We find that the recently proposed ghost-free interaction of a two-form gauge field in four dimensions, which contains derivative couplings in a nonperturbative manner, can be regarded as a resummation of ghostly interaction terms. We investigate the higher derivative structure of this model in a minisuperspace description and demonstrate that the higher derivative terms can be removed by taking appropriate combinations of the Euler-Lagrange equations, while a truncation at a finite order spoils this structure. We also show that this nature is peculiar to four dimensions.

We study the quasinormal mode and stability of optical black holes in moving dielectrics. The results show that the real part of complex frequencies is inversely proportional to the refractive index, but the absolute value of the imaginary part is proportional to the refractive index. We obtain the conditions needed for forming a black hole horizon in moving dielectrics. Moreover, we investigate the evolution behavior of optical black holes with respect to a refractive index at a high overtone number and find that an optical black hole undergoes one phase transition from an unstable mode to a stable one when the refractive index is big enough.

We study memory effects as information backflow for an accelerating two-level detector weakly interacting with a scalar field in the Minkowski vacuum. This is the framework of the well-known Unruh effect: the detector behaves as if it were in a thermal bath with a temperature proportional to its acceleration. Here we show that if we relax the usual assumption of an eternally uniformly accelerating system, and we instead consider the more realistic case in which a finite-size detector starts accelerating at a certain time, information backflow may appear in the dynamics. Our results demonstrate the existence of a connection between the trajectory of the detector in Minkowski space and the behavior of information flow. This allows us to inspect the Unruh effect under a new light, making use of the latest developments in quantum information theory and open quantum systems.

We reconsider and extend the cosmological predictions that can be made under the assumption that the total action of the Universe is finite. When initial and final singularities in curvature invariants are avoided, it leads to singularities in the gravitational action of the Universe. The following properties are required of a universe with finite action: Compact spatial sections (i.e., a closed universe) giving a finite total lifetime for the Universe. Compactification of flat and open universes is excluded. The Universe can contain perfect fluids with $-1<p/\rho <2$ on approach to singularities. The Universe cannot display a bounce or indefinite cyclic behavior to the past or the future. Here, we establish new consequences of imposing finite action: the Universe cannot be dominated by massless scalar fields or the kinetic energy of self-interacting scalar fields or a $p=\rho$ perfect fluid on approach to the initial or final singularity. The ekpyrotic scenario with an effective fluid obeying $p/\rho >2$ in a closed, flat or open universe is excluded. Any bouncing loop quantum gravity model with indefinite past or future evolution is ruled out. The Einstein static and steady-state universes are ruled out along with past or future eternal inflating universes anisotropies of Kasner or Mixmaster type cannot dominate the dynamics on approach to singularities. This excludes density inhomogeneity spectra versus mass, of the form $\delta \rho /\rho \propto M^{-q}$, with $q>2/3$. Higher-order Lagrangian theories of gravity are significantly constrained. Quadratic Lagrangians are excluded with fluids satisfying $p/\rho >-1/3$. Lagrangians with $L_g=R^{1+\delta }$ have infinite actions on approach to a singularity when $2\delta (1-3\gamma )+2-3\gamma <0$, where $p=(\gamma -1)\rho$ for the fluid. As shown by Barrow and Tipler, the Gauss-Bonnet quadratic combination causes a cosmological action singularity even though it does not contribute terms to the field equations. Scalar-tensor theories like Brans-Dicke dominated by the scalar field on approach to singularities have action singularities. Dark energy cannot be a simple cosmological constant, as it would create an action singularity to the future: the Universe cannot be asymptotically de Sitter as $t\to \infty$. The dark energy must be an evolving energy density in a closed universe that produces collapse to a future singularity and cannot be dominated by the kinetic energy of the scalar field.

The recently proposed trans-Planckian censorship conjecture (TCC) seems to require that the energy scale of inflation is significantly lower than the Planck scale $(H_{\inf }<{10}^{-20}{M}_{\mathrm{Pl}})$. This, in turn, implies that the tensor-to-scalar ratio for inflation is negligibly small, independent of assumptions of slow-roll or even of having a single scalar field, thus ruling out inflation if primordial tensor modes are ever observed. After demonstrating the robustness and generality of these bounds, we show that having an excited initial state for cosmological perturbations seems to be a way out of this problem for models of inflation.

