We study the cosmological implications of interactions between spacetime quanta in the group field theory (GFT) approach to quantum gravity from a phenomenological perspective. Our work represents a first step towards understanding early Universe cosmology by studying the dynamics of the emergent continuum spacetime, as obtained from a fundamentally discrete microscopic theory. In particular, we show how GFT interactions lead to a recollapse of the Universe while preserving the bounce replacing the initial singularity, which has already been shown to occur in the free case. It is remarkable that cyclic cosmologies are thus obtained in this framework without any a priori assumption on the geometry of spatial sections of the emergent spacetime. Furthermore, we show how interactions make it possible to have an early epoch of accelerated expansion, which can be made to last for an arbitrarily large number of $e$-folds, without the need to introduce an ad hoc potential for the scalar field.

In this paper, we investigate the scalar quasinormal modes of Hořava-Lifshitz theory with $U(1)$ symmetry in static anti–de Sitter spacetime. The static planar and spherical black hole solutions in lower energy limit are derived in nonprojectable Hořava-Lifshitz gravity. The equation of motion of a scalar field is obtained, and is utilized to study the quasinormal modes of massless scalar particles. We find that the effect of Hořava-Lifshitz correction is to increase the quasinormal period as well as to slow down the decay of the oscillation magnitude. Besides, the scalar field could be unstable when the correction becomes too large.

In classical Hamiltonian theories, entropy may be understood either as a statistical property of canonical systems or as a mechanical property, that is, as a monotonic function of the phase space along trajectories. In classical mechanics, there are theorems which have been proposed for proving the nonexistence of entropy in the latter sense. We explicate, clarify, and extend the proofs of these theorems to some standard matter (scalar and electromagnetic) field theories in curved spacetime, and then we show why these proofs fail in general relativity; due to properties of the gravitational Hamiltonian and phase space measures, the second law of thermodynamics holds. As a concrete application, we focus on the consequences of these results for the gravitational two-body problem, and in particular, we prove the noncompactness of the phase space of perturbed Schwarzschild-Droste spacetimes. We thus identify the lack of recurring orbits in phase space as a distinct sign of dissipation and hence entropy production.

We consider the collision of two particles that move in the equatorial plane near a general stationary rotating axially symmetric extremal black hole. One of the particles is critical (with fine-tuned parameters) and moves in the outward direction. The second particle (usual, not fine-tuned) comes from infinity. We examine the efficiency $\eta$ of the collisional Penrose process. There are two relevant cases here: a particle falling into a black hole after collision (i) is heavy or (ii) has a finite mass. We show that the maximum of $\eta$ in case (ii) is less than or equal to that in case (i). It is argued that for superheavy particles, the bound applies to nonequatorial motion as well. As an example, we analyze collision in the Kerr-Newman background. When the bound is the same for processes (i) and (ii), $\eta =3$ for this metric. For the Kerr black hole, recent results in the literature are reproduced.

Known entropy bounds, and the generalized second law, were recently shown to imply bounds on the information arriving at future null infinity. We complete this derivation by including the contribution from gravitons. We test the bounds in classical settings with gravity and no matter. In Minkowski space, the bounds vanish on any subregion of the future boundary, independently of coordinate choices. More generally, the bounds vanish in regions where no gravitational radiation arrives. In regions that do contain Bondi news, the bounds are compatible with the presence of information, including the information stored in gravitational memory. All of our results are consistent with the equivalence principle, which states that empty Riemann-flat spacetime regions contain no classical information. We also discuss the possibility that Minkowski space has an infinite vacuum degeneracy labeled by a choice of Bondi coordinates (a classical parameter, if physical). We argue that this degeneracy cannot have any observational consequences if the equivalence principle holds. Our bounds are consistent with this conclusion.

