We present a privileged Fock quantization of a massive Dirac field in a closed Friedmann–Robertson–Walker cosmology, partially selected by the criteria of invariance of the vacuum under the symmetries of the field equations, and unitary implementation of the dynamics. When quantizing free scalar fields in homogeneous and isotropic spacetimes with compact spatial sections, these criteria have been shown to pick out a unique Fock representation (up to unitary equivalence). Here, we employ the same criteria for fermion fields and explore whether that uniqueness result can be extended to the case of the Fock quantization of fermions. For the massive Dirac field, we start by introducing a specific choice of the complex structure that determines the Fock representation. Such a structure is invariant under the symmetries of the equations of motion. We then prove that the corresponding representation of the canonical anticommutation relations admits a unitary implementation of the dynamics. Moreover, we construct a rather general class of representations that satisfy the above criteria, and we demonstrate that they are all unitarily equivalent to our previous choice. The complex structures in this class are restricted only by certain conditions on their asymptotic behavior for modes in the ultraviolet sector of the Dirac operator. We finally show that, if one assumes that these asymptotic conditions are in fact trivial once our criteria are fulfilled, then the time-dependent scaling in the definition of the fermionic annihilation and creationlike variables is essentially unique.

We construct supersymmetric extensions for different Skyrme-like models in $3+1$ dimensions. BPS equations and BPS bounds are obtained from supersymmetry in some cases. We discuss also the emergence of several Skyrme-like models from supersymmetric Yang-Mills theory and Born-Infeld theory in 5 dimensions.

Shear viscosity of the classical ${\mathrm{\Phi }}^4$ theory is measured using classical microcanonical simulation. To calculate the Kubo formula, we measure the energy-momentum tensor correlation function and apply the Green-Kubo relation. Given that this is a classical theory, the results depend on the cutoff, which should be chosen in the range of the temperature. Comparison with experimentally accessible systems is also performed.

A recent discovery in 2009 by Bañados, Silk and West (BSW), which generated a lot of interest, involves the arbitrary high center-of-mass (c.m.) energies for free particle collisions at the horizon of an extreme kerr black hole when one of the free particles has a critical value of the angular momentum. In light of this we consider the rotating Kerr-like black hole solution in Brans-Dicke theory and study the motion of scalar test charges in the vicinity of the black hole horizon. We show that the interaction of the test scalar charges with the background scalar field in this spacetime suppresses the c.m. energy for collisions occurring near the event horizon, and the value of the c.m. energy there, is finite irrespective of whether the black hole is extreme or not and its value is also independent of the angular momenta of the colliding test charges.

Starting in 2007, a string of papers argue about if the weak cosmic censorship conjecture (WCCC) can be violated by classically forbidden interactions between particles and slightly subextremal black holes, occurring via the quantum nature of the particles; and where backreaction and/or superradiance are pointed out as effects working in the direction of preserving the WCCC. We correct/modify a backreaction argument, point out that transmission/reflection coefficients for relativistic wave equations are not the respective probabilities, and conclude that superradiance does not prevent single particles from being captured by the black hole; even if this capture would lead to WCCC violation. Then we consider the spontaneous emission (which we call the Zel’dovich-Unruh “ZU” effect) of particles by the black hole, and point out that it completely invalidates the mentioned single- or few-particle thought experiments. We find that at least for scalars, the ZU effect can be understood without second quantization; and reevaluate our previous work on scalar fields interacting with black holes in view of this new understanding, finding that it becomes inconclusive.

We show that there is a classical metric satisfying the Einstein equations outside a finite spacetime region where matter collapses into a black hole and then emerges from a white hole. We compute this metric explicitly. We show how quantum theory determines the (long) time for the process to happen. A black hole can thus quantum tunnel into a white hole. For this to happen, quantum gravity should affect the metric also in a small region outside the horizon; we show that, contrary to what is commonly assumed, this is not forbidden by causality or by the semiclassical approximation, because quantum effects can pile up over a long time. This scenario alters radically the discussion on the black hole information puzzle.

We analyze the constraints that causality imposes on some of the particle detector models employed in quantum field theory in general and, in particular, on those used in quantum optics (or superconducting circuits) to model atoms interacting with light. Namely, we show that disallowing faster-than-light communication can impose severe constraints on the applicability of particle detector models in three different common scenarios: (1) when the detectors are spatially smeared, (2) when a UV cutoff is introduced in the theory and (3) under one of the most typical approximations made in quantum optics: the rotating-wave approximation. We identify the scenarios in which the models’ causal behavior can and cannot be cured.

