We investigate the two-photon transitions $H_{c\overline{c}}\to {\gamma }^{*}\gamma$ of the charmonium system in light-front dynamics. The light-front wave functions were obtained from solving the effective Hamiltonian based on light-front holography and one-gluon exchange interaction within the basis light-front quantization approach. We compute the two-photon transition form factors as well as the two-photon decay widths for S- and P-wave charmonia, ${\eta }_c$ and ${\chi }_{cJ}$ and their excitations. Without introducing any free parameters, our predictions are in good agreement with the recent experimental measurements by BABAR and Belle, shedding light on the relativistic nature of charmonium.

In the discussed paper [N. Starkman, H. Winch, J. S. Sidhu, and G. Starkman, Straight lightning as a signature of macroscopic dark matter, Phys. Rev. D 103, 063024 (2021)], the authors have made several assumptions and statements concerning the initiation and propagation of lightning flashes induced by macroscopic dark matter passing through the atmosphere. The authors suggest that the path of dark matter can be identified by looking for lightning with straight channels, although such channels have not been previously reported. Even though we agree with the suggestion of the authors that macroscopic dark matter could, in theory, give rise to straight lightning channels, there are several statements in the paper that are not sufficiently clear and which could lead to misinterpretation. Our comments on the paper are the following.

The holographic entropy cone (HEC) characterizes the entanglement structure of quantum states which admit geometric bulk duals in holography. Due to its intrinsic complexity, to date it has only been possible to completely characterize the HEC for at most $n=5$ parties. For larger $n$, our knowledge of the HEC falls short of incomplete: almost nothing is known about its extremal elements. Here, we introduce a symmetrization procedure that projects the HEC onto a natural lower dimensional subspace. Upon symmetrization, we are able to conjecture precise properties that its extremal structure exhibits for general $n$. Further, by applying this symmetrization to the quantum entropy cone, we are able to quantify the typicality of symmetrized holographic entropies, which we find to be exponentially rare quantum entropies in the number of parties.

In this work we consider the effects of gravitons and their fluctuations on the dynamics of two masses using the Feynman-Vernon influence functional formalism, applied earlier to nonequilibrium quantum field theory [Calzetta and Hu, Nonequilibrium Quantum Field Theory (Cambridge University Press, Cambridge, England, 2008)] and semiclassical stochastic gravity [Hu and Verdaguer, Semiclassical and Stochastic Gravity: Quantum Field Effects on Curved Spacetime (Cambridge University Press, Cambridge, England, 2020)], and most recently, to this problem by Parikh et al., [Phys. Rev. Lett. 127, 081602 (2021); Phys. Rev. D 104, 046021 (2021)]. The Hadamard function of the gravitons yields the noise kernel acting as a stochastic tensorial force in a Langevin equation governing the motion of the separation of the two masses. The fluctuations of the separation due to the graviton noise are then solved for various quantum states including the Minkowski vacuum, thermal, coherent and squeezed states. The previous considerations of Parikh et al. are only for some selected modes of the graviton, while in this work we have included all graviton modes and polarizations. We comment on the possibility of detecting these fluctuations in primordial gravitons using interferometors with long baselines in deep space experiments.

We study the vacuum polarization effects of the Dirac fermionic field induced by a pointlike global monopole located in the cosmological de Sitter spacetime. First we derive the four orthonormal Dirac modes in this background in a closed form. Quantizing the field using these modes, we then compute the fermionic condensate, $⟨0|\overline{\mathrm{\Psi }}\mathrm{\Psi }|0⟩$, as well as the vacuum expectation value of the energy-momentum tensor for a massive Dirac field, regularized in a particular way. We have used the Abel-Plana summation formula in order to extract the global monopole contribution to these quantities and have investigated their variations numerically with respect to relevant parameters.

For all 4D $\mathcal{N}=4$ Super-Yang-Mills theories with simple gauge groups $G$, we show that a set of residues of the integrands in the $\mathcal{N}=4$ Schur indices, which are related to Gukov-Witten-type surface defects in the theories, equal the vacuum characters of $\mathrm{rank}G$ copies of $bc\beta \gamma$ systems that provide the free field realization of associated $\mathcal{N}=4$ vertex operator algebras in [F. Bonetti, C. Meneghelli, and L. Rastelli, J. High Energy Phys. 05 (2019) 155]. This result predicts that these residues, as module characters, are additional solutions to the flavored modular differential equations satisfied by the original Schur index. The prediction is verified in the $G=SU(2)$ case, where an additional logarithmic solution is constructed.

We compute the third post-Newtonian (3PN) accurate instantaneous contributions to the radiated gravitational wave energy and angular momentum arising from the hyperbolic passages of nonspinning compact objects. The present computations employ 3PN-accurate instantaneous contributions to the far-zone energy and angular momentum fluxes and the 3PN-accurate Keplerian type parametric solution for compact binaries in hyperbolic orbits.

