The problem of kinematic effects in gluon and color dipole cascades is addressed in the large $N_c$ limit of $SU(N_c)$ Yang-Mills theory. We investigate the tree-level multigluon components of the gluon light-cone wave functions in the light-cone gauge keeping the exact kinematics of the gluon emissions. We focus on the components with all helicities identical to the helicity of the incoming gluon. The recurrence relations for the gluon wave functions are derived. In the case when the virtuality of the incoming gluon is neglected the exact form of the multigluon wave function is obtained. Furthermore, we propose an approximate scheme to treat the kinematic effects in the color dipole evolution kernel. The new kernel entangles longitudinal and transverse degrees of freedom and leads to a reduced diffusion in the impact parameter. When evaluated in the next-to-leading logarithmic (NLL) accuracy, the kernel reproduces the correct form of the double logarithmic terms of the dipole size ratios present in the exact NLL dipole kernel. Finally, we analyze the scattering of the incoming gluon light-cone components off a gluon target and the fragmentation of the scattered state into the final state. The equivalence of the resulting amplitudes and the maximally helicity-violating amplitudes is demonstrated in the special case when the target gluon is far in rapidity from the evolved gluon wave function.

We continue the study of heterotic non-Abelian Bogomol’nyi-Prasad-Sommerfield-saturated flux tubes (strings). Previously, such solutions were obtained [M. Shifman and A. Yung, Phys. Rev. D 77, 125016 (2008).] in a particular U(2) gauge theory: $\mathcal{N}=2$ supersymmetric QCD deformed by superpotential terms of a special type breaking $\mathcal{N}=2$ supersymmetry down to $\mathcal{N}=1$. Here we generalize the previous results to $\mathrm{U}(N)$ gauge theories. As was suggested by Edalati and Tong [M. Edalati and D. Tong, J. High Energy Phys. 05 (2007) 005.], the string world-sheet theory is a heterotic $\mathcal{N}=(0,2)$ sigma model, with the $CP(N-1)$ target space for bosonic fields and an extra right-handed fermion which couples to the fermion fields of the $\mathcal{N}=(2,2)$ $CP(N-1)$ model. We derive the heterotic $\mathcal{N}=(0,2)$ world-sheet model directly from the $\mathrm{U}(N)$ bulk theory. Parameters of the bulk theory are related to those of the world-sheet theory. Qualitatively this relation turns out to be the same as in the U(2) case.

We assess the statistical errors in estimating the parameters of nonspinning black hole binaries using ground-based gravitational-wave detectors. While past assessments were based on partial information provided by only the inspiral and/or ring-down pieces of the coalescence signal, the recent progress in analytical and numerical relativity enables us to make more accurate projections using complete inspiral-merger-ring-down waveforms. We employ the Fisher information-matrix formalism to estimate how accurately the source parameters will be measurable using a single interferometric detector as well as a network of interferometers. Those estimates are further vetted by full-fledged Monte Carlo simulations. We find that the parameter accuracies of the complete waveform are, in general, significantly better than those of just the inspiral waveform in the case of binaries with total mass $M\gtrsim 20M_{\odot }$. In particular, for the case of the Advanced LIGO detector, parameter estimation is the most accurate in the $M=100–200M_{\odot }$ range. For an $M=100M_{\odot }$ system, the errors in measuring the total mass and the symmetric mass-ratio are reduced by an order of magnitude or more compared to inspiral waveforms. Furthermore, for binaries located at a fixed luminosity distance $d_L$, and observed with the Advanced LIGO-Advanced Virgo network, the sky-position error is expected to vary widely across the sky: For $M=100M_{\odot }$ systems at $d_L=1\mathrm{Gpc}$, this variation ranges mostly from about a hundredth of a square degree to about a square degree, with an average value of nearly a tenth of a square degree. This is more than 40 times better than the average sky-position accuracy of inspiral waveforms at this mass range. For the mass parameters as well as the sky position, this improvement in accuracy is due partly to the increased signal-to-noise ratio and partly to the information about these parameters harnessed through the post-inspiral phases of the waveform. The error in estimating $d_L$ is dominated by the error in measuring the wave’s polarization and is roughly 43% for low-mass ($M\sim 20M_{\odot }$) binaries and about 23% for high-mass ($M\sim 100M_{\odot }$) binaries located at $d_L=1\mathrm{Gpc}$.

