The Hellings-Downs (HD) curve plays a crucial role in search for nano-hertz gravitational waves (GWs) with pulsar timing arrays. We discuss the angular pattern of correlations for pulsar pairs within a celestial hemisphere. The hemisphere-averaged correlation curve depends upon the sky location of a GW compact source like a binary of supermassive black holes. If a single source is dominant, the variation in the hemisphere-averaged angular correlation is greatest when the hemisphere has its North Pole at the sky location of the GW source. Possible GW amplitude and source distance relevant to the current pulsar timing arrays by using the hemisphere-averaged correlation are also studied.

Using graphs plotted from the Mathematica notebooks posted with our paper “Dynamical Gravastars,” we show that a dynamical gravastar has no hard surface, and that a second light sphere resides in the deep interior where there is maximum time dilation. These facts may permit dynamical gravastars to evade no-go results for exotic compact objects relating to light leakage inside the shadow, and nonlinear instabilities arising from an interior light sphere. Testing these surmises will require further detailed modeling calculations, beyond what we commence in this paper, using the numerical dynamical gravastar solution. We also discuss the effect of replacing the sigmoidal function in the gravastar calculation by a unit step function, and we analyze why the dynamical gravastar evades the singularities predicted by the Penrose and Hawking singularity theorems, despite satisfying both the null and strong energy conditions. We then give a simplified two-step process for tuning the initial value $\nu (0)=\text{nuinit}$ to achieve $\nu (\infty )=0$, and give exact integrals for the pressure differential equation in terms of $\nu (r)$ in the interior and exterior regions. Finally, we briefly discuss an extension of the model that includes an external shell of massive particles.

Massive vector fields can form spatially localized, nonrelativistic, stationary field configurations supported by gravitational interactions. The ground-state configurations ($p$-solitons/vector solitons/dark photon stars/polarized Proca stars) have a time-dependent vector field pointing in the same spatial direction throughout the configuration at any instant of time, can carry macroscopic amounts of spin angular momentum, and are spherically symmetric and monotonic in the energy density. In this paper, we include general relativistic effects, and numerically investigate the stability of compact polarized Proca stars (linear and circularly polarized) and compare them to hedgehog-like field configurations (with radially pointing field directions). Starting with approximate field profiles of such stars, we evolve the system numerically using ($3+1$)-dimensional numerical simulations in general relativity. We find that these initial conditions lead to stable configurations. However, at sufficiently large initial compactness, they can collapse to black holes. We find that the initial compactness that leads to black hole formation is higher for circularly polarized stars (which carry macroscopic spin angular momentum), compared to linearly polarized ones, which in turn is higher than that for hedgehog configurations.

The Aether scalar tensor (AeST) theory is an extension of general relativity, proposed for addressing galactic and cosmological observations without dark matter. The action for the theory includes a function that can currently only be constrained by phenomenological considerations. In antecedent work, forms of this function were considered that led to an effective fluid contribution to the cosmological evolution equations that approximated that of dust more and more closely at late cosmic times. In this work we consider an alternative set of functions that most closely approximate dust at the earliest cosmic times and where deviations from dustlike behavior gradually emerge with time. We use the dynamical system formalism to analyze example models from both possible sets of functions, introducing a complete set of dynamical variables describing the spacetime curvature, energy density parameters of different matter components, and AeST scalar field, and obtain the dynamical equations describing cosmological evolution. The cosmological phase space is found to feature invariant submanifolds associated to the absence of the matter components, as well as equilibrium states associated with well-known cosmological behaviors e.g. matter, radiation, and cosmological-constant dominated epochs. A full numerical integration of the dynamical system is performed for the models and it is shown that each can closely approximate the $\mathrm{\Lambda }\mathrm{CDM}$ model at the level of the cosmic background. Generalizations of the models are considered and it is shown that the new models likely can simultaneously replicate the cosmological successes of cold dark matter whilst satisfying constraints on the theory from the weak-field quasistatic regime.

We calculate the reheating temperature in scenarios where heavy particles are gravitationally produced during a phase transition. We explore two distinct situations: the decay of these particles both during and after the kination phase. Subsequently, we determine the respective reheating temperatures. Finally, we constrain these temperatures based on considerations related to the overproduction of gravitational waves during the phase transition from the end of inflation to the onset of kination.