We present an inequality between two types of distance measures from an observer to a single light source in general relativity. It states that for a given source and observer the distance measured by the trigonometric parallax is never shorter than the angular diameter distance provided that the null energy condition holds and that there are no focal points in between. This result is independent of the details of the spacetime geometry or the motions of the observer and the source. The proof is based on the geodesic bilocal operator formalism and on well-known properties of infinitesimal light ray bundles. Observation of the violation of the distance inequality would mean that on large scales either the null energy condition does not hold or that light does not travel along null geodesics.

As a photon propagates along a null geodesic, the space-time curvature around the geodesic distorts its wave function. We utilize the Fermi coordinates adapted to a general null geodesic and derive the equation for interaction between the Riemann tensor and the photon wave function. The equation is solved by being mapped to a time-dependent Schrödinger equation in ($2+1$) dimensions. The results show that as a Gaussian time-bin wave packet with a narrow bandwidth travels over a null geodesic, it gains an extra phase that is a function of the Riemann tensor evaluated and integrated over the propagation trajectory. This extra phase is calculated for communication between satellites around the Earth and is shown to be measurable by current technology.

We demonstrate that the location of a stable photon sphere (PS) in a compact object is not always an edge such as the inner boundary of a black hole shadow, whereas the location of an unstable PS is known to be the shadow edge; notably, in the Schwarzschild black hole. If a static spherically symmetric (SSS) spacetime has the stable outermost PS, the spacetime cannot be asymptotically flat. A nondivergent deflection is caused for a photon traveling around a stable PS, though a logarithmic divergent behavior is known to appear in most of SSS compact objects with an unstable photon sphere. The reason for the nondivergence is that the closest approach of a photon is prohibited in the immediate vicinity of the stable PS when the photon is emitted from a source (or reaches a receiver) distant from a lens object. The finite gap size depends on the receiver and source distances from the lens as well as the lens parameters. The mild deflection angle of light can be approximated by an arcsine function. A class of SSS solutions in Weyl gravity exemplify the nondivergent deflection near the stable outer PS.

The Gauss-Bonnet curvature invariant has attracted the attention of physicists and mathematicians over the years. In particular, it has recently been proved that black holes can support external matter configurations that are nonminimally coupled to the Gauss-Bonnet invariant of the curved spacetime. Motivated by this physically interesting behavior of black holes in Einstein-Gauss-Bonnet theories, we present a detailed analytical study of the physical and mathematical properties of the Gauss-Bonnet curvature invariant ${\mathcal{G}}_{\text{Kerr}}(r,\cos \theta ;a/M)$ of spinning Kerr black holes in the spacetime region outside the horizon [here $\{r,\theta \}$ are respectively the radial and polar coordinates of the black-hole spacetime, and $a/M$ is the dimensionless angular momentum of the black hole]. Interestingly, we prove that, for all spinning Kerr spacetimes in the physically allowed regime $a/M\in [0,1]$, the spin-dependent maximum curvature of the Gauss-Bonnet invariant is attained at the equator of the black-hole surface. Intriguingly, we reveal that the location of the global minimum of the Gauss-Bonnet invariant has a highly nontrivial functional dependence on the black-hole rotation parameter: (i) For Kerr black holes in the dimensionless slow-rotation $a/M<(a/M{)}_{\mathrm{crit}}^{-}=1/2$ regime, the Gauss-Bonnet curvature invariant attains its global minimum asymptotically at spatial infinity, (ii) for black holes in the intermediate spin regime $1/2=(a/M{)}_{\mathrm{crit}}^{-}\le a/M\le (a/M{)}_{\mathrm{crit}}^{+}=\sqrt{\{7+\sqrt{7}\cos [3^{-1}\arctan (3\sqrt{3})]-\sqrt{21}\sin [3^{-1}\arctan (3\sqrt{3})]\}/12}$, the global minima are located at the black-hole poles, and (iii) Kerr black holes in the supercritical (rapidly-spinning) regime $a/M>(a/M{)}_{\mathrm{crit}}^{+}$ are characterized by a nontrivial (nonmonotonic) functional behavior of the Gauss-Bonnet curvature invariant ${\mathcal{G}}_{\text{Kerr}}(r=r_{+},\cos \theta ;a/M)$ along the black-hole horizon with a spin-dependent polar angle for the global minimum point.

We have developed and implemented a numerical scheme and code that computes the evolution of the massless scalar field wave equation in Kerr spacetime, using double null coordinates. The results show a smooth behavior across the event horizon, making it possible to give initial data outside the event horizon, evolve the scalar field across the horizon, and keep the evolution even inside the black hole. This is the first time that a double null evolution is performed for a scalar field in Kerr spacetime.

