We consider all degenerate scalar-tensor theories that depend quadratically on second-order derivatives of a scalar field, which we have identified in a previous work. These theories, whose degeneracy, in general, ensures the absence of Ostrogradsky’s instability, include the quartic Horndeski Lagrangian and its quartic extension beyond Horndeski, as well as other Lagrangians. We study how all these theories transform under general disformal transformations and find that they can be separated into three main classes that are stable under these transformations. This leads to a complete classification modulo disformal transformations. Finally, we show that these higher order theories include mimetic gravity and some particular khronometric theories. They also contain theories that do not correspond, to our knowledge, to already studied theories, even up to disformal transformations.

Despite stringent constraints set by astrophysical observations, there remain viable scalar-tensor theories that could be distinguished from general relativity with gravitational-wave detectors. A promising signal predicted in these alternative theories is dynamical scalarization, which can dramatically affect the evolution of neutron-star binaries near merger. Motivated by the successful treatment of spontaneous scalarization, we develop a formalism that partially resums the post-Newtonian expansion to capture dynamical scalarization in a mathematically consistent manner. We calculate the post-Newtonian order corrections to the equations of motion and scalar mass of a binary system. Through comparison with quasiequilibrium configuration calculations, we verify that this new approximation scheme can accurately predict the onset and magnitude of dynamical scalarization.

Picking out a DBI scalar field as inflation, the slow-rolling inflationary scenario is studied by attributing an exponential time function to scale factor, known as intermediate inflation. The perturbation parameters of the model are estimated numerically for two different cases, and the final result is compared with Planck data. The diagram of tensor-to-scalar ratio $r$ versus scalar spectra index $n_s$ is illustrated, and it is found that they are within an acceptable range as suggested by Planck. In addition, the acquired values for amplitude of scalar perturbation reveal the ability of the model to depict a good picture of the Universe in one of its earliest stages. As a further argument, the non-Gaussianity is investigated, displaying that the model prediction stands in a 68% C.L. regime according to the latest Planck data.

Cosmological constraints on the scalar-tensor theory of gravity by analyzing the angular power spectrum data of the cosmic microwave background (CMB) obtained from the Planck 2015 results are presented. We consider the harmonic attractor model, in which the scalar field has a harmonic potential with curvature ($\beta$) in the Einstein frame and the theory relaxes toward the Einstein gravity with time. Analyzing the $TT$, $EE$, $TE$ and lensing CMB data from Planck by the Markov chain Monte Carlo method, we find that the present-day deviation from the Einstein gravity (${{\alpha }_0}^2$) is constrained as ${{\alpha }_0}^2<2.5\times {10}^{-4-4.5{\beta }^2}$ (95.45% C.L.) and ${{\alpha }_0}^2<6.3\times {10}^{-4-4.5{\beta }^2}$ (99.99% C.L.) for $0<\beta <0.4$. The time variation of the effective gravitational constant between the recombination and the present epochs is constrained as $G_{\mathrm{rec}}/G_0<1.0056$ (95.45% C.L.) and $G_{\mathrm{rec}}/G_0<1.0115$ (99.99% C.L.). We also find that the constraints are little affected by extending to nonflat cosmological models because the diffusion damping effect revealed by Planck breaks the degeneracy of the projection effect.

We show that the mixing effect of the neutral gauge bosons in the $3\text{−}3\text{−}1\text{−}1$ model comes from two sources. The first one is due to the $3\text{−}3\text{−}1\text{−}1$ gauge symmetry breaking as usual, whereas the second one results from the kinetic mixing between the gauge bosons of $U(1{)}_X$ and $U(1{)}_N$ groups, which are used to determine the electric charge and baryon minus lepton numbers, respectively. Such mixings modify the $\rho$-parameter and the known couplings of $Z$ with fermions. The constraints that arise from flavor-changing neutral currents due to the gauge boson mixings and nonuniversal fermion generations are also given.