We derive the odd-parity perturbation equation for the nonminimal kinetic coupling sector of the general Horndeski theory, where the kinetic term is coupled to the metric and the Einstein tensor. We derive the potential of the perturbation, by identifying a master function and switching to tortoise coordinates. We then prove the mode stability under linear odd-parity perturbations of hairy black holes in this sector of Horndeski theory, when a cosmological constant term in the action is included. Finally, we comment on the existence of slowly rotating black hole solutions in this setup and discuss their implications on the physics of compact object configurations, such as neutron stars.

We present a model where sterile neutrinos with rest masses in the range $\sim \mathrm{keV}$ to $\sim \mathrm{MeV}$ can be the dark matter and be consistent with all laboratory, cosmological, and large-scale structure, as well as x-ray constraints. These sterile neutrinos are assumed to freeze out of thermal and chemical equilibrium with matter and radiation in the very early Universe, prior to an epoch of prodigious entropy generation (“dilution”) from out-of-equilibrium decay of heavy particles. In this work, we consider heavy, entropy-producing particles in the $\sim \mathrm{TeV}$ to $\sim \mathrm{EeV}$ rest-mass range, possibly associated with new physics at high-energy scales. The process of dilution can give the sterile neutrinos the appropriate relic densities, but it also alters their energy spectra so that they could act like cold dark matter, despite relatively low rest masses as compared to conventional dark matter candidates. Moreover, since the model does not rely on active-sterile mixing for producing the relic density, the mixing angles can be small enough to evade current x-ray or lifetime constraints. Nevertheless, we discuss how future x-ray observations, future lepton number constraints, and future observations and sophisticated simulations of large-scale structure could, in conjunction, provide evidence for this model and/or constrain and probe its parameters.

Using limits on photon flux from dwarf spheroidal galaxies, we place bounds on the parameter space of models in which dark matter annihilates into multiple final state particle pair channels. We derive constraints on effective operator models with dark matter couplings to third generation fermions and to pairs of standard model vector bosons. We present limits in various slices of model parameter space along with estimations of the region of maximal validity of the effective operator approach for indirect detection. We visualize our bounds for models with multiple final state annihilations by projecting parameter space constraints onto triangles, a technique familiar from collider physics; and we compare our bounds to collider limits on equivalent models.

We consider a quartic interaction of the Higgs and Goldberger-Wise fields, which connects the mechanism of the extra dimension size stabilization with spontaneous symmetry breaking on our brane and gives rise to a coupling of the Higgs field to the radion and its KK tower. We estimate a possible influence of this coupling on the Higgs-radion mixing and study restrictions on model parameters from the LHC data.

The reweighting method is widely used in numerical studies of QCD, in particular, for the cases in which the conventional Monte Carlo method cannot be applied directly, e.g., finite density QCD. However, the application range of the reweighing method is restricted due to several problems. One of the most severe problems here is the overlap problem. To solve it, we examine a multipoint reweighting method in which simulations at several simulation points are combined in the data analyses. We systematically study the applicability and limitation of the multipoint reweighting method in two-flavor QCD at zero density. Measuring histograms of physical quantities at a series of simulation points, we apply the multipoint reweighting method to calculate the meson masses as continuous functions of the gauge coupling $\beta$ and the hopping parameters $\kappa$. We then determine lines of constant physics and beta functions, which are needed in a calculation of the equation of state at finite temperature.

We determine the critical value of the coupling where the first-order quantum phase transition takes place for lattice $SU(2)$ Yang–Mills theories in dimensions higher than 4. Within a mean-field approach, we derive an approximate law valid for any dimension $d$, and in the context of a Monte Carlo approach, in addition to the already known $d=5$ case, we look at $d=6,7,8$.

We consider the Standard Model with right-handed neutrinos to explain the masses of active neutrinos by the seesaw mechanism. Since active neutrinos as well as heavy neutral leptons are Majorana fermions in this case, the lepton number violating process can be induced. We discuss the inverse neutrinoless double beta decay $e^{-}{e}^{-}\to W^{-}{W}^{-}$ in the framework of the seesaw mechanism and its detectability at future colliders. It is shown that the cross section can be 17 fb for $\sqrt{s}=3\mathrm{TeV}$ even with the stringent constraint from the neutrinoless double beta decays if three (or more) right-handed neutrinos exist. In such a case, the future $e^{-}{e}^{-}$ colliders can test lepton number violation mediated by a right-handed neutrino lighter than about 10 TeV.

