Showing papers in "Physical Review D in 2003"

De Sitter vacua in string theory

Abstract: We outline the construction of metastable de Sitter vacua of type IIB string theory. Our starting point is highly warped IIB compactifications with nontrivial NS and RR three-form fluxes. By incorporating known corrections to the superpotential from Euclidean D-brane instantons or gaugino condensation, one can make models with all moduli fixed, yielding a supersymmetric AdS vacuum. Inclusion of a small number of $\overline{\mathrm{D}3}$-branes in the resulting warped geometry allows one to uplift the AdS minimum and make it a metastable de Sitter ground state. The lifetime of our metastable de Sitter vacua is much greater than the cosmological time scale of ${10}^{10}\mathrm{yr}.$ We also prove, under certain conditions, that the lifetime of dS space in string theory will always be shorter than the recurrence time.

Modified gravity with negative and positive powers of curvature: Unification of inflation and cosmic acceleration

Abstract: A modified gravity, which eliminates the need for dark energy and which seems to be stable, is considered. The terms with positive powers of curvature support the inflationary epoch while the terms with negative powers of curvature serve as effective dark energy, supporting current cosmic acceleration. The equivalent scalar-tensor gravity may be compatible with the simplest solar system experiments.

Chameleon Cosmology

Abstract: The evidence for the accelerated expansion of the universe and the time-dependence of the fine-structure constant suggests the existence of at least one scalar field with a mass of order H_0 If such a field exists, then it is generally assumed that its coupling to matter must be tuned to unnaturally small values in order to satisfy the tests of the Equivalence Principle (EP) In this paper, we present an alternative explanation which allows scalar fields to evolve cosmologically while having couplings to matter of order unity In our scenario, the mass of the fields depends on the local matter density: the interaction range is typically of order 1 mm on Earth (where the density is high) and of order 10-10^4 AU in the solar system (where the density is low) All current bounds from tests of General Relativity are satisfied Nevertheless, we predict that near-future experiments that will test gravity in space will measure an effective Newton's constant different by order unity from that on Earth, as well as EP violations stronger than currently allowed by laboratory experiments Such outcomes would constitute a smoking gun for our scenario

Can the dark energy equation - of - state parameter w be less than -1?

Abstract: Models of dark energy are conveniently characterized by the equation-of-state parameter $w=p/\ensuremath{\rho},$ where $\ensuremath{\rho}$ is the energy density and p is the pressure. Imposing the dominant energy condition, which guarantees stability of the theory, implies that $wg~\ensuremath{-}1.$ Nevertheless, it is conceivable that a well-defined model could (perhaps temporarily) have $wl\ensuremath{-}1 ,$ and indeed such models have been proposed. We study the stability of dynamical models exhibiting $wl\ensuremath{-}1$ by virtue of a negative kinetic term. Although naively unstable, we explore the possibility that these models might be phenomenologically viable if thought of as effective field theories valid only up to a certain momentum cutoff. Under our most optimistic assumptions, we argue that the instability time scale can be greater than the age of the universe, but only if the cutoff is at or below ${10}^{\ensuremath{-}3}\mathrm{eV}.$ We conclude that it is difficult, although not necessarily impossible, to construct viable models of dark energy with $wl\ensuremath{-}1;$ observers should keep an open mind, but the burden is on theorists to demonstrate that any proposed new models are not ruled out by rapid vacuum decay.

The CP-conserving two-Higgs-doublet model: the approach to the decoupling limit

Abstract: A CP-even neutral Higgs boson with standard-model-like couplings may be the lightest scalar of a two-Higgs-doublet model. We study the decoupling limit of the most general CP-conserving two-Higgs-doublet model, where the mass of the lightest Higgs scalar is significantly smaller than the masses of the other Higgs bosons of the model. In this case, the properties of the lightest Higgs boson are nearly indistinguishable from those of the standard model Higgs boson. The first nontrivial corrections to Higgs boson couplings in the approach to the decoupling limit are also evaluated. The importance of detecting such deviations in precision Higgs boson measurements at future colliders is emphasized. We also clarify the case in which a neutral Higgs boson can possess standard-model-like couplings in a regime where the decoupling limit does not apply. The two-Higgs-doublet sector of the minimal supersymmetric model illustrates many of the above features.