A large number of new experimental data on $B$ decay into two light pesudoscalar ($P$) mesons have been collected by the LHCb collaboration. Besides confirming information on $B_{u,d}\to PP$ decays obtained earlier by B-factories at KEK and SLAC, new information on $B_s\to PP$ and also more decay modes with $P$ being $\eta$ or ${\eta }^{\prime }$ have been obtained. Using these new data, we perform a global fit for $B\to PP$ to determine decay amplitudes in the framework of $SU(3)/U(3)$ flavor symmetry. We find that $SU(3)$ flavor symmetry can explain data well. The annihilation amplitudes are found to be small as expected. Several $CP$ violating relations predicted by $SU(3)$ flavor symmetry are in good agreement with data. Current available data can give constraints on the amplitudes which induce $P=\eta ,{\eta }^{\prime }$ decays in the framework of $U(3)$ flavor symmetry, and can also determine the $\eta -{\eta }^{\prime }$ mixing angle $\theta$ with $\theta =(-18.4\pm 1.2)°$. Several $B\to PP$ decay modes which have not been measured are predicted with branching ratios accessible at the LHCb. These decays can provide further tests for the framework of $SU(3)/U(3)$ flavor symmetry for $B$ decays.

We study the effect of scalar leptoquarks in the lepton flavor violating $B$ meson decays induced by the flavor-changing transitions $b\to q{l}_i^{+}{l}_j^{-}$ with $q=s$, $d$. In the standard model, these transitions are extremely rare as they are either two-loop suppressed or proceed via box diagrams with tiny neutrino masses in the loop. However, in the leptoquark model, they can occur at tree level and are expected to have significantly large branching ratios. The leptoquark parameter space is constrained using the experimental limits on the branching ratios of $B_q\to l^{+}{l}^{-}$ processes. Using such constrained parameter space, we predict the branching ratios of LFV semileptonic $B$ meson decays, such as $B^{+}\to K^{+}({\pi }^{+})l_i^{+}{l}_j^{-}$, $B^{+}\to (K^{*+},{\rho }^{+})l_i^{+}{l}_j^{-}$, and $B_s\to \varphi l_i^{+}{l}_j^{-}$, which are found to be within the experimental reach of LHCb and the upcoming Belle II experiments. We also investigate the rare leptonic $K_{L,S}\to {\mu }^{+}{\mu }^{-}(e^{+}{e}^{-})$ and $K_L\to {\mu }^{\mp }{e}^{\pm }$ decays in the leptoquark model.

The Laser Interferometer Gravitational Wave Observatory (LIGO) consists of two widely separated 4 km laser interferometers designed to detect gravitational waves from distant astrophysical sources in the frequency range from 10 Hz to 10 kHz. The first observation run of the Advanced LIGO detectors started in September 2015 and ended in January 2016. A strain sensitivity of better than ${10}^{-23}/\sqrt{\mathrm{Hz}}$ was achieved around 100 Hz. Understanding both the fundamental and the technical noise sources was critical for increasing the astrophysical strain sensitivity. The average distance at which coalescing binary black hole systems with individual masses of $30M_{\odot }$ could be detected above a signal-to-noise ratio (SNR) of 8 was 1.3 Gpc, and the range for binary neutron star inspirals was about 75 Mpc. With respect to the initial detectors, the observable volume of the Universe increased by a factor 69 and 43, respectively. These improvements helped Advanced LIGO to detect the gravitational wave signal from the binary black hole coalescence, known as GW150914.

We report a measurement of the forward-backward asymmetry, $A_{\mathrm{FB}}$, in $b\overline{b}$ pairs produced in proton-antiproton collisions and identified by muons from semileptonic $b$-hadron decays. The event sample is collected at a center-of-mass energy of $\sqrt{s}=1.96\mathrm{TeV}$ with the CDF II detector and corresponds to $6.9{\mathrm{fb}}^{-1}$ of integrated luminosity. We obtain an integrated asymmetry of $A_{\mathrm{FB}}(b\overline{b})=(1.2\pm 0.7)\%$ at the particle level for $b$-quark pairs with invariant mass, $m_{b\overline{b}}$, down to $40\mathrm{GeV}/c^2$ and measure the dependence of $A_{\mathrm{FB}}(b\overline{b})$ on $m_{b\overline{b}}$. The results are compatible with expectations from the standard model.

