The quartic self-coupling of the Standard Model Higgs boson can only be measured by observing the triple-Higgs production process, but it is challenging for the LHC Run 2 or International Linear Collider (ILC) at a few TeV because of its extremely small production rate. In this paper, we present a detailed Monte Carlo simulation study of the triple-Higgs production through gluon fusion at a 100 TeV hadron collider and explore the feasibility of observing this production mode. We focus on the decay channel $HHH\to b\overline{b}b\overline{b}\gamma \gamma$, investigating detector effects and optimizing the kinematic cuts to discriminate the signal from the backgrounds. Our study shows that, in order to observe the Standard Model triple-Higgs signal, the integrated luminosity of a 100 TeV hadron collider should be greater than $1.8\times {10}^4{\mathrm{ab}}^{-1}$. We also explore the dependence of the cross section upon the trilinear (${\lambda }_3$) and quartic (${\lambda }_4$) self-couplings of the Higgs. We find that, through a search in the triple-Higgs production, the parameters ${\lambda }_3$ and ${\lambda }_4$ can be restricted to the ranges $[-1,5]$ and $[-20,30]$, respectively. We also examine how new physics can change the production rate of triple-Higgs events. For example, in the singlet extension of the Standard Model, we find that the triple-Higgs production rate can be increased by a factor of $\mathcal{O}(10)$.

We consider the D1-D5 CFT near the orbifold point, specifically the computation of correlators involving twist sector fields using covering surface techniques. As is well known, certain twists introduce spin fields on the cover. Here we consider the bosonization of fermions to facilitate computations involving the spin fields. We find a set of cocycle operators that satisfy constraints coming from various $SU(2)$ symmetries, including the $SU(2{)}_L\times SU(2{)}_R$ R-symmetry. Using these cocycles, we consider the correlator of four spin fields on the cover, and show that it is invariant under all of the $SU(2)$ symmetries of the theory. We consider the mutual locality of operators, and compute several three-point functions. These computations lead us to a notion of radial ordering on the cover that is inherited from the original computation before lifting. Further, we note that summing over orbifold images sets certain branch-cut ambiguous correlators to zero.

We study electroweak interactions in the multiscale theory with $q$-derivatives, a framework where spacetime has the typical features of a multifractal. In the simplest case with only one characteristic time, length, and energy scale $t_{*}$, ${\ell }_{*}$, and $E_{*}$, we consider (i) the muon decay rate and (ii) the Lamb shift in the hydrogen atom, and constrain the corrections to the ordinary results. We obtain the independent absolute upper bounds (i) $t_{*}<10^{-13}\mathrm{s}$ and (ii) $E_{*}>35\mathrm{MeV}$. Under some mild theoretical assumptions, the Lamb shift alone yields the even tighter ranges $t_{*}<10^{-27}\mathrm{s}$, ${\ell }_{*}<10^{-19}\mathrm{m}$, and $E_{*}>450\mathrm{GeV}$. To date, these are the first robust constraints on the scales at which the multifractal features of the geometry can become important in a physical process.

We perform general-relativistic hydrodynamical simulations of dynamical capture binary neutron star mergers, emphasizing the role played by the neutron star spin. Dynamical capture mergers may take place in globular clusters, as well as other dense stellar systems, where most neutron stars have large spins. We find significant variability in the merger outcome as a function of initial neutron star spin. For cases where the spin is aligned with the orbital angular momentum, the additional centrifugal support in the remnant hypermassive neutron star can prevent the prompt collapse to a black hole, while for antialigned cases the decreased total angular momentum can facilitate the collapse to a black hole. We show that even moderate spins can significantly increase the amount of ejected material, including the amount unbound with velocities greater than half the speed of light, leading to brighter electromagnetic signatures associated with kilonovae and interaction of the ejecta with the interstellar medium. Furthermore, we find that the initial neutron star spin can strongly affect the already rich phenomenology in the postmerger gravitational wave signatures that arise from the oscillation modes of the hypermassive neutron star. In several of our simulations, the resulting hypermassive neutron star develops the one-arm ($m=1$) spiral instability, the most pronounced cases being those with small but non-negligible neutron star spins. For long-lived hypermassive neutron stars, the presence of this instability leads to improved prospects for detecting these events through gravitational waves, and thus may give information about the neutron star equation of state.

