We reexamine the determination of $|V_{cb}|$ from $B\to Dl\nu$ in view of recent experimental and theoretical progress, discussing the parameterization of the form factors and studying the role played by the unitarity constraints. Our fit to experimental and lattice results for $B\to D\ell \nu$ gives $|V_{cb}|=40.49(97){10}^{-3}$ and to $R(D)=0.299(3)$.

Measurements of normalized differential cross sections of top quark pair ($t\overline{t}$) production are presented as a function of the mass, the transverse momentum and the rapidity of the $t\overline{t}$ system in proton-proton collisions at center-of-mass energies of $\sqrt{s}=7$ and 8 TeV. The data set corresponds to an integrated luminosity of $4.6{\mathrm{fb}}^{-1}$ at 7 TeV and $20.2{\mathrm{fb}}^{-1}$ at 8 TeV, recorded with the ATLAS detector at the Large Hadron Collider. Events with top quark pair signatures are selected in the dilepton final state, requiring exactly two charged leptons and at least two jets with at least one of the jets identified as likely to contain a $b$ hadron. The measured distributions are corrected for detector effects and selection efficiency to cross sections at the parton level. The differential cross sections are compared with different Monte Carlo generators and theoretical calculations of $t\overline{t}$ production. The results are consistent with the majority of predictions in a wide kinematic range.

A phenomenologically viable theory of quantum gravity must accommodate all observed matter degrees of freedom and their properties. Here, we explore whether a toy model of the Higgs-Yukawa sector of the Standard Model is compatible with asymptotically safe quantum gravity. We discuss the phenomenological implications of our result in the context of the Standard Model. We analyze the quantum scaling dimension of the system and find an irrelevant Yukawa coupling at a joint gravity-matter fixed point. Further, we explore the impact of gravity-induced couplings between scalars and fermions, which are nonvanishing in asymptotically safe gravity.

Modeling of gravitational waves (GWs) from binary black hole inspiral brings together early post-Newtonian waveforms and late quasinormal ringing waveforms. Attempts to bridge the two limits without recourse to numerical relativity involve predicting the time of the peak GW amplitude. This prediction will require solving the question of why the peak of the “source,” i.e., the peak of the binary angular velocity, does not correspond to the peak of the GW amplitude. We show here that this offset can be understood as due to the existence of two distinct components of the radiation: the “direct” radiation analogous to that in flat spacetime and “scattered” radiation associated with curved spacetime. The time dependence of these two components and of their relative phases determines the location of the peak amplitude. We use a highly simplified model to clarify the two-component nature of the source, then demonstrate that the explanation is valid also for an extreme mass-ratio binary inspiral.

We study the 4D holomorphic spin foam amplitude on arbitrary connected 2-complexes and degrees of divergence. With recently developed tools and a truncation scheme, we derive a formula for a certain class of graphs, which allows us to write down the value of bulk amplitudes simply based on graph properties. We then generalize the result to arbitrarily connected 2-complexes and extract a simple expression for the degree of divergence only in terms of combinatorial properties and topological invariants. The distinct behaviors of the model in different regions of parameter space signal phase transitions. In the regime which is of physical interest for recovering diffeomorphism symmetry in the continuum limit, the most divergent configurations are melonic graphs. We end with a discussion of physical implications.

Gravitational waves emitted by distorted black holes—such as those arising from the coalescence of two neutron stars or black holes—carry not only information about the corresponding spacetime but also about the underlying theory of gravity. Although general relativity remains the simplest, most elegant, and viable theory of gravitation, there are generic and robust arguments indicating that it is not the ultimate description of the gravitational universe. Here, we focus on a particularly appealing extension of general relativity, which corrects Einstein’s theory through the addition of terms which are second order in curvature: the topological Gauss-Bonnet invariant coupled to a dilaton. We study gravitational-wave emission from black holes in this theory and (i) find strong evidence that black holes are linearly (mode) stable against both axial and polar perturbations, (ii) discuss how the quasinormal modes of black holes can be excited during collisions involving black holes, and finally (iii) show that future ringdown detections with a large signal-to-noise ratio would improve current constraints on the coupling parameter of the theory.