The production of $Z$ bosons with one or two isolated high-energy photons is studied using $pp$ collisions at $\sqrt{s}=8\mathrm{TeV}$. The analyses use a data sample with an integrated luminosity of $20.3{\mathrm{fb}}^{-1}$ collected by the ATLAS detector during the 2012 LHC data taking. The $Z\gamma$ and $Z\gamma \gamma$ production cross sections are measured with leptonic ($e^{+}{e}^{-}$, ${\mu }^{+}{\mu }^{-}$, $\nu \overline{\nu }$) decays of the $Z$ boson, in extended fiducial regions defined in terms of the lepton and photon acceptance. They are then compared to cross-section predictions from the Standard Model, where the sources of the photons are radiation off initial-state quarks and radiative $Z$-boson decay to charged leptons, and from fragmentation of final-state quarks and gluons into photons. The yields of events with photon transverse energy $E_{\mathrm{T}}>250\mathrm{GeV}$ from ${\ell }^{+}{\ell }^{-}\gamma$ events and with $E_{\mathrm{T}}>400\mathrm{GeV}$ from $\nu \overline{\nu }\gamma$ events are used to search for anomalous triple gauge-boson couplings $ZZ\gamma$ and $Z\gamma \gamma$. The yields of events with diphoton invariant mass $m_{\gamma \gamma }>200\mathrm{GeV}$ from ${\ell }^{+}{\ell }^{-}\gamma \gamma$ events and with $m_{\gamma \gamma }>300\mathrm{GeV}$ from $\nu \overline{\nu }\gamma \gamma$ events are used to search for anomalous quartic gauge-boson couplings $ZZ\gamma \gamma$ and $Z\gamma \gamma \gamma$. No deviations from Standard Model predictions are observed and limits are placed on parameters used to describe anomalous triple and quartic gauge-boson couplings.

A direct correspondence of quantum mechanics as a minisuperspace model for a self-interacting scalar quantum-field theory is established by computing, in several models, the infrared contributions to 1-loop effective potentials of Coleman-Weinberg type. A minisuperspace approximation rather than truncation is thereby obtained. By this approximation, the spatial averaging scale of minisuperspace models is identified with an infrared scale (but not a regulator or cutoff) delimiting the modes included in the minisuperspace model. Some versions of the models studied here have discrete space or modifications of the Hamiltonian expected from proposals of loop quantum gravity. They shed light on the question of how minisuperspace models of quantum cosmology can capture features of full quantum gravity. While it is shown that modifications of the Hamiltonian can be well described by minisuperspace truncations, some related phenomena such as signature change, confirmed and clarified here for modified scalar field theories, require at least a perturbative treatment of inhomogeneity beyond a strict minisuperspace model. The new methods suggest a systematic extension of minisuperspace models by a canonical effective formulation of perturbative inhomogeneity.

Screened modified gravity (SMG) is a kind of scalar-tensor theory with screening mechanisms, which can generate a screening effect to suppress the fifth force in high density environments and pass the solar system tests. Meanwhile, the potential of the scalar field in the theories can drive the acceleration of the late Universe. In this paper, we calculate the parametrized post-Newtonian (PPN) parameters $\gamma$ and $\beta$, the effective gravitational constant $G_{\mathrm{eff}}$, and the effective cosmological constant $\mathrm{\Lambda }$ for SMG with a general potential $V$ and coupling function $A$. The dependence of these parameters on the model parameters of SMG and/or the physical properties of the source object are clearly presented. As an application of these results, we focus on three specific theories of SMG (chameleon, symmetron, and dilaton models). Using the formulas to calculate their PPN parameters and cosmological constant, we derive the constraints on the model parameters by combining the observations on solar system and cosmological scales.

We consider nonspherically symmetric perturbations of the uncharged black string/flat black brane in the large dimension (D) limit of general relativity. We express the perturbations in a simplified form using variables introduced by Ishibashi and Kodama. We apply the large D limit to the equations and show that this leads to decoupling of the equations in the near-horizon and asymptotic regions. It also enables use of matched asymptotic expansions to obtain approximate analytical solutions and to analyze stability of the black string/brane. For a large class of nonspherically symmetric perturbations, we prove that there are no instabilities in the large D limit. For the rest, we provide additional matching arguments that indicate that the black string/brane is stable. In the static limit, we show that for all nonspherically symmetric perturbations, there is no instability. This is proof that the Gross-Perry-Yaffe mode for semiclassical black hole perturbations is the unique unstable mode even in the large D limit. This work is also a direct analytical indication that the only instability of the black string is the Gregory-Laflamme instability.

The Kerr solution in coordinates corotating with the horizon is studied as a testbed for a spacetime with a helical Killing vector in the Ernst picture. The solution is numerically constructed by solving the Ernst equation with a spectral method and a Newton iteration. We discuss convergence of the iteration for several initial iterates and different values of the Kerr parameters.