We study gauge (in)dependence of the gravitational waves (GWs) induced from curvature perturbations. For the GWs produced in a radiation-dominated era, we find that the observable (late-time) GWs in the transverse-traceless (synchronous) gauge and in the Newtonian gauge are the same in contrast to a claim in the literature. We also mention the interpretation of the gauge dependence of the tensor perturbations which appears in the context of the induced GWs.

Gravitational waves searches for compact binary mergers with LIGO and Virgo are presently a two stage process. First, a gravitational wave signal is identified. Then, an exhaustive search over possible signal parameters is performed. It is critical that the identification stage is efficient in order to maximize the number of gravitational wave sources that are identified. Initial identification of gravitational wave signals with LIGO and Virgo happens in real-time which requires that less than one second of computational time must be used for each one second of gravitational wave data collected. In contrast, subsequent parameter estimation may require hundreds of hours of computational time to analyze the same one second of gravitational wave data. The real-time identification requirement necessitates efficient and often approximate methods for signal analysis. We describe one piece of real-time gravitational-wave identification: an efficient method for ascertaining a signal’s consistency between multiple gravitational wave detectors suitable for real-time gravitational wave searches for compact binary mergers. This technique was used in analyses of Advanced LIGO’s second observing run and Advanced Virgo’s first observing run.

There are eight possible Pin groups that can be used to describe the transformation behavior of fermions under parity and time reversal. We show that only two of these are compatible with general relativity, in the sense that the configuration space of fermions coupled to gravity transforms appropriately under the space-time diffeomorphism group.

We present and study a new mechanism of interaction between solitons based on the exchange interaction mediated by the localized fermion states. As particular examples, we consider solutions of simple ($1+1$)-dimensional scalar field theories with self-interaction potentials, including the sine-Gordon model and the polynomial ${\varphi }^4$ and ${\varphi }^6$ models, coupled to the Dirac fermions with a backreaction. We discover that there is an additional fermion exchange interaction between the solitons, and it leads to the formation of static multisoliton bound states. Furthermore, we argue that similar mechanisms of formation of stable coupled multisoliton configurations can be observed for a wide class of physical systems.

In the limit of extremely intense electromagnetic fields the Maxwell equations are modified due to the photon-photon scattering that makes the vacuum refraction index depend on the field amplitude. In the presence of electromagnetic waves with small but finite wave numbers the vacuum behaves as a dispersive medium. We show that the interplay between the vacuum polarization and the nonlinear effects in the interaction of counter-propagating electromagnetic waves can result in the formation of Kadomtsev-Petviashvily solitons and, in one-dimension configuration, of Korteveg-de-Vries type solitons that can propagate over a large distance without changing their shape.

Neutrinos may acquire small Dirac or Majorana masses by new low-energy physics in terms of the chiral gravitational anomaly, as proposed by Dvali and Funcke (2016). This model predicts fast neutrino decays, ${\nu }_i\to {\nu }_j+\varphi$ and ${\nu }_i\to {\overline{\nu }}_j+\varphi$, where the gravi-Majorons $\varphi$ are pseudoscalar Nambu-Goldstone bosons. The final-state neutrino and antineutrino distributions differ depending on the Dirac or Majorana mass of the initial state. This opens a channel for distinguishing these cases, for example in the spectrum of high-energy astrophysical neutrinos. In particular, we put bounds on the neutrino lifetimes in the Majorana case, ${\tau }_2/m_2>1.1\times {10}^{-3}(6.7\times {10}^{-4})\mathrm{s}/\mathrm{eV}$ and ${\tau }_3/m_3>2.2\times {10}^{-5}(1.3\times {10}^{-4})\mathrm{s}/\mathrm{eV}$ at 90% CL for hierarchical (degenerate) masses, using data from experiments searching for antineutrino appearance from the Sun.