In a recent paper [P. V. P. Cunha, C. A. R. Herdeiro, E. Radu, and H. F. Runarsson, Phys. Rev. Lett. 115, 211102 (2015).], it was shown that the lensing of light around rotating boson stars and Kerr black holes with scalar hair can exhibit chaotic patterns. Since no separation of variables is known (or expected) for geodesic motion on these backgrounds, we examine the 2D effective potentials for photon trajectories, to obtain a deeper understanding of this phenomenon. We find that the emergence of stable light rings on the background spacetimes allows the formation of “pockets” in one of the effective potentials, for open sets of impact parameters, leading to an effective trapping of some trajectories, dubbed “quasibound orbits.” We conclude that pocket formation induces chaotic scattering, although not all chaotic orbits are associated to pockets. These and other features are illustrated in a gallery of examples, obtained with a new ray-tracing code, pyhole, which includes tools for a simple, simultaneous visualization of the effective potential, together with the spacetime trajectory, for any given point in a lensing image. An analysis of photon orbits allows us to further establish a positive correlation between photon orbits in chaotic regions and those with more than one turning point in the radial direction; we recall that the latter is not possible around Kerr black holes. Moreover, we observe that the existence of several light rings around a horizon (several fundamental orbits, including a stable one), is a central ingredient for the existence of multiple shadows of a single hairy black hole. We also exhibit the lensing and shadows by Kerr black holes with scalar hair, observed away from the equatorial plane, obtained with pyhole.

We describe an atom interferometric gravitational wave detector design that can operate in a resonant mode for increased sensitivity. By oscillating the positions of the atomic wave packets, this resonant detection mode allows for coherently enhanced, narrow-band sensitivity at target frequencies. The proposed detector is flexible and can be rapidly switched between broadband and narrow-band detection modes. For instance, a binary discovered in broadband mode can subsequently be studied further as the inspiral evolves by using a tailored narrow-band detector response. In addition to functioning like a lock-in amplifier for astrophysical events, the enhanced sensitivity of the resonant approach also opens up the possibility of searching for important cosmological signals, including the stochastic gravitational wave background produced by inflation. We give an example of detector parameters which would allow detection of inflationary gravitational waves down to ${\mathrm{\Omega }}_{\mathrm{GW}}\sim {10}^{-14}$ for a two-satellite space-based detector.

Current measurements of the Higgs boson mass and top Yukawa coupling suggest that the effective Higgs potential develops an instability below the Planck scale. If the energy scale of inflation is as high as the grand unified theory (GUT) scale, inflationary quantum fluctuations of the Higgs field can easily destabilize the standard electroweak vacuum and produce a lot of anti–de Sitter (AdS) domains. This destabilization during inflation can be avoided if a relatively large nonminimal Higgs-gravity or inflaton-Higgs coupling is introduced. Such couplings generate a large effective mass term for the Higgs, which can raise the effective Higgs potential and suppress the vacuum fluctuation of the Higgs field. After primordial inflation, however, such effective masses drops rapidly and the nonminimal Higgs-gravity or inflaton-Higgs coupling can cause large fluctuations of the Higgs field to be generated via parametric resonance, thus producing AdS domains in the preheating stage. Furthermore, thermal fluctuations of the Higgs field cannot be neglected in the proceeding reheating epoch. We discuss the Higgs vacuum fluctuations during inflation, preheating, and reheating, and show that the Higgs metastability problem is severe unless the energy scale of the inflaton potential is much lower than the GUT scale.

When modeling inflaton fluctuations as a free quantum scalar field, the initial vacuum is conventionally imposed at the infinite past. This is called the Bunch-Davies (BD) vacuum. If however an asymptotically Minkowskian past does not exist, this requires modifications. We derive corrections to the scalar spectral index $n_s$ and the tensor tilt $n_t$ descending from arbitrary mixed states or from explicit non-BD initial conditions. The former may stem from some preinflationary background and can redshift away whereas the latter are induced by a timelike hypersurface parametrizing a physical cutoff. In both cases, we find that corrections scale in parts or fully as $\mathcal{O}(\epsilon )$ where $\epsilon$ is the first slow-roll parameter. The precise observational footprint is hence dependent on the model driving inflation. Further, we show how the inflationary consistency relation is altered. We thus provide an analytic handle on possible high scale or preinflationary physics.

Axions may make a significant contribution to the dark matter of the Universe. It has been suggested that these dark matter axions may condense into localized clumps, called “axion stars.” In this paper we argue that collisions of dilute axion stars with neutron stars, of the type known as “magnetars,” may be the origin of most of the observed fast radio bursts. This idea is a variation of an idea originally proposed by Iwazaki. However, instead of the surface effect of Iwazaki, we propose a perhaps stronger volume effect caused by the induced time dependent electric dipole moment of neutrons.

We introduce a new type of gamma-ray spectral feature, which we denominate gamma-ray triangle. This spectral feature arises in scenarios where dark matter self-annihilates via a chiral interaction into two Dirac fermions, which subsequently decay in flight into another fermion and a photon. The resulting photon spectrum resembles a sharp triangle and can be readily searched for in the gamma-ray sky. Using data from the Fermi-LAT and H.E.S.S. instruments, we find no evidence for such a spectral feature and, therefore, set strong upper bounds on the corresponding annihilation cross section. A concrete realization of a scenario yielding gamma-ray triangles consists of an asymmetric dark matter model where the dark matter particle carries lepton number. We show explicitly that this class of models can lead to intense gamma-ray spectral features, potentially at the reach of upcoming gamma-ray telescopes, opening a new window to explore asymmetric dark matter through indirect searches.