Recent observations suggest that there are violations of the isotropy of the Universe at large scales, which is an important part of the cosmological principle. In this paper, we use the cosmic microwave background (CMB) data to search for spatial variations of the cosmological parameters in the $\mathrm{\Lambda }\mathrm{CDM}$ model. We fit the Planck temperature angular power spectrum ${\mathcal{C}}_{\ell }^{TT}$ for 48 different half-skies, centering on 48 different directions, to search for directional dependences of the standard cosmological parameters. There are $3[2]\sigma$-level directional variations in ${\mathrm{\Omega }}_b{h}^2$, ${\mathrm{\Omega }}_c{h}^2$, $n_s$, $100{\theta }_{\mathrm{MC}}$, and $H_0$ [$\tau$ and $\ln ({10}^{10}{A}_s)$]. Furthermore, the directional distributions of the parameters follow a dipole form to good approximation. The Bayes factor between the isotropic and anisotropic hypotheses is 0.0041, strongly disfavoring the former. The best-fit dipole axes for ${\mathrm{\Omega }}_b{h}^2$, ${\mathrm{\Omega }}_c{h}^2$, $n_s$, $100{\theta }_{\mathrm{MC}}$, and $A_s{e}^{-2\tau }$ all generally align with the mean direction of $\mathit{V}\equiv (l={48.8}_{-14.4}^{+14.3}°,b=-{5.6}_{-17.4}^{+17.0}°)$, which is roughly perpendicular to the dipole of the variation in the fine-structure constant, and is about 45° to the directions of the CMB kinematic dipole, CMB parity asymmetry, and polarization of quasars. Our results suggest either a significant violation of the cosmological principle, or previously unknown systematic errors in the standard CMB analysis.

The runaway collapse phase of a small dark matter cluster inside a white dwarf star encompasses a reversible stage, where heat can be transferred back and forth between nuclear and dark matter. Induced nuclear burning phases are stable and early carbon depletion undermines previous claims of type Ia supernova ignition. Instead, mini black holes are formed at the center of the star that either evaporate or accrete stellar material until a macroscopic sub-Chandrasekhar-mass black hole is formed. In the latter case, a 0.1 to 1 second lasting electromagnetic transient signal can be detected upon ejection of the white dwarf’s potential magnetic field. Binary systems that transmute to black holes and subsequently merge emit gravitational waves. Advanced LIGO should detect one such sub-Chandrasekhar binary black hole inspiral per year, while future Einstein telescope-like facilities will detect thousands per year. The effective spin parameter distribution is peaked at 0.2 and permits future studies to disentangle from primordial sub-Chandrasekhar black holes. Such signatures are compatible with current direct detection constraints, as well as with neutron star constraints in the case of bosonic dark matter, even though they remain in conflict with the fermionic case for part of the parameter space.

Several pulsar-timing array (PTA) collaborations are finding tantalizing hints for a stochastic gravitational wave background signal in the nano-Hertz regime. So far, though, no convincing evidence for the expected Hellings-Downs quadrupolar correlations has been found. While this issue might get fixed at the light of more accurate, forthcoming data, it is important to keep an eye open on different possibilities and explore scenarios able to produce different types of PTA angular correlations. We point out that a stationary non-Gaussian component to the gravitational wave background can modulate the two-point PTA overlap reduction function, adding contributions that can help in fitting the angular distribution of PTA data. We discuss possible sources for such non-Gaussian signal in terms of cosmological processes occurring after inflation ends, and we investigate further tests for this idea.

High-significance measurements of the monopole thermal Sunyaev-Zel’dovich cosmic microwave background spectral distortions have the potential to tightly constrain poorly understood baryonic feedback processes. The sky-averaged Compton-$y$ distortion and its relativistic correction are measures of the total thermal energy in electrons in the observable universe and their mean temperature. We use the CAMELS suite of hydrodynamic simulations to explore possible constraints on parameters describing the subgrid implementation of feedback from active galactic nuclei and supernovae, assuming a PIXIE-like measurement. The small $25h^{-1}\mathrm{Mpc}$ CAMELS boxes present challenges due to significant sample variance. We utilize machine learning to construct interpolators through the noisy simulation data. Using the halo model, we translate the simulation halo mass functions into correction factors to reduce sample variance where required. Our results depend on the subgrid model. In the case of IllustrisTNG, we find that the best-determined parameter combination can be measured to $\simeq 2\%$ and corresponds to a product of active galactic nuclei (AGN) and supernova (SN) feedback. In the case of SIMBA, the tightest constraint is $\simeq 0.2\%$ on a ratio between AGN and SN feedback. A second orthogonal parameter combination can be measured to $\simeq 8\%$. Our results demonstrate the significant constraining power a measurement of the late-time spectral distortion monopoles would have for baryonic feedback models.