Corrections to general relativity that introduce long-ranged scalar fields which are nonminimally coupled to curvature typically predict that neutron stars possess a nontrivial scalar field profile anchored to the star. An observer far from a star is most sensitive to the spherically symmetric piece of this profile that decays linearly with the inverse of the distance to the source, the so-called scalar monopole charge, which is related to the emission of dipolar radiation from compact binary systems. The presence of dipolar radiation has the potential to rule out or very strongly constrain extended theories of gravity. These facts may lead people to believe that gravitational theories that introduce long-ranged scalar fields have already been constrained strongly from binary pulsar observations. Here we challenge this “lore” by investigating the decoupling limit of Gauss-Bonnet gravity as an example, in which the scalar field couples linearly to the Gauss-Bonnet density in the action. We prove a theorem that neutron stars in this theory cannot possess a scalar charge, due to the topological nature of the Gauss-Bonnet density. Thus Gauss-Bonnet gravity evades the strong binary pulsar constraints on dipole radiation. We discuss the astrophysical systems which will yield the best constraints on Gauss-Bonnet gravity and related quadratic gravity theories. To achieve this we compute the scalar charge in quadratic gravity theories by performing explicit analytic and numerical matching calculations for slowly rotating neutron stars. In generic quadratic gravity theories, either neutron star–binary or neutron star–black hole systems can be used to constrain the theory, but because of the vanishing charge, Gauss-Bonnet gravity evades the neutron star–binary constraints. However, in contrast to neutron stars, black holes in Gauss-Bonnet gravity do anchor scalar charge, because of the difference in topology. The best constraints on Gauss-Bonnet gravity will thus come from accurate black hole observations, for example through gravitational waves from inspiraling binaries or the timing of pulsar–black hole binaries with radio telescopes. We estimate these constraints to be a factor of 10 better than the current estimated bound, and also include estimated constraints on generic quadratic gravity theories from pulsar timing.

In the adiabatic post-Newtonian (PN) approximation, the phase evolution of gravitational waves (GWs) from inspiraling compact binaries in quasicircular orbits is computed by equating the change in binding energy with the GW flux. This energy balance equation can be solved in different ways, which result in multiple approximants of the PN waveforms. Due to the poor convergence of the PN expansion, these approximants tend to differ from each other during the late inspiral. Which of these approximants should be chosen as templates for detection and parameter estimation of GWs from inspiraling compact binaries is not obvious. In this paper, we present estimates of the effective higher-order (beyond the currently available 4 and 3.5PN) nonspinning terms in the PN expansion of the binding energy and the GW flux that minimize the difference of multiple PN approximants (TaylorT1, TaylorT2, TaylorT4, TaylorF2) with effective one-body waveforms calibrated to numerical relativity (EOBNR). We show that PN approximants constructed using the effective higher-order terms show significantly better agreement (as compared to 3.5PN) with the inspiral part of the EOBNR. For nonspinning binaries with component masses $m_{1,2}\in [1.4M_{\odot },15M_{\odot }]$, most of the approximants have a match (faithfulness) of better than 99% with both EOBNR and each other.

Besides black holes, the phase space of three-dimensional massive gravity about warped anti–de Sitter space contains solutions that decay exponentially in time. They describe evanescent graviton configurations that, while governed by a wave equation with nonvanishing effective mass, do not carry net gravitational energy. Explicit examples of such solutions have been found in the case of topologically massive gravity; here, we generalize them to a much more general ghost-free massive deformation, with the difference being that the decay rate gets corrected due to the presence of higher-order terms.

