We revisit the current experimental bounds on fourth generation Majorana neutrino masses, including the effects of right-handed neutrinos. Current bounds from LEP II are significantly altered by a global analysis. We show that the current bounds on fourth generation neutrinos decaying to $eW$ or $\mu W$ can be reduced to about 80 GeV (from the current bound of 90 GeV), while a neutrino decaying to $\tau W$ can be as light as 62.1 GeV. The weakened bound opens up a neutrino decay channel for intermediate mass Higgs, and interesting multiparticle final states for Higgs and fourth generation lepton decays.

A new combined next to leading order QCD analysis of the polarized inclusive and semi-inclusive deep inelastic lepton-hadron scattering (DIS) data is presented. In contrast to previous combined analyses, the $1/Q^2$ terms (kinematic—target mass corrections, and dynamic—higher twist corrections) in the expression for the nucleon spin structure function $g_1$ are taken into account. The new COMPASS data are included in the analysis. The impact of the semi-inclusive data on the polarized parton densities (PDFs) and on the higher twist corrections is discussed. The new results for the PDFs are compared to our (Leader, Sidorov, Stamenov) LSS’06 PDFs, obtained from the fit to the inclusive DIS data alone, and to those obtained from the de Florian, Sassot, Stratmann, and Vogelsang global analysis.

A $d$-wave holographic superconductor in the presence of a constant magnetic field is studied by the perturbation method. We obtain both droplet and triangular vortex lattice solutions. The results are the same as that of an $s$-wave holographic superconductor. The non-Abelian holographic superconductor with $p+ip$-wave background in the presence of a magnetic field is also studied. Unlike the $d$-wave and $s$-wave models, it is found that the non-Abelian model has only a droplet solution.

Junctions on cosmic string loops give rise to the proliferation of sharp kinks. We study the effect of this proliferation on the gravitational wave (GW) signals emitted from string networks with junctions, assuming a scaling solution. We calculate the rate of occurrence and the distribution in amplitude of the GW bursts emitted at cusps and kinks in the frequency bands of LIGO/VIRGO and LISA as a function of the string tension, the number of sharp kinks on loops with junctions, and the fraction of loops in the cosmological network which have junctions. Combining our results with current observational constraints, we find that pulsar data rule out a significant number of kinks on loops for strings with tensions $G\mu \gtrsim {10}^{-12}$. By contrast, for smaller tensions current observations allow for a large number of kinks on loops. If this is the case, the incoherent superposition of small bursts emitted at kink-kink encounters leads to an enhanced GW background that hides the strong individual bursts from kinks and cusps.

We identify Moore-Read wave functions, describing non-Abelian statistics in fractional quantum Hall systems, with the instanton partition of $\mathcal{N}=2$ superconformal quiver gauge theories at suitable values of masses and $\Omega$-background parameters. This is obtained by extending to rational conformal field theories the $SU(2)$ gauge quiver/Liouville field theory duality recently found by Alday-Gaiotto-Tachikawa. A direct link between the Moore-Read Hall $n$-body wave functions and ${\mathbb{Z}}_n$-equivariant Donaldson polynomials is pointed out.

We study how the introduction of a two-form field flux modifies the dynamics of a T-duality invariant string gas cosmology model of Greene, Kabat, and Marnerides which replaces the Newtonian-like kinetic terms in the dilaton gravity action by their relativistic counterparts. It induces a repulsive potential term in the effective action for the scale factor of the spatial dimensions. Without the two-form field flux, the Universe fails to expand when the pressure due to string modes vanishes. With the presence of a homogeneous two-form field flux, it propels three spatial dimensions to grow into a macroscopic four-dimensional space-time. We find that it triggers an expansion of a universe away from the oscillating phase around the self-dual radius. We also investigate the effects of a constant two-form field. We can obtain an expanding four-dimensional space-time by tuning it at the critical value.

We discuss the phenomenological consequences of requiring the cancellation of quadratic divergences up to the leading two-loop order within the two-Higgs-doublet model. Taking into account existing experimental constraints, allowed regions in the parameter space, permitting the cancellation, are determined. A degeneracy between masses of scalar bosons is observed for $\tan \beta \gtrsim 40$. The possibility for $CP$ violation in the scalar potential is discussed and regions of $\tan \beta –M_{H^{\pm }}$ with a substantial amount of $CP$ violation are determined. In order to provide a source for dark matter in a minimal manner, a scalar gauge singlet is introduced and discussed. The model allows to ameliorate the little hierarchy problem by lifting the minimal scalar Higgs-boson mass and by suppressing the quadratic corrections to scalar masses. The cutoff originating from the naturality arguments is therefore lifted from $\sim 0.6\mathrm{TeV}$ in the standard model to $\gtrsim 2.5\mathrm{TeV}$ in two-Higgs-doublet model depending on the mass of the lightest scalar.