The observation of a possible electromagnetic counterpart by the Fermi GBM group to the aLIGO detection of the merger of a black hole binary has spawned a number of ideas about its source. Furthermore, observations of fast radio bursts (FRBs) have similarly resulted in a range of new models that might endow black hole binaries with electromagnetic signatures. In this context, even the unlikely idea that astrophysical black holes may have significant charge is worth exploring, and here we present results from the simulation of weakly charged black holes as they orbit and merge. Our simulations suggest that a black hole binary with mass comparable to that observed in GW150914 could produce the level of electromagnetic luminosity observed by Fermi GBM ($10^{49}\mathrm{ergs}/\mathrm{s}$) with a nondimensional charge of $q\equiv Q/M={10}^{-4}$ assuming good radiative efficiency. However even a charge such as this is difficult to imagine avoiding neutralization long enough for the binary to produce its electromagnetic counterpart, and so this value would likely serve simply as an upper bound. On the other hand, one can equivalently consider the black holes as having acquired a magnetic monopole charge that would be easy to maintain and would generate an identical electromagnetic signature as the electric charges. The observation of such a binary would have significant cosmological implications, not the least of which would be an explanation for the quantization of charge itself. We also study such a magnetically charged binary in the force-free regime and find it much more radiative, reducing even further the requirements to produce the counterpart.

In Randall-Sundrum models, one typically expects the radion to be the lightest new “gravity” state, as it is dual to a composite pseudo-Goldstone boson associated with conformal symmetry breaking in the IR. Here, we investigate the effects of localized brane curvature on the properties of the radion in Goldberger-Wise stabilized Randall-Sundrum models. We point out that both the radion mass and coupling to brane matter are sensitive to the brane curvature. Radion/Higgs kinetic mixing, via an IR-localized nonminimal coupling to the Higgs, is also investigated, in relation to the ghostlike radion that can occur for $\mathcal{O}(10)$ values of the IR curvature (as required to significantly suppress the first Kaluza-Klein graviton mass). We also discuss a class of IR-localized terms involving the radion. Basic comments regarding the dual four-dimensional theory are offered.

Galactic gamma ray astronomy at very high energy ($E_{\gamma }\gtrsim 30\mathrm{TeV}$) is a vital tool in the study of the nonthermal universe. The interpretation of the observations in this energy region requires the precise modeling of the attenuation of photons due to pair production interactions ($\gamma \gamma \to e^{+}{e}^{-}$) where the targets are the radiation fields present in interstellar space. For gamma rays with energy $E_{\gamma }\gtrsim 300\mathrm{TeV}$ the attenuation is mostly due to the photons of the cosmic microwave background radiation. At lower energy the most important targets are infrared photons with wavelengths in the range $\lambda \simeq 50–500\mu \mathrm{m}$ emitted by dust. The evaluation of the attenuation requires a good knowledge of the density, and energy and angular distributions of the target photons for all positions in the Galaxy. In this work we discuss a simple model for the infrared radiation that depends on only few parameters associated to the space and temperature distributions of the emitting dust. The model allows to compute with good accuracy the effects of absorption for any space and energy distribution of the diffuse Galactic gamma ray emission. The absorption probability due to the Galactic infrared radiation is maximum for $E_{\gamma }\simeq 150\mathrm{TeV}$, and can be as large as $P_{\text{abs}}\simeq 0.45$ for distant sources on lines of sight that pass close to the Galactic center. The systematic uncertainties on the absorption probability are estimated as $\mathrm{\Delta }{P}_{\text{abs}}\lesssim 0.08$.

Realistic models of neutron and quark stars in the framework of mimetic gravity with a Lagrange multiplier constraint are presented. We discuss the effect of a mimetic scalar aiming to describe dark matter on the mass-radius relation and the moment of inertia for slowly rotating relativistic stars. The mass-radius relation and moment of inertia depend on the value of the mimetic scalar in the center of the star. This fact leads to the ambiguity in the mass-radius relation for a given equation of state. Such ambiguity allows us to explain some observational facts better than in standard general relativity. The case of mimetic potential $V(\varphi )\sim A{e}^{C{\varphi }^2}$ is considered in detail. The relative deviation of the maximal moment of inertia is approximately twice as large as the relative deviation of the maximal stellar mass. We also briefly discuss the mimetic $f(R)$ gravity. In the case of $f(R)=R+a{R}^2$ mimetic gravity, it is expected that the increase of maximal mass and maximal moment of inertia due to the mimetic scalar becomes much stronger with bigger parameter $a$. The influence of the scalar field in mimetic gravity can lead to the possible existence of extreme neutron stars with large masses.

