The models for the internal structure of the newly found four-quark charmoniumlike resonance $Z_c(3900)$ are discussed: the molecular model as well as the hadro-charmonium and tetraquark schemes. It is argued that it would be possible to resolve between these models by combining measurements of the quantum numbers of the resonance and of its decay rates into yet unseen channels $\pi {\psi }^{\prime }$, $\pi h_c$, $\rho {\eta }_c$ and into pairs of heavy mesons $D^{*}\overline{D}$ and $D{\overline{D}}^{*}$. The models also predict different related four-quark states, which can be sought for in the existing and future data.

We consider general 2D orbifold CFTs of the form $M^N/S_N$, with $M$ a target space manifold and $S_N$ the symmetric group, and generalize the Lunin-Mathur covering space technique in two ways. First, we consider excitations of twist operators by modes of fields that are not twisted by that operator, and show how to account for these excitations when computing correlation functions in the covering space. Second, we consider nontwist sector operators and show how to include the effects of these insertions in the covering space. We work two examples, one using a simple bosonic CFT, and one using the D1-D5 CFT at the orbifold point. We show that the resulting correlators have the correct form for a 2D CFT.

In this paper a model in a space-time with compact extra dimension is presented, describing five-dimensional fermion fields interacting with an electromagnetic field localized on a brane. This model can be considered as a toy model for examining possible consequences of the localization of gauge fields on a brane. It is shown that in the limit of infinite extra dimension, the lowest-order amplitudes of some processes in the resulting four-dimensional effective theory are divergent. Such a “localization catastrophe” can be inherent to more realistic brane world models with infinite extra dimensions.

We compute the greybody factors for nonminimally coupled scalar fields in four-dimensional Schwarzschild–de Sitter spacetime. In particular, we demonstrate that the zero-angular-momentum greybody factor generically tends to zero in the zero-frequency limit like frequency squared if there is nonvanishing coupling to the scalar curvature. This is in contrast with the minimally coupled case, where this greybody factor is known to tend to a finite constant. We also study the Hawking radiation for nonminimally coupled massless scalar fields in Schwarzschild–de Sitter spacetime, formulate a sensible notion of a generalized absorption cross section and investigate its properties.

We explore the collision between two concentric spherical thin shells. The inner shell is charged, whereas the outer one is either neutral or charged. In the situation we consider, the charge of the inner shell is larger than its gravitational mass, and the inside of it is empty and regular. Hence, the domain just outside it is described by the overcharged Reissner-Nordström geometry, whereas the inside of it is Minkowskian. First, the inner shell starts to shrink from infinity with finite kinetic energy, and then the outer shell starts to shrink from infinity with vanishing kinetic energy. The inner shell bounces on the potential wall and collides with the ingoing outer shell. The energy of collision between these shells at “their center-of-mass frame” does not exceed the total energy of the system. By contrast, by virtue of the very large gamma factor of the relative velocity of the shells, the energy of collision between two of the constituent particles of these shells at their center-of-mass frame can be much larger than the Planck scale. This result suggests that the black hole or naked singularity is not necessary for the ultrahigh energy collision of particles.

We consider the static, spherically symmetric black hole solutions in the bigravity theory for a minimal model with a condition $f_{\mu \nu }=C^2{g}_{\mu \nu }$ and evaluate the entropy for black holes. In this condition, we show that there exists the Schwarzschild solution for $C^2=1$, which is a unique consistent solution. We examine how the massive spin-2 field contributes to and affects the Bekenstein-Hawking entropy corresponding to Einstein gravity. In order to obtain the black hole entropy, we use a recently proposed approach, which uses Virasoro algebra and central charge corresponding to the surface term in the gravitational action.

Virial relations are satisfied for equilibria and quasiequilibria of self-gravitating systems in general relativity and Newtonian gravity. First-law relations are also satisfied along sequences of equilibria and quasiequilibria of self-gravitating systems. These relations are useful for analyzing numerical solutions of equilibria and quasiequilibria. We derive these relations in scalar-tensor theories of gravity in an explicit form. The implication of these relations to compact binary evolution is also discussed.

We show that dark matter annihilation around the time of recombination can lead to growing ionization fraction perturbations, that track the linear collapse of matter over-densities. This amplifies small scale cosmological perturbations to the free electron density by a significant amount compared to the usual acoustic oscillations. Electron density perturbations distort the cosmic microwave background (CMB), inducing secondary non-Gaussianity. We calculate the CMB bispectrum from recombination, that is marginally observable by Planck. Even though electron perturbations can be markedly boosted compared with the standard model prediction, the dark matter effect in the CMB bispectrum turns out to be small and will be difficult to disentangle from the standard model in the foreseeable future.