Global symmetries are expected to be violated by gravity, which may cause a serious problem to models based on these symmetries. A famous example is the Peccei-Quinn solution to the strong $CP$ problem in QCD axion models; it requires a global U(1) symmetry to be respected with high quality, and even the Planck-scale suppressed operators that violate the U(1) symmetry would spoil this solution. Indeed, it is known that gravitational instantons do break the U(1) Peccei-Quinn symmetry and induce the U(1)-violating operators, bringing back the strong $CP$ problem to the axion models. This conclusion is, however, highly dependent on the structure of the theory around the Planck scale and, therefore, may be evaded if we go beyond the minimal setup. In this paper, we study the effect of a nonminimal coupling of the Peccei-Quinn field to gravity, $\xi$, on the gravitational instanton effect. This setup is frequently considered in cosmology as it can realize a successful inflation if $\xi \lesssim {10}^5$. We find that the U(1)-breaking effect of the gravitational instantons can sufficiently be suppressed for $\xi \gtrsim 2\times {10}^3$, which suggests that the U(1) symmetry may be maintained with high quality even in the presence of gravity. Our result thus points to a new way to avoid the quality problem of global symmetries.

We systematically calculate the radiative corrections of order $\alpha /\pi$ to elastic muon-proton scattering at low momentum transfers. These include vacuum polarization, photon-loop form factors of the muon and the proton, two-photon exchange corrections and soft photon radiation. In particular, we discuss these corrections for the kinematics of the upcoming AMBER experiment with a 100 GeV muon beam. It is found that for the ratio to the Born cross section, only the minor terms from the photon-loop form factors of the proton and two-photon exchange depend on the proton structure as predetermined by the strong interactions. The inclusion of soft photon radiation (below 20 MeV), which as such is mandatory to cancel the infrared divergences arising from virtual photon loops, points to a prominent role of bremsstrahlung among the radiative corrections of order $\alpha /\pi$. Therefore, the calculation of the process ${\mu }^{\mp }p\to {\mu }^{\mp }p\gamma$ must be extended beyond the soft photon approximation and tailored to the specific experimental conditions for a proper analysis of the upcoming AMBER data.

This paper investigates the $e^{-}{e}^{+}\to Z{h}_1$ sensitivity for a Higgs boson’s rare decay into heavy neutrinos $h_1\to NN$ at the proposed electron-positron collider, with the focus on multilepton final states that contain same-sign lepton pairs. $h_1\to NN$ decay can derive from Higgs bosons mixing with new physics scalar(s) in a way that is complementary to the contribution from active-sterile neutrino mixings. We consider the scenario with a singlet scalar that gives the heavy neutrino mass and has a small mixing with the SM Higgs boson. We analyze the semileptonic, fully leptonic, and mixed $NN$ decay scenarios, and we categorize the signal based on the number of leptons in the final state: ${\ell }^{\pm }{\ell }^{\pm }$ with at least three jets, ${\ell }^{\pm }{\ell }^{\pm }\ell$ with at least two jets, and $e^{\pm }{e}^{\pm }{\mu }^{\mp }{\mu }^{\mp }$ with at least one jet, each containing one or two same-sign dilepton system(s). Selection cuts are optimized for the presence of the associated $Z$ boson, which leads to additional backgrounds at the $e^{-}{e}^{+}$ collider. The Standard Model background channels are systematically analyzed. Sensitivity limits on $h_1\to NN$ branching fractions are derived for signals with two to four final leptons, assuming the heavy neutrino masses are between 10 and 60 GeV. With 240 GeV center-of-mass energy and $5.6{\mathrm{ab}}^{-1}$ design luminosity, the $h_1\to NN$ branching fraction can be probed to $2\times {10}^{-4}$ in the $2\ell$ and $3\ell$ channels, and to $6\times {10}^{-4}$ in the $4\ell$ channel at a 95% confidence level. The $3\ell$ and $4\ell$ channels expect one background event or fewer, and their sensitivities saturate the statistic limit at $5.6{\mathrm{ab}}^{-1}$ luminosity. A same-sign trilepton (${\ell }^{\pm }{\ell }^{\pm }{\ell }^{\pm }$) signal in the $3\ell$ channel is also discussed. Our search strategy provides an approach to discovering the singlet scalar and exploring the origin of neutrino masses at future $e^{-}{e}^{+}$ colliders.

We point out unique relations between the decay widths for leptoquarks and Higgs bosons that can be used to test the unification of quarks and leptons at the TeV scale. We discuss the main predictions of the minimal theory for quark-lepton unification and show how the different decays for leptoquarks and Higgses are related by the symmetry of the theory.