In dynamical spacetimes apparent and event horizons do not coincide. In this paper we propose a geometrical measure of the distance between those horizons and investigate it for the case of the Vaidya spacetime. We show that it is well defined, physically meaningful, and has the expected behavior in the near-equilibrium limit. We consider its implications for our understanding of black hole physics.

Black holes of mass $M$ must have a spin angular momentum $S$ below the Kerr limit ($\chi \equiv S/M^2\le 1$), but whether astrophysical black holes can attain this limiting spin depends on their accretion history. Gas accretion from a thin disk limits the black-hole spin to ${\chi }_{\mathrm{gas}}\lesssim 0.9980\pm 0.0002$, as electromagnetic radiation from this disk with retrograde angular momentum is preferentially absorbed by the black hole. Extrapolation of numerical-relativity simulations of equal-mass binary black-hole mergers to maximum initial spins suggests these mergers yield a maximum spin ${\chi }_{\mathrm{eq}}\lesssim 0.95$. Here we show that for smaller mass ratios $q\equiv m/M\ll 1$, the superradiant extraction of angular momentum from the larger black hole imposes a fundamental limit ${\chi }_{\lim }\lesssim 0.9979\pm 0.0001$ on the final black-hole spin even in the test-particle limit ($q\to 0$) of binary black-hole mergers. The nearly equal values of ${\chi }_{\mathrm{gas}}$ and ${\chi }_{\lim }$ imply that measurement of supermassive black-hole spins cannot distinguish a black hole built by gas accretion from one assembled by the gravitational inspiral of a disk of compact stellar remnants. We also show how superradiant scattering alters the mass and spin predicted by models derived from extrapolating test-particle mergers to finite mass ratios.

The cosmology of general fourth-order corrections to Einstein gravity is considered, both for a homogeneous and isotropic background and for general tensor perturbations. It is explicitly shown how the standard cosmological history can be (approximately) reproduced and under what condition the evolution of the tensor modes remain (approximately) unchanged. Requiring that the deviations from general relativity are small during inflation sharpens the current constraints on such corrections terms by some thirty orders of magnitude. Taking a more conservative approach and requiring only that cosmology be approximately that of general relativity during big bang nucleosynthesis, the constraints are improved by 4–6 orders of magnitude.

We follow the Feynman procedure to obtain a path integral formulation of loop quantum cosmology starting from the Hilbert space framework. Quantum geometry effects modify the weight associated with each path so that the effective measure on the space of paths is different from that used in the Wheeler-DeWitt theory. These differences introduce some conceptual subtleties in arriving at the WKB approximation. But the approximation is well defined and provides intuition for the differences between loop quantum cosmology and the Wheeler-DeWitt theory from a path integral perspective.

The general analysis of the relations between masses and angular momenta in the configurations composed of two balancing extremal Kerr particles is made on the basis of two exact solutions arising as extreme limits of the well-known double-Kerr spacetime. We show that the inequality $M^2\ge |J|$ characteristic of an isolated Kerr black hole is verified by all the extremal components of the Tomimatsu and Dietz-Hoenselaers solutions. At the same time, the inequality can be violated by the total masses and total angular momenta of these binary systems, and we identify all the cases when such violation occurs.

The $f(R)$ gravity models formulated in the Einstein conformal frame are equivalent to Einstein gravity together with a minimally coupled scalar field. The scalar field couples with the matter sector and the coupling term is given by the conformal factor. We use this interacting model to derive a necessary condition for alleviating the coincidence problem.