We calculate the production of the gravitational waves from a double inflation model with lattice simulations. Between the two inflationary stages, gravitational waves with a characteristic frequency are produced by fluctuations of the scalar fields enhanced through parametric resonance. The wavelength of the produced gravitational waves gets extra redshift during the second inflationary stage and it can be in the observable range for the direct gravitational wave detectors. It is found that there is a possibility for the produced gravitational waves to be detected in the planned experiments.

We derive consistency relations for correlators of scalar cosmological perturbations that hold in the “squeezed limit” in which one or more of the external momenta become soft. Our results are formulated as relations between suitably defined one-particle-irreducible $N$-point and ($N-1$)-point functions that follow from residual spatial conformal diffeomorphisms of the unitary gauge Lagrangian. As such, some of these relations are exact to all orders in perturbation theory and do not rely on approximate de Sitter invariance or other dynamical assumptions (e.g., properties of the operator product expansion or the behavior of modes at the horizon crossing). The consistency relations apply model-independently to cosmological scenarios in which the time evolution is driven by a single scalar field. Besides reproducing the known results for single-field inflation in the slow-roll limit, we verify that our consistency relations hold more generally, for instance, in ghost condensate models in flat space. We comment on possible extensions of our results to multifield models.

ATLAS and CMS have discovered a standard model (SM) Higgs-like particle. One of the main discovery channels is the Higgs decay to two photons, which, at the moment, seems to be considerably enhanced over the standard model expectation. Models with additional charged matter coupling to the Higgs sector can enhance or decrease the $h\to \gamma \gamma$ branching ratio. We take this as motivation to confront the so-called Georgi-Machacek model of Higgs triplets with the results of recent searches for a SM Higgs boson performed at the LHC. We also investigate the model in regions of the allowed parameter space relevant for a SM-like phenomenology. The Georgi-Machacek model avoids tree-level issues of the $T$ parameter, while offering a vastly modified Higgs phenomenology compared to the standard model. This comes at the price of introducing another fine-tuning problem related to electroweak precision measurements. We investigate the collider phenomenology of the Georgi-Machacek model in the light of existing collider constraints beyond any effective field theory approximation and contextualize our findings with electroweak precision constraints.

We investigate the charmed-baryon mass spectrum using the relativistic heavy-quark action on $2+1$ flavor PACS-CS configurations previously generated on a ${32}^3\times 64$ lattice. The dynamical up-down- and strange-quark masses are tuned to their physical values, reweighted from those employed in the configuration generation. At the physical point, the inverse lattice spacing determined from the $\Omega$ baryon mass gives $a^{-1}=2.194(10)\mathrm{GeV}$, and thus the spatial extent equals $L=32a=2.88(1)\mathrm{fm}$. Our results for the charmed-baryon masses are consistent with experimental values, except for the mass of ${\Xi }_{cc}$, which has been measured by only one experimental group so far and has not been confirmed yet by others. In addition, we report values of other doubly and triply charmed baryon masses, which have never been measured experimentally.

In the first-order formalism of gravitational theories, the spacetime connection is considered as an independent variable to vary together with the metric. However, the metric still generates its Levi-Civita connection that turns out to determine the geodesics of matter. Recently, “hybrid” gravity theories have been introduced by constructing actions involving both the independent Palatini connection and the metric Levi-Civita connection. In this study a method is developed to analyze the field content of such theories, in particular to determine whether the propagating degrees of freedom are ghosts or tachyons. New types of second-, fourth- and sixth-order derivative gravity theories are investigated and the so-called $f(X)$ theories are singled out as a viable class of “hybrid” extensions of general relativity. These are the theories in which the corrections to Einstein’s theory are written in terms of the deviations from the usual trace equation $X=R-{\kappa }^{2}T$.

In the near future, gravitational wave detection is set to become an important observational tool for astrophysics. It will provide us with an excellent means to distinguish different gravitational theories. In the effective form, many gravitational theories can be cast into an $f(R)$ theory. In this article, we study the dynamics and gravitational waveform of an equal-mass binary black hole system in $f(R)$ theory. We reduce the equations of motion in $f(R)$ theory to the Einstein-Klein-Gordon coupled equations. In this form, it is straightforward to modify our existing numerical relativistic codes to simulate binary black hole mergers in $f(R)$ theory. We consider a scalar field with the shape of a spherical shell containing binary black holes scalar field. We solve the initial data numerically using the Olliptic code. The evolution part is calculated using the extended AMSS-NCKU code. Both codes were updated and tested to solve the problem of binary black holes in $f(R)$ theory. Our results show that the binary black hole dynamics in $f(R)$ theory is more complex than in general relativity. In particular, the trajectory and merger time are strongly affected. Via the gravitational wave, it is possible to constrain the quadratic part parameter of $f(R)$ theory in the range $|a_2|<{10}^{11}{\mathrm{m}}^2$. In principle, a gravitational wave detector can distinguish between a merger of a binary black hole in $f(R)$ theory and the respective merger in general relativity. Moreover, it is possible to use gravitational wave detection to distinguish between $f(R)$ theory and a non–self-interacting scalar field model in general relativity.