We study inflation driven by a dilaton and an axion, both of which are coupled to a SU(2) gauge field. We find that the inflation driven by the dilaton occurs in the early stage of inflation during which the gauge field grows due to the gauge-kinetic function. When the energy density of magnetic fields catches up with that of electric fields, chromonatural inflation takes over in the late stage of inflation, which we call delayed chromonatural inflation. Thus, the delayed chromonatural inflation driven by the axion and the gauge field is induced by the dilaton. The interesting outcome of the model is the generation of chiral primordial gravitational waves on small scales. Since the gauge field is inert in the early stage of inflation, it is viable in contrast to the conventional chromonatural inflation. We find the parameter region where chiral gravitational waves are generated in a frequency range higher than nHz, which are potentially detectable in future gravitational wave interferometers and pulsar-timing arrays such as DECi-hertz Interferometer Gravitational wave Observatory (DECIGO), evolved Laser Interferometer Space Antenna (eLISA), and Square Kilometer Array (SKA).

We study the stability of cosmic string wakes against the disruption by the dominant Gaussian fluctuations which are present in cosmological models. We find that for a string tension given by $G\mu ={10}^{-7}$ wakes remain locally stable until a redshift of $z=6$, and for a value of $G\mu ={10}^{-14}$ they are stable beyond a redshift of $z=20$. We study a global stability criterion which shows that wakes created by strings at times after $t_{\mathrm{eq}}$ are identifiable up to the present time, independent of the value of $G\mu$. Taking into account our criteria it is possible to develop strategies to search for the distinctive position space signals in cosmological maps which are induced by wakes.

We revisit the problem of perturbations of Schwarzschild-${\mathrm{AdS}}_4$ black holes by using a combination of the Martel-Poisson formalism for perturbations of four-dimensional spherically symmetric spacetimes [K. Martel and E. Poisson, Phys. Rev. D 71, 104003 (2005).] and the Kodama-Ishibashi formalism [H. Kodama and A. Ishibashi, Prog. Theor. Phys. 110, 701 (2003).]. We clarify the relationship between both formalisms and express the Brown-York-Balasubramanian-Krauss boundary stress-energy tensor, ${\overline{T}}_{\mu \nu }$, on a finite-$r$ surface purely in terms of the even and odd master functions. Then, on these surfaces we find that the spacelike components of the conservation equation ${\overline{\mathcal{D}}}^{\mu }{\overline{T}}_{\mu \nu }=0$ are equivalent to the wave equations for the master functions. The renormalized stress-energy tensor at the boundary $\frac{r}{L}\underset{r\to \infty }{\lim }{\overline{T}}_{\mu \nu }$ is calculated directly in terms of the master functions.

One strategy for reducing the online computational cost of matched-filter searches for gravitational waves is to introduce a compressed basis for the waveform template bank in a grid-based search. In this paper, we propose and investigate several tunable compression schemes for a general template bank. Through offline compression, such schemes are shown to yield faster detection and localization of signals, along with moderately improved sensitivity and accuracy over coarsened banks at the same level of computational cost. This is potentially useful for any search involving template banks, and especially in the analysis of data from future space-based detectors such as eLISA, for which online grid searches are difficult due to the long-duration waveforms and large parameter spaces.

A generic prediction of scenarios with extra dimensions accessible in TeV-scale collisions is the existence of Kaluza-Klein excitations of the graviton. For a broad class of strongly warped scenarios one expects to initially find an isolated resonance, whose phenomenology in the simplest cases is described by a simplified model with two parameters, its mass, and a constant $\mathrm{\Lambda }$ with units of mass parametrizing its coupling to the Standard Model stress tensor. These parameters are in turn determined by the geometrical configuration of the warped compactification. We explore the possibility that the 750 GeV excess recently seen in 13 TeV data at ATLAS and CMS could be such a warped Kaluza-Klein graviton, and find a best-fit value $\mathrm{\Lambda }\approx 60\mathrm{TeV}$. We find that while there is some tension between this interpretation and data from 8 TeV and from the dilepton channel at 13 TeV, it is not strongly excluded. However, in the simplest scenarios of this kind, such a signal should soon become apparent in both diphoton and dilepton channels.