The influence of spacetime foam on a broad class of bosonic fields with arbitrary numbers of particles in the low-energy regime is investigated. Based on a recently formulated general description of open quantum gravitational systems, we analyze the propagation of scalar, electromagnetic, and gravitational waves on both long and short time scales with respect to their mean frequencies. For the long time propagation, the Markov approximation that neglects the effects of initial conditions of these waves is employed. In this case, despite intuitively expected decoherence and dissipation from the noisy spacetime, we show that such phenomena turn out to be completely suppressed for scalar bosons, photons, and gravitons, which are coupled to gravity but otherwise free. The short time effects are then recovered through the transient non-Markovian evolution. Focusing on scalar bosons in initially incoherent states, we find that the resulting quantum dissipation depends strongly on the distribution of the particle momentum states. We further identify a hitherto undiscovered collective antidissipation mechanism for a large number of particles. The surprising new effect tends to “bundle” identical particles within sharply distributed momentum states having a width inversely proportional to the particle number due to the thermal fluctuations, or its square root due to the vacuum fluctuations of spacetime.

We propose a simplified model to study the possible new heavy diphoton resonance from the cascade decay of a heavier particle at colliders, which may be related to dark matter or other new physics beyond the Standard Model. Model-independent constraints and predictions on the allowed couplings for generating the observed diphoton data are studied in detail. We demonstrate that this scenario can be tested by the possible four-photon signal or the $WW/ZZ/Z\gamma$ resonances. Meanwhile, this cascade decay scenario also provides us with dark matter candidates, which is consistent with the observed dark matter relic density.

We revisit the existing limits on the gaugino masses in various supersymmetric (SUSY) scenarios derived from Run-I data of the LHC. These limits obtained from the various final states rely heavily on the simplified assumptions regarding the masses, compositions, and decay branching ratios of the gauginos. The most severe exclusion limits on the gaugino masses are obtained from trilepton final states, while the second lightest neutralino (${\tilde{\chi }}_2^0$) decaying into the SM-like Higgs and lightest SUSY particle results in the weakest bounds. Our aim is to assess the extent of deviation of these exclusion limits in more realistic scenarios. After a brief discussion on the various decay modes of the ${\tilde{\chi }}_2^0$ and the lightest chargino (${\tilde{\chi }}_1^{\pm }$), we proceed to validate the ATLAS exclusion limits obtained from trilepton, $l\gamma \gamma$, and $lb\overline{b}$ final states associated with missing energy. We then consider different combinations of the relevant branching ratios to study their impact on the existing bounds. The results are presented alongside the existing exclusion limits to showcase the extent of the obtained deviation. We also observe that the three-body decay modes of ${\tilde{\chi }}_2^0$ and ${\tilde{\chi }}_1^{\pm }$ via off-shell slepton decays resulting in trilepton final states provide bounds that are far more severe in some parts of the available parameter space than those obtained from the off-shell gauge boson decays.

In this paper, we investigate anomalous interactions of the Higgs boson with heavy fermions, employing shapes of kinematic distributions. We study the processes $pp\to t\overline{t}+H$, $b\overline{b}+H$, $tq+H$, and $pp\to H\to {\tau }^{+}{\tau }^{-}$ and present applications of event generation, reweighting techniques for fast simulation of anomalous couplings, as well as matrix element techniques for optimal sensitivity. We extend the matrix element likelihood approach (MELA) technique, which proved to be a powerful matrix element tool for Higgs boson discovery and characterization during Run I of the LHC, and implement all analysis tools in the JHU generator framework. A next-to-leading-order QCD description of the $pp\to t\overline{t}+H$ process allows us to investigate the performance of the MELA in the presence of extra radiation. Finally, projections for LHC measurements through the end of Run III are presented.