The measurement of the cosmic ray energy spectrum, in particular for individual species of nuclei, is an important tool to investigate cosmic ray production and propagation mechanisms. The determination of the “knees” in the spectra of different species remains one of the main challenges in cosmic ray physics. In fact, experimental results are still conflicting. In this paper we report a measurement of the mixed proton and helium energy spectrum, obtained with the combined data of the ARGO-YBJ experiment and a wide field of view Cherenkov telescope, a prototype of the future LHAASO experiment. By means of a multiparameter technique, we have selected a high-purity proton plus helium sample. The reconstructed energy resolution is found to be about 25% throughout the investigated energy range from 100 TeV to 3 PeV, with a systematic uncertainty in the absolute energy scale of 9.7%. The found energy spectrum can be fitted with a broken power-law function, with a break at the energy ${\mathrm{E}}_k=700\pm 230(\mathrm{stat})\pm 70(\mathrm{sys})\mathrm{TeV}$, where the spectral index changes from $-2.56\pm 0.05$ to $-3.24\pm 0.36$. The statistical significance of the observed spectral break is 4.2 standard deviations.

Within massive gravity, we construct a gravity dual for the insulator/metal phase transition and colossal magnetoresistance effect found in some manganese oxides materials. In the heavy graviton limit, a remarkable magnetic-field-sensitive DC resistivity peak appears at the Curie temperature, where an insulator/metal phase transition happens and the magnetoresistance is scaled with the square of field-induced magnetization. We find that metallic and insulating phases coexist below the Curie point and the relation with the electronic phase separation is discussed.

We present the explicit de Sitter supergravity action describing the interaction of supergravity with an arbitrary number of chiral and vector multiplets as well as one nilpotent chiral multiplet. The action has a non-Gaussian dependence on the auxiliary field of the nilpotent multiplet; however, it can be integrated out for an arbitrary matter-coupled supergravity. The general supergravity action with a given Kähler potential $K$, superpotential $W$ and vector matrix $f_{AB}$ interacting with a nilpotent chiral multiplet consists of the standard supergravity action defined by $K$, $W$ and $f_{AB}$ where the scalar in the nilpotent multiplet has to be replaced by a bilinear combination of the fermion in the nilpotent multiplet divided by the Gaussian value of the auxiliary field. All additional contributions to the action start with terms quartic and higher order in the fermion of the nilpotent multiplet. These are given by a simple universal closed form expression.

By applying Darboux-Crum transformations to a Lax pair formulation of the Korteweg–de Vries (KdV) equation, we construct new sets of multisoliton solutions to it as well as to the modified Korteweg–de Vries (mKdV) equation. The obtained solutions exhibit a chiral asymmetry in the propagation of different types of defects in crystalline backgrounds. We show that the KdV solitons of pulse and compression modulation types—which support bound states in, respectively, semi-infinite and finite forbidden bands in the spectrum of the perturbed quantum one-gap Lamé system—propagate in opposite directions with respect to the asymptotically periodic background. A similar but more complicated picture also appears for multi-kink-antikink mKdV solitons that propagate with a privileged direction over the topologically trivial or topologically nontrivial crystalline background depending on the position of the energy levels of the trapped bound states in the spectral gaps of the associated Dirac system. An exotic $N=4$ nonlinear supersymmetric structure incorporating Lax-Novikov integrals of a pair of perturbed Lamé systems is shown to underlie the Miura-Darboux-Crum construction. It unifies the KdV and mKdV solutions, detects the defects and distinguishes their types, and identifies the types of crystalline backgrounds.

In this paper we perform stability analysis for exponential solutions in Einstein-Gauss-Bonnet and cubic Einstein-Lovelock gravity. We report our findings, provide areas on parameter space, and discuss similarities and differences between cases. Analysis suggests that only several cases out of numerous found solutions have stable specific perturbation. Overall, all solutions fall into three groups—the above mentioned group with at least one stable specific perturbation; another group with at least one unstable perturbation, which makes these solutions unstable; and the largest group with marginal or neutral stability. In particular, for the cases with three-dimensional isotropic subspace which could give rise to successful compactification, only one general case and one additional partial solution in the cubic Lovelock case could have stable perturbation.

For a set of $N$ asymptotically flat black holes with arbitrary charges and masses, all initially at rest and well separated, we prove the following extremum principle: the extremal charge configuration ($|q_i|=m_i$ for each black hole) can be derived by extremizing the total energy, for variations of the black hole apparent horizon areas, at fixed charges and fixed Euclidean separations. We prove this result through second order in an expansion in the inverse separations. If all charges have the same sign, this result is a variational principle that reinterprets the static equilibrium of the Majumdar-Papapetrou-Hartle-Hawking solution as an extremum of total energy, rather than as a balance of forces; this result augments a list of related variational principles for other static black holes, and is consistent with the independently known Bogomol’nyi-Prasad-Sommerfield (BPS) energy minimum.