LHC searches for nonstandard scalars in vector boson fusion (VBF) production processes can be particularly efficient in probing scalars belonging to triplet or higher multiplet representations of the Standard Model $SU(2{)}_L$ gauge group. They can be especially relevant for models where the additional scalars do not have any tree level couplings to the Standard Model fermions, rendering VBF as their primary production mode at the LHC. In this work we employ the latest LHC data from VBF resonance searches to constrain the properties of nonstandard scalars, taking the Georgi-Machacek model as a prototypical example. We take into account the theoretical constraints on the potential from unitarity and boundedness from below as well as indirect constraints coming from the signal strength measurements of the 125 GeV Higgs boson at the LHC. To facilitate the phenomenological analysis we advocate a convenient reparametrization of the trilinear couplings in the scalar potential. We derive simple correlations among the model parameters corresponding to the decoupling limit of the model. We explicitly demonstrate how a combination of theoretical and phenomenological constraints can push the Georgi-Machacek model towards the decoupling limit. Our analysis suggests that the VBF searches can provide key insights into the composition of the electroweak vacuum expectation value.

We study the possible impact of dark photons on lepton flavor phenomenology. We derive the constraints on nondiagonal dark photon couplings with leptons by analyzing corresponding contributions to lepton anomalous magnetic moments, rare lepton decays, and the prospects of fixed-target experiments aimed at searching for light dark matter based on missing energy/momentum techniques.

In the presence of an anomalous $CP$ phase in a U(1) gauge theory, a monopole becomes a dyon via the Witten effect. When the anomalous $CP$ phase is promoted to a dynamical field, the axion, the electric charge of the dyon changes according to the coherent motion of the axion oscillation. Once the electric charge exceeds a certain threshold, the Schwinger pair production of charged particles becomes efficient near the surface of the dyon. These nonperturbative effects lead to the backreaction of the axion dynamics by causing the dissipation of the axion oscillation energy and the change of the effective potential due to the Witten effect. Taking these effects into account, we consider the dynamics of the whole system, including the axion, monopole, and charged heavy vector bosons, and discuss to what extent the axion abundance is modified. We also discuss the electric dipole radiation from a bound state of a monopole-antimonopole pair due to the axion coherent oscillations.

Axions are a compelling dark matter candidate, and one of the primary techniques employed to search for them is the axion haloscope, in which a resonant cavity is deployed inside a strong magnetic field so that some of the surrounding axions may convert into photons via the inverse Primakoff effect and become trapped inside the resonator. Resonant cavity design is critical to the sensitivity of a haloscope, and several geometries have been utilized and proposed. Here we consider a relatively simple concept—a rectangular resonant cavity with a tunable wall—and compare it to the standard tuning rod-type resonators employed in the field. We find that the rectangular cavities support similar modes to cylindrical tuning rod cavities, and have some advantages in terms of axion sensitivity and practicality, particularly when moving to higher frequencies which are of great and growing interest in the international axion dark matter community.

The internal image of the proton is unveiled by examining the generalized parton distributions (GPDs) at zero skewness, within the basis light-front quantized environment. Several distributions emerge when a quark is sampled with different currents depending upon the helicity arrangements of the active quark and the proton target. We investigate six of the eight leading-twist proton GPDs of the valence quarks, the helicity conserving distributions $(H,E,\tilde{H})$ and the helicity nonconserving $(H_T,E_T,{\tilde{H}}_T)$ distributions at skewness set to zero ($\zeta =0$). We consider purely transverse momentum transfer and, hence, obtain results describe only the proton’s two-dimensional structure in the transverse plane. We present the Mellin moments of these distribution functions, where the first moment produces a form factor and the second Mellin moments help extract the information on partonic contributions to the hadronic angular momentum. We compare our results for the Mellin moments with those from lattice QCD and other approaches where available. We also present the GPDs in transverse position space.

Anomalous contributions to the electric and magnetic dipole moments of the $\tau$ lepton from new physics scenarios have brought renewed interest in the development of new charge-parity violating signatures in $\tau$-pair production at Belle II energies, and also at higher energies of the Large Hadron Collider and the Future Circular Collider. In this paper, we discuss the effects of spin correlations, including transverse degrees of freedom, in the $\tau$-pair production and decay. These studies include calculating analytical formulas, obtaining numerical results, and building semirealistic observables sensitive to the transverse spin correlations induced by the dipole moments of the $\tau$ lepton. The effects of such anomalous contributions to the dipole moments are introduced on top of precision simulations of $e^{-}{e}^{+}\to {\tau }^{-}{\tau }^{+}$, $q\overline{q}\to {\tau }^{-}{\tau }^{+}$ and $\gamma \gamma \to {\tau }^{-}{\tau }^{+}$ processes, involving multibody final states. The $\tau$ decays are simulated along with radiative corrections, in particular electroweak box contributions of $WW$ and $ZZ$ exchanges are taken into account. Respective extensions of the Standard Model amplitudes and the reweighting algorithms are implemented into the kkmc Monte Carlo, which is used to simulate $\tau$-pair production in $e^{-}{e}^{+}$ collisions, and the tauspinner program, which is used to reweight events with $\tau$ pair produced in $pp$ collisions.