We investigate the static $Q\overline{Q}$ potential at zero and finite temperature in the presence of a constant and uniform external magnetic field $\overrightarrow{B}$, for several values of the lattice spacing and for different orientations with respect to $\overrightarrow{B}$. As a byproduct, we provide continuum limit extrapolated results for the string tension, the Coulomb coupling and the Sommer parameter at $T=0$ and $B=0$. We confirm the presence in the continuum of a $B$-induced anisotropy, regarding essentially the string tension, for which it is of the order of 15% at $|e|B\sim 1{\mathrm{GeV}}^2$ and would suggest, if extrapolated to larger fields, a vanishing string tension along the magnetic field for $|e|B\gtrsim 4{\mathrm{GeV}}^2$. The angular dependence for $|e|B\lesssim 1{\mathrm{GeV}}^2$ can be nicely parametrized by the first allowed term in an angular Fourier expansion, corresponding to a quadrupole deformation. Finally, for $T\ne 0$, the main effect of the magnetic field is a general suppression of the string tension, leading to a early loss of the confining properties: this happens even before the appearance of inverse magnetic catalysis in the chiral condensate, supporting the idea that the influence of the magnetic field on the confining properties is the leading effect originating the decrease of $T_c$ as a function of $B$.

F-theory attempts to include all U-dualities manifestly. Unlike its T-dual manifest partner, which is based on string current algebra, F-theory is based on higher dimensional brane current algebra. Like the T-dual manifest theory, which has $O(D-1,1{)}^2$ unbroken symmetry, the F-theory vacuum also enjoys certain symmetries (“$H$”). One of its important and exotic properties is that worldvolume indices are also spacetime indices. This makes the global brane current algebra incompatible with $H$ symmetry currents. The solution is to introduce worldvolume covariant derivatives, which depend on the $H$ coordinates even in a “flat” background. We will also give as an explicit example the 5-brane case.

The chiral and deconfinement phase transitions are investigated within the variational Hamiltonian approach to QCD in Coulomb gauge. The temperature ${\beta }^{-1}$ is introduced by compactifying a spatial dimension. Thereby the whole temperature dependence is encoded in the vacuum state on the spatial manifold ${\mathbb{R}}^2\times S^1(\beta )$. The chiral quark condensate and the dual quark condensate (dressed Polyakov loop) are calculated as a function of the temperature. From their inflection points the pseudocritical temperatures for the chiral and deconfinement crossover transitions are determined. Using the zero-temperature quark and gluon propagators obtained within the variational approach as input, we find 170 and 198 MeV, respectively, for the chiral and deconfinement transition.

Three static and axially symmetric (Weyl-type) ring singularities—the Majumdar-Papapetrou–type (extremally charged) ring, the Bach-Weyl ring, and the Appell ring—are studied in general relativity in order to show how remarkably the geometries in their vicinity differ from each other. This is demonstrated on basic measures of the rings and on invariant characteristics given by the metric and by its first and second derivatives (lapse, gravitational acceleration, and curvature), and also on geodesic motion. The results are also compared against the Kerr space-time which possesses a ring singularity too. The Kerr solution is only stationary, not static, but in spite of the consequent complication by dragging, its ring appears to be simpler than the static rings. We show that this mainly applies to the Bach-Weyl ring, although this straightforward counterpart of the Newtonian homogeneous circular ring is by default being taken as the simplest ring solution, and although the other two static ring sources may seem more “artificial.” The weird, directional deformation around the Bach-Weyl ring probably indicates that a more adequate coordinate representation and interpretation of this source should exist.

We use fully nonlinear numerical relativity techniques to estimate the maximum gravitational radiation emitted by high energy head-on collisions of nonspinning, equal-mass black holes. Our simulations include improvements in the construction of initial data, subsequent full numerical evolutions, and the computation of waveforms at infinity. The new initial data significantly reduce the spurious radiation content, allowing for initial speeds much closer to the speed of light, i.e., $v\sim 0.99c$. Using these new techniques, we estimate the maximum radiated energy from head-on collisions to be $E_{\max }/M_{\mathrm{ADM}}=0.13\pm 0.01$. This value differs from the second-order perturbative (0.164) and zero-frequency-limit (0.17) analytic computations but is close to those obtained by thermodynamic arguments (0.134) and by previous numerical estimates $(0.14\pm 0.03)$.

A common feature in the thermodynamic analysis of homogeneous and isotropic world models is the assumption that the temperature of the fluids inside the cosmic horizon (including dark energy) coincides with the temperature of the latter, whether it be either the event or the apparent horizon. We examine up to what extent this assumption may be justified, given that these temperatures evolve under different time-temperature laws. We argue that while radiation cannot reach thermal equilibrium with the horizon, nonrelativistic matter may, and dark energy might though only approximately.