No process without a standard model (SM) background has been observed so far. As a tool in the study of a class of models containing doubly charged vector bileptons, we propose one such process that, violating lepton flavor number conservation, has no contribution from the SM: $pp\to {\mu }^{+}{\mu }^{+}{\mu }^{-}{e}^{-}$. By carefully isolating the parameters to keep them free, we are able to acquire a notion of how a possible PMNS-like matrix present in the relevant charged current parametrization could affect the observables. We find that an interesting section of the space of parameters can be explored at the current LHC in the specified conditions.

The scalar potential of the $N$-Higgs-doublet model (NHDM) is best analyzed not in the space of $N$ complex doublets ${\varphi }_a$ but in the $N^2$-dimensional space of real-valued bilinears constructed of ${\varphi }_a^{†}{\varphi }_b$. In particular, many insights have been gained into $CP$ violation in the 2HDM and 3HDM by studying how generalized $CP$ transformations (GCPs) act in this bilinear space. These insights relied on the fact that GCPs, which involved an odd number of mirror reflection, could be clearly distinguished from Higgs family transformations by the sign of the determinant of the transformation matrix. It was recently pointed out that this criterion fails starting from 4HDM, where the reflection/rotation dichotomy does not exist anymore. In this paper, we restore intuition by finding a different quantity which faithfully discriminates between GCPs and Higgs family transformations in the bilinear space for any number of Higgs doublets. We also establish the necessary and sufficient conditions for an orthogonal transformation in the bilinear space to represent a viable transformation back in the space of $N$ doublets, which is helpful if one prefers to build an NHDM directly in the bilinear space.

There are strong similarities between charge-parity ($CP$) violating observables in hadronic $B$ decays (in particular $\mathrm{\Delta }{A}_{CP}^{-}$ in $B\to K\pi$) and direct $CP$ violation in kaon decays (${\epsilon }^{\prime }$): All these observables are very sensitive to new physics (NP) which is at the same time $CP$ and isospin violating (i.e., NP with complex couplings which are different for up quarks and down quarks). Intriguingly, both the measurements of ${\epsilon }^{\prime }$ and $\mathrm{\Delta }{A}_{CP}^{-}$ show deviations from their Standard Model predictions, calling for a common explanation (the latter is known as the $B\to K\pi$ puzzle). For addressing this point, we parametrize NP using a gauge invariant effective field theory approach combined with a global $U(2{)}^3$ flavor symmetry in the quark sector (also known as less-minimal flavor violation). We first determine the operators which can provide a common explanation of ${\epsilon }^{\prime }$ and $\mathrm{\Delta }{A}_{CP}^{-}$ and then perform a global fit of their Wilson coefficients to the data from hadronic $B$ decays. Here we also include e.g., the recently measured $CP$ asymmetry in $B_s\to KK$ as well as the purely isospin violating decay $B_s\to \varphi {\rho }^0$, finding a consistent NP pattern providing a very good fit to data. Furthermore, we can at the same time explain ${\epsilon }^{\prime }/\epsilon$ for natural values of the free parameters within our $U(2{)}^3$ flavor approach, and this symmetry gives interesting predictions for hadronic decays involving $b\to d$ transitions.

Extensions of the Standard Model (SM) gauge group with a new $U(1{)}_X$ predict an additional gauge boson. Through kinetic mixing with the SM photons featured by a coupling $\epsilon$, the ensuing so-called dark photons ${\gamma }^{\prime }$, which acquire mass as a result of the breaking of the gauge group $U(1{)}_X$, can interact with the SM field content. These massive dark photons can therefore decay to pairs of leptons, hadrons, or quarks, depending on their mass $m_{{\gamma }^{\prime }}$. In this work, we discuss searches for dark photons in the mass range around and below 1 GeV at the LHeC and FCC-he colliders. The signal is given by the displaced decays of the long-lived dark photon into two charged particles. We discuss the impact of conceivable irreducible (SM and machine-related) backgrounds and different signal efficiencies. Our estimates show that the LHeC and FCC-he can test a domain that is complementary to other present and planned experiments.