We study the influence of astrophysical formation scenarios on the precessional dynamics of spinning black-hole binaries by the time they enter the observational window of second- and third-generation gravitational-wave detectors, such as Advanced LIGO/Virgo, LIGO-India, KAGRA, and the Einstein Telescope. Under the plausible assumption that tidal interactions are efficient at aligning the spins of few-solar mass black-hole progenitors with the orbital angular momentum, we find that black-hole spins should be expected to preferentially lie in a plane when they become detectable by gravitational-wave interferometers. This “resonant plane” is identified by the conditions $\Delta \Phi =0°$ or $\Delta \Phi =\pm 180°$, where $\Delta \Phi$ is the angle between the components of the black-hole spins in the plane orthogonal to the orbital angular momentum. If the angles $\Delta \Phi$ can be accurately measured for a large sample of gravitational-wave detections, their distribution will constrain models of compact binary formation. In particular, it will tell us whether tidal interactions are efficient and whether a mechanism such as mass transfer, stellar winds, or supernovae can induce a mass-ratio reversal (so that the heavier black hole is produced by the initially lighter stellar progenitor). Therefore, our model offers a concrete observational link between gravitational-wave measurements and astrophysics. We also hope that it will stimulate further studies of precessional dynamics, gravitational-wave template placement, and parameter estimation for binaries locked in the resonant plane.

We study the restoration of spontaneously broken symmetry at nonzero temperature in the framework of the $O(2)$ model using polar coordinates. We apply the Cornwall-Jackiw-Tomboulis formalism to calculate the masses and the condensate in the double-bubble approximation, both with and without a term that explicitly breaks the $O(2)$ symmetry. We find that, in the case with explicitly broken symmetry, the mass of the angular degree of freedom becomes tachyonic above a temperature of about 300 MeV. Taking the term that explicitly breaks the symmetry to be infinitesimally small, we find that the Goldstone theorem is respected below the critical temperature. However, this limit cannot be performed for temperatures above the phase transition. We find that, no matter whether we break the symmetry explicitly or not, there is no region of temperature in which the radial and the angular degree of freedom become degenerate in mass. These results hold also when the mass of the radial mode is sent to infinity.

Searches for dark matter production at particle colliders are complementary to direct-detection and indirect-detection experiments and especially powerful for small masses, $m_{\chi }<100\mathrm{GeV}$. An important collider dark matter signature is due to the production of a pair of these invisible particles with the initial-state radiation of a standard model particle. Currently, collider searches use individual and nearly orthogonal final states to search for initial-state jets, photons or massive gauge bosons. We combine these results across final states and across experiments to give the strongest current collider-based limits in the context of effective field theories and map these to limits on dark matter interactions with nuclei and to dark matter self-annhiliation.

A state-of-the-art analysis of the pion-photon transition form factor is presented based on an improved theoretical calculation that includes the effect of a finite virtuality of the quasireal photon in the method of light-cone sum rules. We carry out a detailed statistical analysis of the existing experimental data using this method and by employing pion distribution amplitudes with up to three Gegenbauer coefficients $a_2$, $a_4$, $a_6$. Allowing for an error range in the coefficient $a_6\approx 0$, the theoretical predictions for ${\gamma }^{*}\gamma \to {\pi }^0$ obtained with nonlocal QCD sum rules are found to be in good agreement with all data that support a scaling behavior of the transition form factor at higher $Q^2$, like those of the Belle Collaboration. The data on ${\gamma }^{*}\gamma \to \eta /{\eta }^{\prime }$ from CLEO and BABAR are also reproduced, while there is a strong conflict with the auxetic trend of the BABAR data above $10{\mathrm{GeV}}^2$. The broader implications of these findings are discussed.

We provide a simple method for writing the Dirac-Born-Infeld equations of a Dp-brane in an arbitrary static background whose metric depends only on the holographic radial coordinate $z$. Using this method we revisit the Karch-O’Bannon procedure to calculate the dc conductivity in the presence of constant electric and magnetic fields for backgrounds where the boundary is four- or three-dimensional and satisfies homogeneity and isotropy. We find a frame-independent expression for the dc conductivity tensor. For particular backgrounds we recover previous results on holographic metals and strange metals.