Ultralight axion or axionlike particles are one of the most promising candidates for dark matter, because they are a well-motivated solution for the theoretical strong $CP$ problem and observational issues on small scales, i.e., the core-cusp problem and the satellite problem. A tiny coupling of axions and photons induces birefringence. We propose the differential birefringence measurements of multiple images of gravitationally lensed fast radio burst (FRB) systems as probes of the galactic axion dark matter (ADM) background. In addition to general advantages of lensing systems, i.e., alleviating systematics and intrinsic astrophysical dependencies, precise measurements of lensing time delay and polarization angle in gravitationally lensed FRB systems make them a more robust and powerful probe. We show that, with a single lensed FRB system (which may be detected in large numbers in the Square Kilometre Array era), the axion-photon coupling under the ADM background could be constrained to be $g_{a\gamma }<7.3\times {10}^{-11}{\mathrm{GeV}}^{-1}$ for an axion mass $m_a\sim {10}^{-20}\mathrm{eV}$. This will be of great significance in achieving synergistic searches of the galactic ADM with other astrophysical probes and laboratorial experiments.

In a recent paper [Phys. Rev. D 102, 016020 (2020)], pseudo quantum electrodynamics was used to simulate the Coulomb interaction between electrons and determine the mass renormalization in a two-dimensional Dirac-like system. In the present paper, we expand upon these findings by examining this system in a certain distance from a planar interface separating two dielectrics. Using the random-phase approximation, we calculate the renormalization group functions and show how the behaviors of the mass, Fermi velocity, and the anomalous dimension of the fermion field are affected by the presence of this interface. To exemplify an application of our formulas, we calculate the influence of this interface on the renormalized energy band gap in a two-dimensional material. In the appropriate limits, our results recover the corresponding ones reported in the aforementioned publication, as well as in others.

We have discovered a class of dynamically stable coherent states for motion on the half line. The regularization of the half line boundary and the consequent quantum motion are expounded within the framework of covariant affine quantization, although alternative approaches are also feasible. The former approach is rooted in affine coherent states and offers a consistent semiclassical representation of quantum motion. However, this method has been known to possess two shortcomings: (a) the dependence of affine coherent states on the choice of a vector, denoted as the “fiducial vector” (which remains unspecified), introduces significant arbitrariness in boundary regularization, and (b) regardless of the choice of fiducial vector, affine coherent states fail to evolve parametrically under the Schrödinger equation, thus limiting the accuracy of the semiclassical description. This limitation, in particular, hampers their suitability for approximating the evolution of compound observables. We demonstrate that a distinct and more refined definition of affine coherent states can simultaneously address both of these issues. In other words, these new affine coherent states exhibit parametric evolution only when the fiducial vector, denoted as $|{\psi }_0⟩$, possesses a highly specific character, such as being an eigenstate of a well-defined Hamiltonian. Our discovery holds significant relevance in the field of quantum cosmology, particularly in scenarios where the positive variable is the scale factor of the universe, and its regularized motion plays a crucial role in avoiding the big-bang singularity.

We consider a $k$-essence model in which a single scalar field can be responsible for both primordial inflation and the present observed acceleration of the cosmological background geometry, while also admitting a nonsingular de Sitter beginning of the Universe (it arises from de Sitter and ends in de Sitter). The early one is driven by a slow-roll potential, and the late one is driven by a dynamical dimensional reduction process which freezes the scalar field in a degenerate surface, turning it into a cosmological constant. This is done by proposing a realizable stable cosmic time crystal, although giving a different interpretation to the “moving ground state”, in which there is no motion because the system loses degrees of freedom. Furthermore, the model is free of pathologies such as propagating superluminal perturbations, negative energies, and perturbation instabilities.

