulas which are, to the best of our knowledge, not in the literature. We also examine how the supersymmetric infinite dimensional soft algebras behave compared to the purely bosonic cases.

Motivated, on the one hand, by recent results on isochronous dynamical systems, and on the other, by quantum gravity applications of complex metrics, we show that, if such an enlarged class of metrics is considered, one can easily obtain periodic or bouncing complex solutions of Einstein’s equations. It is found that, for any given solution $g_{\mu \nu }$ of the Einstein’s equations, by means of a complex periodic change of time, one can construct infinitely many periodic or bouncing complex solutions ${\widehat{g}}_{\mu \nu }$ that are physically indistinguishable from $g_{\mu \nu }$ over an arbitrarily long time interval. These results, which are based on the use of complex diffeomorphisms, point out an unacceptable arbitrariness in the theory. As we will show, a condition on the class of physically meaningful complex metrics proposed in [M. Kontsevich and G. B. Segal, Q. J. Math. 72, 673 (2021)] and discussed in [E. Witten, arXiv:2111.06514] solves this problem, restricting the family of admissible complex diffeomorphisms. We conclude by arguing that this condition can be viewed as a quantum-gravity generalization of the equivalence principle to complex space-times.

The study of toy models in loop quantum gravity, defined as truncations of the full theory, is relevant to both the development of the loop quantum gravity phenomenology, in cosmology and astrophysics, and the progress towards the resolution of the open issues of the theory, in particular the implementation of the dynamics. Here, we study the dynamics of spin network states of quantum geometry defined on the family of graphs consisting in 2 vertices linked by an arbitrary number of edges, or 2-vertex model in short. A symmetry reduced sector of this model—to isotropic and homogeneous geometries—was successfully studied in the past, where interesting cosmological insights were found. We now study the evolution of the classical trajectories for this system in the general case, for arbitrary number of edges with random initial configurations. We use the spinorial formalism and its clear interpretation of spin networks in terms of discrete twisted geometries, with the quantum 3D space made of superpositions of polyhedra glued together by faces of equal area. Remarkably, oscillatory and divergent regimes are found with a universal dependence on the coupling constants of the Hamiltonian and independent of the initial spinors or the number of edges. Furthermore, we explore the evolution of the associated polyhedra as well as their volumes and areas.