This paper deals mainly with the application of the second-order symplectic implicit midpoint rule and its symmetric compositions to a post-Newtonian Hamiltonian formulation with canonical spin variables in relativistic compact binaries. The midpoint rule, as a basic algorithm, is directly used to integrate the completely canonical Hamiltonian system. On the other hand, there are symmetric composite methods based on a splitting of the Hamiltonian into two parts: the Newtonian part associated with a Kepler motion, and a perturbation part involving the orbital post-Newtonian and spin contributions, where the Kepler flow has an analytic solution and the perturbation can be calculated by the midpoint rule. An example is the second-order mixed leapfrog symplectic integrator with one stage integration of the perturbation flow and two semistage computations of the Kepler flow at every integration step. Also, higher-order composite methods such as the Forest-Ruth fourth-order symplectic integrator and its optimized algorithm are applicable. Various numerical tests including simulations of chaotic orbits show that the mixed leapfrog integrator is always superior to the midpoint rule in energy accuracy, while both of them are almost equivalent in computational efficiency. Particularly, the optimized fourth-order algorithm compared with the mixed leapfrog scheme provides good precision and needs no expensive additional computational time. As a result, it is worth performing a more detailed and careful examination of the dynamical structure of chaos and order in the parameter windows and phase space of the binary system.

In this paper we study how the distortion generated by a static and neutral distribution of external matter affects a five-dimensional Schwarzschild-Tangherlini black hole. A solution representing a particular class of such distorted black holes admits an ${\mathbb{R}}^1\times U(1)\times U(1)$ isometry group. We show that there exists a certain duality transformation between the black hole horizon surface and the stretched singularity surface. The space-time near the distorted black hole singularity has the same topology and Kasner exponents as those of a five-dimensional Schwarzschild-Tangherlini black hole. We calculate the maximal proper time of free fall of a test particle from the distorted black hole horizon to its singularity and find that, depending on the distortion, it can be less, equal to, or greater than that of a Schwarzschild-Tangherlini black hole of the same horizon area. This implies that due to the distortion, the singularity of a Schwarzschild-Tangherlini black hole can come close to its horizon. A relation between the Kretschmann scalar calculated on the horizon of a five-dimensional static, asymmetric, distorted black hole and the trace of the square of the Ricci tensor of the horizon surface is derived.

We investigate the low-frequency absorption cross section of the electromagnetic waves for extreme Reissner-Nordström black holes in higher dimensions. We first construct the exact solutions to the relevant wave equations in the zero-frequency limit. In most cases it is possible to use these solutions to find the transmission coefficients of partial waves in the low-frequency limit. We use these transmission coefficients to calculate the low-frequency absorption cross section in five and six spacetime dimensions. We find that this cross section is dominated by the modes with $\ell =2$ in the spherical-harmonic expansion rather than those with $\ell =1$, as might have been expected, because of the mixing between the electromagnetic and gravitational waves. We also find an upper limit for the low-frequency absorption cross section in dimensions higher than six.

In this paper we extend our previous works on spherically symmetric accretion onto black holes and superspinars to the case in which the fluid has a finite angular momentum initially. We run 2.5D and 3D general relativistic hydrodynamic simulations of the accretion of a fat disk. We study how the accretion process changes by changing the values of the parameters of our model. We show that the value of the fluid angular momentum critically determines turn-on and off the production of powerful equatorial outflows around superspinars. For corotating disks, equatorial outflows are efficiently generated, even for relatively low spin parameters or relatively large superspinar radii. For counterrotating disks, equatorial outflows are instead significantly suppressed, and they are possible only in limited cases. We also study accretion around a tilted disk.

We summarize, simplify and extend recent work showing that small deviations from exact thermality in Hawking radiation, first uncovered by Kraus and Wilczek, have the capacity to carry off the maximum information content of a black hole. This goes a considerable way toward resolving a long-standing “information loss paradox.”

In the context of the teleparallel equivalent of general relativity, we establish the Hamiltonian formulation of the unimodular theory of gravity. Here we do not carry out the usual $3+1$ decomposition of the field quantities in terms of the lapse and shift functions, as in the ADM formalism. The corresponding Lagrange multiplier is the timelike component of the tetrad field. The dynamics is determined by the Hamiltonian constraint ${\mathcal{H}}_0^{\prime }$ and a set of primary constraints. The constraints are first class and satisfy an algebra that is similar to the algebra of the Poincaré group.

We present wormhole solutions to Hořava nonrelativistic gravity theory in vacuum. We show that, if the parameter $\lambda$ is set to one, traversable wormholes connecting two asymptotically de Sitter or anti-de Sitter regions exist. In the case of arbitrary $\lambda$, the asymptotic regions have a more complicated metric with constant curvature. We also show that, when the detailed balance condition is violated softly, traversable and asymptotically Minkowski, de Sitter or anti-de Sitter wormholes exist.