The possibility of an expanding decelerating Universe in the distant future is investigated in the context of a quintessence scalar field cosmology. Such a conceivable evolution is tested against SNe Ia and $H(z)$ cosmic chronometers data, and also through a model independent method based on Gaussian processes. The scalar field model is an extension of the exponential Ratra-Peebles (RP) quintessential cosmology whose potential now depends on a pair of parameters ($\alpha ,\lambda$) and predicts a decelerated expansion in the future. Different from the RP approach the $\alpha$ parameter allows for a decelerating cosmology in the future thereby frustrating the inevitable evolution for a de Sitter cosmology as predicted by the cosmic concordance model ($\mathrm{\Lambda }\mathrm{CDM}$). The statistical model analysis is updated with the most recent SNe Ia and $H(z)$ data thereby obtaining $H_0=68.6\pm 3.7\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$, ${\mathrm{\Omega }}_{\mathrm{\Phi }0}=0.735_{-0.069}^{+0.083}$, $\alpha <6.56$ and $\lambda <0.879$ (at $2\sigma$ c.l.). It is also found that the extended RP model allows for a future deceleration both for $H(z)$ and SNe Ia data. In the (model-independent) Gaussian processes analysis, however, future deceleration is allowed only in the case of $H(z)$ data.

Relations between neutron star properties that do not depend on the nuclear equation of state offer insights on neutron star physics and have practical applications in data analysis. Such relations are obtained by fitting to a range of phenomenological or nuclear physics equation of state models, each of which may have varying degrees of accuracy. In this study we revisit commonly used relations and reassess them with a very flexible set of phenomenological nonparametric equation of state models that are based on Gaussian processes. Our models correspond to two sets: equations of state which mimic hadronic models, and equations of state with rapidly changing behavior that resemble phase transitions. We quantify the accuracy of relations under both sets and discuss their applicability with respect to expected upcoming statistical uncertainties of astrophysical observations. We further propose a goodness-of-fit metric which provides an estimate for the systematic error introduced by using the relation to model a certain equation-of-state set. Overall, the nonparametric distribution is more poorly fit with existing relations, with the I–Love–Q relations retaining the highest degree of universality. Fits degrade for relations involving the tidal deformability, such as the binary-Love and compactness-Love relations, and when introducing phase transition phenomenology. For most relations, systematic errors are comparable to current statistical uncertainties under the nonparametric equation of state distributions.

We revisit the calculation of the Casimir effect from the perspective of scale limited resolutions of quantum fields. We use the continuous wavelet transform to introduce a scale degree of freedom and then restrict it to simulate either an observational or fundamental limitation of resolution. The Casimir force is derived in this setting for a free complex massless scalar field between two infinite plates with both Dirichlet and periodic boundary conditions. The dependence of the force on the choice of wavelet and size of scale cutoff is extensively discussed for several examples of wavelets.

This paper concerns a method of describing hadrons that starts with the canonical front form Hamiltonian of QCD. The method is developed in the relatively simple context of QCD with only heavy quarks. We regulate its canonical Hamiltonian by introducing a vanishingly small gluon mass $m_g$. For positive $m_g$, the small-$x$ gluon divergences become ultraviolet and hence they are renormalized in the same way the ultraviolet transverse divergences are. This is done using the renormalization group procedure for effective particles. Up to the second order of expansion of the renormalized Hamiltonian in powers of the quark-gluon coupling constant $g$, only the quark mass-squared and gluon-exchange divergences require counterterms. In these circumstances, we calculate an effective potential between quarks in heavy quarkonia in an elementary way, replacing all the quarkonium-state components with gluons of mass $m_g$ by only one component with just one gluon that is assigned a mass $m_G$, comparable to or exceeding the scale of typical relative momenta of bound quarks. In the limit of $m_g\to 0$ and large $m_G$ two results are obtained. (1) While the color-singlet quarkonium mass eigenvalue stays finite and physically reasonable in that limit, the eigenvalues for single quarks and octet quarkonia are infinite. (2) Besides the coulomb terms, the effective quark-antiquark potential is quadratic as a function of the distance and spherically symmetric for typical separations between quarks but becomes logarithmic and no longer spherically symmetric for large separations. Our conclusion indicates how to systematically improve upon the approximations made in this paper.

Branching fractions of decays $\tau \to K^0{\pi }^{-}\eta {\nu }_{\tau }$, $\tau \to K^{-}{\pi }^0\eta {\nu }_{\tau }$, $\tau \to K^{-}{K}^0\eta {\nu }_{\tau }$, and $\tau \to K^{-}\eta \eta {\nu }_{\tau }$ are calculated in the $U(3)\times U(3)$ chiral Nambu–Jona-Lasinio quark model. The contact, vector, axial-vector, and pseudoscalar channels are considered. It is shown that the axial vector channel is dominant. The obtained results are in satisfactory agreement with experiment.