There is an apparent power deficit relative to the $\mathrm{\Lambda }\mathrm{CDM}$ prediction of the cosmic microwave background spectrum at large scales, which, though not yet statistically significant, persists from WMAP to Planck data. Proposals that invoke some form of initial condition for the inflation have been made to address this apparent power suppression, albeit with conflicting conclusions. By studying the curvature perturbations of a scalar field in the Friedmann-Lemaître-Robertson-Walker universe parameterized by the equation of state parameter $w$, we find that the large-scale spectrum at the end of inflation reflects the superhorizon spectrum of the initial state. The large-scale spectrum is suppressed if the universe begins with the adiabatic vacuum in a superinflation ($w<-1$) or positive-pressure ($w>0$) era. In the latter case, there is however no causal mechanism to establish the initial adiabatic vacuum. On the other hand, as long as the universe begins with the adiabatic vacuum in an era with $-1<w<0$, even if there exists an intermediate positive-pressure era, the large-scale spectrum would be enhanced rather than suppressed. We further calculate the spectrum of a two-stage inflation model with a two-field potential and show that the result agrees with that obtained from the ad hoc single-field analysis.

We construct a $2+1$ dimensional model that sustains superconductivity at all temperatures. This is achieved by introducing a Chern-Simons mixing term between two Abelian gauge fields $\mathcal{A}$ and $\mathcal{Z}$. The superfluid is described by a complex scalar charged under $\mathcal{Z}$, whereas a sufficiently strong magnetic field of $\mathcal{A}$ forces the superconducting condensate to form at all temperatures. In fact, at finite temperature, the theory exhibits Berezinsky-Kosterlitz-Thouless phase transition due to proliferation of topological vortices admitted by our construction. However, the critical temperature is proportional to the magnetic field of $\mathcal{A}$, and thus, the phase transition can be postponed to high temperatures by increasing the strength of the magnetic field.

We study $W$ boson scattering in the same- and opposite-sign channels under the assumption that no resonances are present in the collider processes $pp\to l^{\pm }{\nu }_l{l}^{\pm }{\nu }_{l}jj$ and $pp\to l^{\pm }{\nu }_l{l}^{\mp }{\nu }_{l}jj$, respectively. Basic selection cuts together with a restriction on the combination of the final lepton and jet momenta (the Warsaw cut) make it possible to argue that at the LHC a luminosity of $100{\mathrm{fb}}^{-1}$ and a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$ will allow us to constrain the leading effective Lagrangian coefficients at the permil level. We also discuss limits on the other coefficients of the effective Lagrangian as well as stronger constraints provided by higher energy and luminosity.

We perform a coupled-channel analysis of pseudodata for the $D^{+}\to K^{-}{\pi }^{+}{\pi }^{+}$ Dalitz plot. The pseudodata are generated from the isobar model of the E791 Collaboration, and are reasonably realistic. We demonstrate that it is feasible to analyze the high-quality data within a coupled-channel framework that describes the final state interaction of $D^{+}\to K^{-}{\pi }^{+}{\pi }^{+}$ as multiple rescatterings of three pseudoscalar mesons through two-pseudoscalar-meson interactions in accordance with the two-body and three-body unitarity. The two-pseudoscalar-meson interactions are designed to reproduce empirical $\pi \pi$ and $\pi \overline{K}$ scattering amplitudes. Furthermore, we also include mechanisms that are beyond simple iterations of the two-body interactions, i.e., a three-meson force, derived from the hidden local symmetry model. A picture of hadronic dynamics in $D^{+}\to K^{-}{\pi }^{+}{\pi }^{+}$ described by our coupled-channel model is found to be quite different from those of the previous isobar-type analyses. For example, we find that the $D^{+}\to K^{-}{\pi }^{+}{\pi }^{+}$ decay width can get almost triplicated when the rescattering mechanisms are turned on. Among the rescattering mechanisms, those associated with the $\rho (770){\overline{K}}^0$ channel, which contribute to $D^{+}\to K^{-}{\pi }^{+}{\pi }^{+}$ only through a channel coupling, give a large contribution and significantly improve the quality of the fits. The $K^{-}{\pi }^{+}$ $s$-wave amplitude from our analysis is reasonably consistent with those extracted from the E791 model independent partial-wave analysis; the hadronic rescattering and the coupling to the $\rho (770){\overline{K}}^0$ channel play a major role here. We also find that the dressed $D^{+}$ decay vertices have phases, induced by the strong rescatterings, that strongly depend on the momenta of the final pseudoscalar mesons. Although the conventional isobar-type analyses have assumed the phases to be constant, this common assumption is not supported by our more microscopic viewpoint.