It is generally accepted that Einstein’s theory will get some as yet unknown corrections, possibly large in the strong-field regime. An ideal place to look for these modifications is in the vicinities of compact objects such as black holes. Here, we study dilatonic black holes, which arise in the framework of Gauss-Bonnet couplings and one-loop corrected four-dimensional effective theory of heterotic superstrings at low energies. These are interesting objects as a prototype for alternative, yet well-behaved gravity theories: they evade the “no-hair” theorem of general relativity but were proven to be stable against radial perturbations. We investigate the viability of these black holes as astrophysical objects and try to provide some means to distinguish them from black holes in general relativity. We start by extending previous works and establishing the stability of these black holes against axial perturbations. We then look for solutions of the field equations describing slowly rotating black holes and study geodesic motion around this geometry. Depending on the values of mass, dilaton charge, and angular momentum of the solution, one can have differences in the innermost-stable-circular-orbit location and orbital frequency, relative to black holes in general relativity. In the most favorable cases, the difference amounts to a few percent. Given the current state-of-the-art, we discuss the difficulty of distinguishing the correct theory of gravity from electromagnetic observations or even with gravitational-wave detectors.

A low frequency stochastic background of gravitational waves may be detected by pulsar timing experiments in the next 5 to 10 yr. Using methods developed to analyze interferometric gravitational wave data, in this paper we lay out the optimal techniques to detect a background of gravitational waves using a pulsar timing array. We show that for pulsar distances and gravitational wave frequencies typical of pulsar timing experiments, neglecting the effect of the metric perturbation at the pulsar does not result in a significant deviation from optimality. We discuss methods for setting upper limits using the optimal statistic, show how to construct skymaps using the pulsar timing array, and consider several issues associated with realistic analysis of pulsar timing data.

We develop a new formalism for the treatment of gravitational backreaction in the cosmological setting. The approach is inspired by projective techniques in nonequilibrium statistical mechanics. We employ group averaging with respect to the action of the isotropy group of homogeneous and isotropic spacetimes (rather than spatial averaging), in order to define effective Friedmann-Robertson-Walker variables for a generic spacetime. Using the Hamiltonian formalism for gravitating perfect fluids, we obtain a set of evolution equations for the evolution of the effective variables; these equations incorporate the effects of backreaction by the inhomogeneities. Specializing to dust-filled spacetimes, we find regimes that lead to a closed set of backreaction equations, which we solve for small inhomogeneities. We then study the case of large inhomogeneities in relation to the proposal that backreaction can lead to accelerated expansion. In particular, we identify regions of the gravitational state space that correspond to effective cosmic acceleration. Necessary conditions are (i) a strong expansion of the congruences corresponding to comoving observers, and (ii) a large negative value of a dissipation variable that appears in the effective equations (i.e., an effective “antidissipation”).

We explore the problem of braneworld black holes in the heterotic braneworld scenario of Lukas, Ovrut, Stelle and Waldram (LOSW). We show that black string solutions are unstable, and demonstrate some unusual asymptotics in the linearized metric. We also solve the fully coupled brane and bulk Einstein equations, finding an exact, though singular, solution which corresponds to a brane black hole in which the branes spike apart at the Schwarzschild radius.

We demonstrate that the Sommerfeld correction to cold dark matter (CDM) annihilations can be appreciable if even a small component of the dark matter is extremely cold. Subhalo substructure provides such a possibility given that the smallest clumps are relatively cold and contain even colder substructure due to incomplete phase space mixing. Leptonic channels can be enhanced for plausible models and the solar neighborhood boost required to account for PAMELA/ATIC data is plausibly obtained, especially in the case of a few TeV mass neutralino for which the Sommerfeld-corrected boost is found to be $\sim {10}^4–{10}^5$. Saturation of the Sommerfeld effect is shown to occur below $\beta \sim {10}^{-4}$, thereby making this result largely independent on the presence of substructures below $\sim {10}^5{M}_{\odot }$. We find that the associated diffuse gamma-ray signal from annihilations would exceed EGRET constraints unless the channels annihilating to heavy quarks or to gauge bosons are suppressed. The lepton channel gamma rays are potentially detectable by the FERMI satellite, not from the inner galaxy where substructures are tidally disrupted, but rather as a quasi-isotropic background from the outer halo, unless the outer substructures are much less concentrated than the inner substructures and/or the CDM density profile out to the virial radius steepens significantly.