In models with extended scalars and $CP$ violation, resonance searches in double Higgs final states stand in competition with related searches in top quark final states as optimal channels for the discovery of beyond the Standard Model (BSM) physics. This complementarity is particularly relevant for benchmark scenarios that aim to highlight multi-Higgs production as a standard candle for the study of BSM phenomena. In this paper, we compare interference effects in $t\overline{t}$ final states with correlated phenomena in double Higgs production in the complex singlet and the complex two-Higgs-doublet models. Our results indicate that the BSM discovery potential in di-Higgs searches can be underestimated in comparison to $t\overline{t}$ resonance searches. Top pair final states are typically suppressed due to destructive signal-background interference, while $hh$ final states can be enhanced due to signal-signal interference. For parameter choices where the two heavy Higgs resonances are well separated in mass, top final states are suppressed relative to the naive signal expectation, while estimates of the production cross section times branching ratio remain accurate at the $\mathcal{O}(10\%)$ level for double Higgs final states.

We study the possibility of observing lepton number violation in the right-handed sneutrino sector of the next-to-minimal supersymmetric Standard Model extended with right-handed neutrinos. The scalar potential introduces a lepton number violating mass term for the right-handed sneutrinos, which generates a phase difference that results in oscillations between the sneutrino and antisneutrino. If we have light Higgsinos and right-handed sneutrinos, the sneutrino decay width is determined by the tiny Yukawa couplings, which allows the phase difference to accumulate before the sneutrino decays. We investigate the possibilities of producing sneutrino pairs resonantly through a heavy Higgs of such a model and the ability of seeing a lepton number violating signature emerging from sneutrinos at the Large Hadron Collider. We also discuss how a possible future signal of this type could be used to determine the neutrino Yukawa couplings.

We study 2-to-3 meson-meson scattering based on the process that a gluon is created from a constituent quark or antiquark and subsequently the gluon creates a quark-antiquark pair. The transition potential for the process is derived in QCD. Eight Feynman diagrams at tree level are involved in the 2-to-3 meson-meson scattering. Starting from the $S$-matrix element, we derive the unpolarized cross section from the eight transition amplitudes corresponding to the eight Feynman diagrams. The transition amplitudes contain color, spin, and flavor matrix elements. The 2-to-3 meson-meson scattering includes $\pi \pi \to \pi K\overline{K}$, $\pi K\to \pi \pi K$, $\pi K\to KK\overline{K}$, $KK\to \pi KK$, and $K\overline{K}\to \pi K\overline{K}$. Cross sections for the reactions are calculated. The cross sections depend on temperature obviously, and the cross section for $\pi K\to \pi \pi K$ for total isospin $I=3/2$ at zero temperature is compared to experimental data. By comparison with inelastic 2-to-2 meson-meson scattering, we find that 2-to-3 meson-meson scattering may be as important as inelastic 2-to-2 meson-meson scattering.

In this paper we propose a solution to the long standing problem present in the color glass condensate/saturation approach: i.e., the powerlike fall off of the scattering amplitudes at large $b$. We propose a new nonlinear equation, which takes into account random walks both in transverse momenta of the produced gluons, and in their impact parameter. We demonstrate, that this equation is in accord with previous attempts to include diffusion in impact parameters in the Balitsky-Fadin-Kuraev-Lipatov evolution equation. We show in the paper, that the solution to a new equation results in the exponential decrease of the scattering amplitude at large impact parameter, and in the restoration of the Froissart theorem.

Nowadays, it is generally accepted that the $DK$ interaction in isospin zero is strongly attractive, and the $D_{s0}^{*}(2317)$ can be described as a $DK$ molecular state. Recent studies have shown that the three-body $DDK$ system binds as well with a binding energy about 60–70 MeV. The $DDK$ bound state has an isospin of $1/2$ and a spin-parity of $0^{-}$. If discovered either experimentally or in lattice QCD, it will not only provide further support for the molecular nature of the $D_{s0}^{*}(2317)$, but also provide a way to understand other exotic hadrons expected to be of a molecular nature. In the present work, we study its two-body strong decay widths via triangle diagrams. We find that the partial decay width into $D{D}_s\pi$ is at the order of 2–3 MeV, which seems to be within the reach of the current experiments such as Belle II. As a result, we strongly recommend that this decay channel of the $DDK$ bound state is searched for experimentally.