A possible interplay between the two terms of the general type-II seesaw formula is exercised which leads to the generation of nonzero ${\theta }_{13}$. The specific flavor structure of the model, guided by the $A_4\times Z_4\times Z_3$ symmetry and accompanied with the Standard Model singlet flavons, yields the conventional seesaw contribution to produce the tribimaximal lepton mixing which is further corrected by the presence of the $SU(2{)}_L$ triplet contribution to accommodate ${\theta }_{13}$. We consider the $CP$ symmetry to be spontaneously broken by the complex vacuum expectation value (vev) of a singlet field $S$. While the magnitude of its complex vev is responsible for generating ${\theta }_{13}$, its phase part induces the low energy $CP$ violating phase ($\delta$) and the $CP$ violation required for leptogenesis. Hence the triplet contribution, although subdominant, plays a crucial role in providing a common source for nonzero ${\theta }_{13}$, $\delta$ and $CP$-violation required for leptogenesis. We find that the recent hint for $\delta$ close to $3\pi /2$ is somewhat favored in this setup though it excludes the exact equality with $3\pi /2$. We also discuss the generation of lepton asymmetry in this scenario.

We study the electron as a system composed of an electron and a photon, using lowest-order perturbation theory. We derive the leading-twist transverse-momentum-dependent distribution functions for both the electron and photon in the dressed electron, thereby offering a three-dimensional description of the dressed electron in momentum space. To obtain the distribution functions, we apply both the formalism of the light-front wave function overlap representation and the diagrammatic approach. We perform the calculations both in light-cone gauge and Feynman gauge, and we present a detailed discussion of the role of the Wilson lines to obtain gauge-independent results. We provide numerical results and plots for many of the computed distributions.

A search is performed for the production of heavy resonances decaying into top-antitop quark pairs in proton-proton collisions at $\sqrt{s}=8\mathrm{TeV}$. Data used for the analyses were collected with the CMS detector and correspond to an integrated luminosity of $19.7{\mathrm{fb}}^{-1}$. The search is performed using events with three different final states, defined by the number of leptons (electrons and muons) from the $t\overline{t}\to WbWb$ decay. The analyses are optimized for reconstruction of top quarks with high Lorentz boosts, where jet substructure techniques are used to enhance the sensitivity. Results are presented for all channels and a combination is performed. No significant excess of events relative to the expected yield from standard model processes is observed. Upper limits on the production cross section of heavy resonances decaying to $t\overline{t}$ are calculated. A narrow leptophobic topcolor $Z^{\prime }$ resonance with a mass below 2.4 TeV is excluded at 95% confidence level. Limits are also derived for a broad $Z^{\prime }$ resonance with a 10% width relative to the resonance mass, and a Kaluza-Klein excitation of the gluon in the Randall-Sundrum model. These are the most stringent limits to date on heavy resonances decaying into top-antitop quark pairs.

We study the action of the Burg-Metzner-Sachs (BMS) group in critical, bosonic string theory living on a target space of the form ${\mathbb{M}}^d\times C$. Here $M^d$ is $d$-dimensional (asymptotically) flat spacetime and $C$ is an arbitrary compactification. We provide a treatment of generalized Ward-Takahashi identities and derive consistent boundary conditions for any $d$ from string theory considerations. Finally, we derive BMS transformations in higher-dimensional spacetimes and show that the generalized Ward-Takahashi identity of BMS produces Weinberg’s soft theorem in string theory.