Fast robust methods for calculating likelihoods from cosmic microwave background observations on small scales generally rely on approximations based on a set of power spectrum estimators and their covariances. We investigate the optimality of these approximations, how accurate the covariance needs to be, and how to estimate the covariance from simulations. For a simple case with azimuthal symmetry we compare optimality of hybrid pseudo-$C_l$ cosmic microwave background power spectrum estimators with the exact result, indicating that the loss of information is not negligible, but neither is it enough to have a large effect on standard parameter constraints. We then discuss the number of samples required to estimate the covariance from simulations, with and without a good analytic approximation, and assess the use of shrinkage estimators. Finally we discuss how to combine an approximate high-$l$ likelihood with a more exact low-$l$ harmonic-space likelihood as a practical method for accurate likelihood calculation on all scales.

In order to quantitatively test the ability of averaged inhomogeneous cosmologies to correctly describe observations of the large-scale properties of the Universe, we introduce a smoothed template metric corresponding to a constant spatial curvature model at any time, but with an evolving curvature parameter. This metric is used to compute quantities along an approximate effective light cone of the averaged model of the Universe. As opposed to the standard Friedmann model, we parametrize this template metric by exact scaling properties of an averaged inhomogeneous cosmology, and we also motivate this form of the metric by results on a geometrical smoothing of inhomogeneous cosmological hypersurfaces. The purpose of the paper is not to demonstrate that the backreaction effect is actually responsible for the dark energy phenomenon by explicitly calculating the effect from a local model of the geometry and the distribution of matter, but rather to propose a way to deal with observations in the backreaction context, and to understand what kind of generic properties have to hold in order for a backreaction model to explain the observed features of the Universe on large scales. We test our hypothesis for the template metric against supernova data and the position of the cosmic microwave background peaks, and infer the goodness of fit and parameter uncertainties. We find that averaged inhomogeneous models can reproduce the observations without requiring an additional dark energy component (though a volume acceleration is still needed), and that current data do not disfavor our main assumption on the effective light cone structure. We also show that the experimental uncertainties on the angular diameter distance and the Hubble parameter from baryon acoustic oscillations measurements—forseen in future surveys like the proposed EUCLID satellite project—are sufficiently small to distinguish between a Friedmann-Lemaître-Robertson-Walker template geometry and the template geometry with consistently evolving curvature.

In this paper we compute the flavor-dependent chiral-logarithmic corrections to the decay $Z\to b\overline{b}$ in the three-site Higgsless model. We compute these corrections diagrammatically in the gaugeless limit in which the electroweak couplings vanish. We also compute the chiral-logarithmic corrections to the decay $Z\to b\overline{b}$ using a renormalization group equation analysis in effective field theory, and show that the results agree. In the process of this computation, we compute the form of the chiral current in the gaugeless limit of the three-site model, and consider the generalization to the $N$-site case. We elucidate the Ward-Takahashi identities which underlie the gaugeless limit calculation in the three-site model, and describe how the result for the $Z\to b\overline{b}$ amplitude is obtained in unitary gauge in the full theory. We find that the phenomenological constraints on the three-site Higgsless model arising from measurements of $Z\to b\overline{b}$ are relatively mild, requiring only that the heavy Dirac fermion be heavier than 1 TeV or so, and are satisfied automatically in the range of parameters allowed by other precision electroweak data.

We study effects of the scale-invariant hidden sector, the unparticle, proposed by Georgi, on top spin correlation at the Large Hadron Collider. Assuming no flavor-changing interaction between the unparticles and the standard model particles, the top-antitop quark pair production process arises through virtual unparticle exchanges in the $s$ channel, in addition to the standard model processes. In particular, we consider contributions of scalar and vector unparticles and find that these make sizable deviations of the top spin correlation from the standard model one.

We consider the prospects for detecting effects due to the Higgs exchange diagram in high energy ${\mu }^{+}{\mu }^{-}$, $e^{+}{e}^{-}$, and ${\tau }^{+}{\tau }^{-}$ collisions producing a pair of $W$ bosons. The processes $l^{+}{l}^{-}\to W^{+}{W}^{-}$ (with $l=\mu$, $e$, $\tau$) are analyzed, analytically and via numerical simulations, to determine the center of mass energy, $\sqrt{s_H}$, where the effects from Higgs exchange become relevant. The scaling of $\sqrt{s_H}$ with the mass of the incoming leptons is also studied. Special consideration is given to the $W^{+}{W}^{-}\to l^{\pm }{\nu }_{l}jj$ final state after experimental acceptance cuts are imposed. Angular cuts are shown to be able to significantly lower $\sqrt{s_H}$. The center of mass energy for which Higgs exchange effects become relevant in $l^{+}{l}^{-}\to W^{+}{W}^{-}$ is found to be much higher than that in $l^{+}{l}^{-}\to \overline{\nu }\nu W^{+}{W}^{-}$. Higgs boson exchange effects in $W$-pair production thus will not be relevant for the $e^{+}{e}^{-}$ and ${\mu }^{+}{\mu }^{-}$ colliders currently on the drawing board.