In this article, we consider the standard model extended by a number of (light) right-handed neutrinos, and assume the presence of some heavy physics that cannot be directly produced, but can be probed by its low-energy effective interactions. Within this scenario, we obtain all the gauge-invariant dimension-7 effective operators, and determine whether each of the operators can be generated at tree level by the heavy physics, or whether it is necessarily loop generated. We then use the tree-generated operators, including those containing right-handed neutrinos, to put limits on the scale of new physics $\mathrm{\Lambda }$ using low-energy measurements. We also study the production of same-sign dileptons at the Large Hadron Collider and determine the constraints on the heavy physics that can be derived from existing data, as well as the reach in probing $\mathrm{\Lambda }$ expected from future runs of this collider.

We present results for the isovector axial, scalar, and tensor charges $g_A^{u-d}$, $g_S^{u-d}$, and $g_T^{u-d}$ of the nucleon needed to probe the Standard Model and novel physics. The axial charge is a fundamental parameter describing the weak interactions of nucleons. The scalar and tensor charges probe novel interactions at the TeV scale in neutron and nuclear $\beta$-decays, and the flavor-diagonal tensor charges $g_T^u$, $g_T^d$, and $g_T^s$ are needed to quantify the contribution of the quark electric dipole moment (EDM) to the neutron EDM. The lattice-QCD calculations were done using nine ensembles of gauge configurations generated by the MILC Collaboration using the highly improved staggered quarks action with $2+1+1$ dynamical flavors. These ensembles span three lattice spacings $a\approx 0.06,0.09$, and 0.12 fm and light-quark masses corresponding to the pion masses $M_{\pi }\approx 135,225$, and 315 MeV. High-statistics estimates on five ensembles using the all-mode-averaging method allow us to quantify all systematic uncertainties and perform a simultaneous extrapolation in the lattice spacing, lattice volume, and light-quark masses for the connected contributions. Our final estimates, in the $\overline{\mathrm{MS}}$ scheme at 2 GeV, of the isovector charges are $g_A^{u-d}=1.195(33)(20)$, $g_S^{u-d}=0.97(12)(6)$, and $g_T^{u-d}=0.987(51)(20)$. The first error includes statistical and all systematic uncertainties except that due to the extrapolation Ansatz, which is given by the second error estimate. Combining our estimate for $g_S^{u-d}$ with the difference of light quarks masses $(m_d-m_u{)}^{\mathrm{QCD}}=2.67(35)\mathrm{MeV}$ given by the Flavor Lattice Average Group, we obtain $(M_N-M_P{)}^{\mathrm{QCD}}=2.59(49)\mathrm{MeV}$. Estimates of the connected part of the flavor-diagonal tensor charges of the proton are $g_T^u=0.792(42)$ and $g_T^d=-0.194(14)$. Combining our new estimates with precision low-energy experiments, we present updated constraints on novel scalar and tensor interactions, ${\epsilon }_{S,T}$, at the TeV scale.

The inclusive gluon production at midrapidities is described in the color glass condensate formalism using the $k_T$-factorization formula, which was derived at fixed coupling constant considering the scattering of a dilute system of partons with a dense one. Recent analysis demonstrated that this approach provides a satisfactory description of the experimental data for the inclusive hadron production in $pp/pA/AA$ collisions. However, these studies are based on the fixed coupling $k_T$-factorization formula. This formula does not take into account the running coupling corrections, which are important to set the scales present in the cross section. In this paper we consider the running coupling corrected $k_T$-factorization formula conjectured some years ago and investigate the impact of the running coupling corrections on the observables. In particular, the pseudorapidity distributions and charged hadrons’ multiplicity are calculated considering $pp$, $dAu/pPb$, and $AuAu/PbPb$ collisions at RHIC and LHC energies. We compare the corrected running coupling predictions with those obtained using the original $k_T$-factorization assuming a fixed coupling or a prescription for the inclusion of the running of the coupling. Considering the Kharzeev-Levin-Nardi unintegrated gluon distribution and a simplified model for the nuclear geometry, we demonstrate that the distinct predictions are similar for the pseudorapidity distributions in $pp/pA/AA$ collisions and for the charged hadrons’ multiplicity in $pp/pA$ collisions. On the other hand, the running coupling corrected $k_T$-factorization formula predicts a smoother energy dependence for $dN/d\eta$ in $AA$ collisions.