A holographic realization for ferromagnetic systems is constructed. Owing to the holographic dictionary proposed on the basis of this realization, we obtain relevant thermodynamic quantities such as magnetization, magnetic susceptibility, and free energy. This holographic model reproduces the behavior of the mean field theory near the critical temperature. At low temperatures, the results automatically incorporate the contributions from spin wave excitations and conduction electrons.

With the physical Higgs mass the standard model symmetry restoration phase transition is a smooth cross-over. We study the thermodynamics of the cross-over using numerical lattice Monte Carlo simulations of an effective $\mathrm{SU}(2)\times \mathrm{U}(1)$ $\text{gauge}+\text{Higgs}$ theory, significantly improving on previously published results. We measure the Higgs field expectation value, thermodynamic quantities like pressure, energy density, speed of sound and heat capacity, and screening masses associated with the Higgs and $Z$ fields. While the cross-over is smooth, it is very well defined with a width of only $\sim 5\mathrm{GeV}$. We measure the cross-over temperature from the maximum of the susceptibility of the Higgs condensate, with the result $T_c=159.5\pm 1.5\mathrm{GeV}$. Outside of the narrow cross-over region the perturbative results agree well with nonperturbative ones.

The question of the uniqueness of energy-momentum tensors in the linearized general relativity and in the linear massive gravity is analyzed without using variational techniques. We start from a natural ansatz for the form of the tensor (for example, that it is a linear combination of the terms quadratic in the first derivatives), and require it to be conserved as a consequence of field equations. In the case of the linear gravity in a general gauge we find a four-parametric system of conserved second-rank tensors which contains a unique symmetric tensor. This turns out to be the linearized Landau-Lifshitz pseudotensor employed often in full general relativity. We elucidate the relation of the four-parametric system to the expression proposed recently by Butcher et al. “on physical grounds” in harmonic gauge, and we show that the results coincide in the case of high-frequency waves in vacuum after a suitable averaging. In the massive gravity we show how one can arrive at the expression which coincides with the “generalized linear symmetric Landau-Lifshitz” tensor. However, there exists another uniquely given simpler symmetric tensor which can be obtained by adding the divergence of a suitable superpotential to the canonical energy-momentum tensor following from the Fierz-Pauli action. In contrast to the symmetric tensor derived by the Belinfante procedure which involves the second derivatives of the field variables, this expression contains only the field and its first derivatives. It is simpler than the generalized Landau-Lifshitz tensor but both yield the same total quantities since they differ by the divergence of a superpotential. We also discuss the role of the gauge conditions in the proofs of the uniqueness. In the Appendix, the symbolic tensor manipulation software cadabra is briefly described. It is very effective in obtaining various results which would otherwise require lengthy calculations.

We propose a dark energy model where a scalar field $\varphi$ has nonlinear self-interactions in the presence of a dilatonic coupling with the Ricci scalar. This belongs to a subclass of theories beyond Horndeski, which accommodates covariant Galileons and Brans-Dicke theories as specific cases. We derive conditions under which the scalar sound speed squared $c_{\mathrm{s}}^2$ is positive from the radiation era to today. Since $c_{\mathrm{s}}^2$ remains smaller than the order of 1, the deviation from Horndeski theories does not cause heavy oscillations of gauge-invariant gravitational potentials. In this case, the evolution of matter perturbations at low redshifts is similar to that in the coupled dark energy scenario with an enhanced gravitational interaction. On the spherically symmetric background with a matter source, the existence of field self-interactions suppresses the propagation of the fifth force inside a Vainshtein radius. We estimate an allowed parameter space in which the model can be compatible with Solar System constraints while satisfying conditions for the cosmological viability of background and perturbations.

Solutions of the field equations of theories of gravity which admit distinct conformal frame representations can look very different in these frames. We show that Brans class IV solutions describe wormholes in the Jordan frame (in a certain parameter range) but correspond to horizonless geometries in the Einstein frame. The reasons for such a change of behavior under conformal mappings are elucidated in general, using Brans IV solutions as an example. We end with a brief discussion of the Schwarzschild case and a conformally related metric as a further example of some of the peculiarities of the behavior of horizons under conformal transformations.