We extend the nonrelativistic AdS/CFT correspondence to the fermionic fields. In particular, we study the two point function of a fermionic operator in nonrelativistic CFTs by making use of a massive fermion propagating in geometries with Schrödinger group isometry. Although the boundary of the geometries with Schrödinger group isometry differ from that in AdS geometries where the dictionary of AdS/CFT is established, using the general procedure of AdS/CFT correspondence, we see that the resultant two point function has the expected form for fermionic operators in nonrelativistic CFTs, though a nontrivial regularization may be needed.

As is well known, when $D6$ branes wrap a special Lagrangian cycle on a noncompact Calabi-Yau threefold in such a way that the internal string frame metric is a Kähler one there exists a dual description, which is given in terms of a purely geometrical 11-dimensional background with an internal metric of $G_2$ holonomy. It is also known that when $D6$ branes wrap a coassociative cycle of a noncompact $G_2$ manifold in the presence of a self-dual two-form strength the internal part of the string frame metric is conformal to the $G_2$ metric and there exists a dual description, which is expressed in terms of a purely geometrical 11-dimensional background with an internal noncompact metric of spin(7) holonomy. In the present work it is shown that any $G_2$ metric participating in the first of these dualities necessarily participates in one of the second type. Additionally, several explicit spin(7) holonomy metrics admitting a $G_2$ holonomy reduction along one isometry are constructed. These metrics can be described as $R$ fibrations over a 6-dimensional Kähler metric, thus realizing the pattern $\mathrm{\text{spin}}(7)\to G_2\to (\mathrm{\text{Kahler}})$ mentioned above. Several of these examples are further described as fibrations over the Eguchi-Hanson gravitational instanton and, to the best of our knowledge, have not been previously considered in the literature.

We construct an infinite number of new holonomic quasilocal gravitational-energy-momentum density pseudotensors with good limits asymptotically and in small regions, both materially and in vacuum. For small vacuum regions they are all a positive multiple of the Bel-Robinson tensor and consequently have positive energy.

Recent measurements of cosmic-ray electron and positron fluxes by PAMELA and ATIC experiments may indicate the existence of annihilating dark matter with large annihilation cross section. We show that the dark matter annihilation in the big-bang nucleosynthesis epoch affects the light element abundances, and it gives stringent constraints on such annihilating dark matter scenarios.

We perform a complete “Michel parameter” analysis of all possible helicity structures which can appear in the process $B\to X_c\ell {\overline{\nu }}_{\ell }$. We take into account the full set of operators parametrizing the effective Hamiltonian and include the complete one-loop QCD corrections as well as the nonperturbative contributions. The moments of the leptonic energy as well as the combined moments of the hadronic energy and hadronic invariant mass are calculated including the nonstandard contributions.

We propose a new framework for understanding the hierarchies of fermion masses and mixings. The masses and mixings of all standard model (SM) charged fermions other than top arise from higher dimensional operators involving a messenger scalar $S$ and flavon scalars $F_i$. The flavons spontaneously break SM flavor symmetries at around the TeV scale. The SM singlet scalar $S$ couples directly to the Higgs $H$ and spontaneously breaks another $U(1)$ at the electroweak scale. At the TeV scale, SM quarks and charged leptons have renormalizable couplings to $S$, but not to $H$ or $F_i$. These couplings involve new heavy vectorlike fermions. Integrating out these fermions produces a pattern of higher dimensional operators that reproduce the observed hierarchies of the SM masses and mixings in terms of powers of the “little hierarchy”: the ratio of the electroweak scale to the flavor-breaking scale. The framework has important phenomenological implications. Flavor-changing neutral currents are within experimental limits but $D^0-{\overline{D}}^0$ mixing and $B_s\to {\mu }^{+}{\mu }^{-}$ could be close to current sensitivities. The neutral scalar $s$ of the messenger field mixes with the light Higgs of the SM, which can have strong effects on Higgs decay branching fractions. The $s$ mass eigenstate may be lighter than the Higgs, and could be detected at the Tevatron or the LHC.