The interference between Cabibbo-favored and Cabibbo-suppressed $B\to D\pi$ decay amplitudes provides sensitivity to the CKM angle $\gamma$. The relative size of the interfering amplitudes is an important ingredient in the determination of $\gamma$. Using branching fractions from various $B\to Dh$ decays, and the measured value for $r_B^{DK}$, the magnitude of the amplitude ratio of $B^{+}\to D^0{\pi }^{+}$ and $B^{+}\to {\overline{D}}^0{\pi }^{+}$ decays is estimated to be $r_B^{D\pi }=0.0053\pm 0.0007$.

We present a holographic perspective on momentum transport in strongly coupled, anisotropic non-Abelian plasmas in the presence of strong magnetic fields. We compute the anisotropic heavy quark drag forces and Langevin diffusion coefficients and also the anisotropic shear viscosities for two different holographic models, namely, a top-down deformation of strongly coupled $\mathcal{N}=4$ super-Yang-Mills theory triggered by an external Abelian magnetic field, and a bottom-up Einstein-Maxwell-dilaton (EMD) model which is able to provide a quantitative description of lattice QCD thermodynamics with ($2+1$) flavors at both zero and nonzero magnetic fields. We find that, in general, energy loss and momentum diffusion through strongly coupled anisotropic plasmas are enhanced by a magnetic field being larger in transverse directions than in the direction parallel to the magnetic field. Moreover, the anisotropic shear viscosity coefficient is smaller in the direction of the magnetic field than in the plane perpendicular to the field, which indicates that strongly coupled anisotropic plasmas become closer to the perfect fluid limit along the magnetic field. We also present, in the context of the EMD model, holographic predictions for the entropy density and the crossover critical temperature in a wider region of the ($T$, $B$) phase diagram that has not yet been covered by lattice simulations. Our results for the transport coefficients in the phenomenologically realistic magnetic EMD model could be readily used as inputs in numerical codes for magnetohydrodynamics.

We study the thermomagnetic properties of the strong coupling constant $G$ and quark mass $M$ entering the Nambu–Jona-Lasinio model. For this purpose, we compute the quark condensate and compare it to lattice QCD (LQCD) results to extract the behavior of $G$ and $M$ as functions of the magnetic field strength and temperature. We find that at zero temperature, where the LQCD condensate is found to monotonically increase with the field strength, $M$ also increases whereas $G$ remains approximately constant. However, for temperatures above the chiral/deconfinement phase transitions, where the LQCD condensate is found to monotonically decrease with increasing field, $M$ and $G$ also decrease monotonically. For finite temperatures, below the transition temperature, we find that both $G$ and $M$ initially grow and then decrease with increasing field strength. To study possible consequences of the extracted temperature and magnetic field dependence of $G$ and $M$, we compute the pressure and compare to LQCD results, finding an excellent qualitative agreement. In particular, we show that the transverse pressure, as a function of the field strength, is always negative for temperatures below the transition temperature whereas it starts off being positive and then becomes negative for temperatures above the transition temperature, also in agreement with LQCD results. We also show that for the longitudinal pressure to agree with LQCD calculations, the system should be described as a diamagnet. We argue that the turnover of $M$ and $G$ as functions of temperature and field strength is a key element that drives the behavior of the quark condensate going across the transition temperature and provides clues for a better understanding of the inverse magnetic catalysis phenomenon.

We study the fusion processes $W^{-}{W}^{+}\to t\overline{t}$ and $ZZ\to t\overline{t}$ observable at a future $e^{-}{e}^{+}$ collider and we discuss their sensitivity to an $Htt$ form factor which may be due to compositeness, in particular when the $H$ and the top quark have common constituents. We make an amplitude analysis and illustrate which helicity amplitudes and cross sections for specific final $t\overline{t}$ polarizations are especially sensitive to this form factor.