Primordial density perturbation in the curvaton scenario

Abstract: We analyze the primordial density perturbation when it is generated by a ``curvaton'' field different from the inflaton. In some cases this perturbation may have large isocurvature components, fully correlated or anticorrelated with the adiabatic component. It may also have a significant non-Gaussian component. All of these effects are calculated in a form which will enable direct comparison with current and forthcoming observational data.

Generalized Lorentz invariance with an invariant energy scale

Abstract: The hypothesis that the Lorentz transformations may be modified at Planck scale energies is further explored. We present a general formalism for theories which preserve the relativity of inertial frames with a nonlinear action of the Lorentz transformations on momentum space. Several examples are discussed in which the speed of light varies with energy and elementary particles have a maximum momenta and/or energy. Energy and momentum conservation are suitably generalized and a proposal is made for how the new transformation laws apply to composite systems. We then use these results to explain the ultrahigh-energy cosmic ray anomaly and we find a form of the theory that explains the anomaly, and leads also to a maximum momentum and a speed of light that diverges with energy. We finally propose that the spatial coordinates be identified as the generators of translation in Minkowski spacetime. In some examples this leads to a commutative geometry, but with an energy dependent Planck constant.

Chern-Simons modification of general relativity

Abstract: General relativity is extended by promoting the three-dimensional gravitational Chern-Simons term to four dimensions. This entails choosing an embedding coordinate ${v}_{\ensuremath{\mu}}$---an external quantity, which we fix to be a nonvanishing constant in its time component. The theory is identical to one in which the embedding coordinate is itself a dynamical variable, rather than a fixed, external quantity. Consequently diffeomorphism symmetry breaking is hidden in the modified theory: the Schwarzschild metric is a solution; gravitational waves possess two polarizations, each traveling at the velocity of light; a conserved energy-momentum (pseudo)tensor can be constructed. The modification is visible in the intensity of gravitational radiation: the two polarizations of a gravity wave carry intensities that are suppressed or enhanced by the extension.

High resolution foreground cleaned CMB map from WMAP

Abstract: We perform an independent foreground analysis of the Wilkinson Microwave Anisotropy Probe (WMAP) maps to produce a cleaned cosmic microwave background (CMB) map (available online) useful for cross-correlation with, e.g., galaxy and x-ray maps. We use a variant of the Tegmark-Efstathiou technique that assumes that the CMB has a blackbody spectrum, but is otherwise completely blind, making no assumptions about the CMB power spectrum, the foregrounds, WMAP detector noise or external templates. Compared with the foreground-cleaned internal linear combination map produced by the WMAP team, our map has the advantage of containing less non-CMB power (from foregrounds and detector noise) outside the Galactic plane. The difference is most important on the angular scale of the first acoustic peak and below, since our cleaned map is at the highest ${(12.6}^{\ensuremath{'}})$ rather than lowest ${(49.2}^{\ensuremath{'}})$ WMAP resolution. We also produce a Wiener filtered CMB map, representing our best guess as to what the CMB sky actually looks like, as well as CMB-free maps at the five WMAP frequencies useful for foreground studies. We argue that our CMB map is clean enough that the lowest multipoles can be measured without any galaxy cut, and obtain a quadrupole value that is slightly less low than that from the cut-sky WMAP team analysis. This can be understood from a map of the CMB quadrupole, which shows much of its power falling within the Galaxy cut region, seemingly coincidentally. Intriguingly, both the quadrupole and the octopole are seen to have power suppressed along a particular spatial axis, which lines up between the two, roughly towards $(l,b)\ensuremath{\sim}(\ensuremath{-}110\ifmmode^\circ\else\textdegree\fi{},60\ifmmode^\circ\else\textdegree\fi{})$ in Virgo.

Quasinormal behavior of the D -dimensional Schwarzschild black hole and the higher order WKB approach

Abstract: We study characteristic (quasinormal) modes of a D-dimensional Schwarzschild black hole. It is shown that the real parts of the complex quasinormal modes, representing the real oscillation frequencies, are proportional to the product of the number of dimensions and inverse horizon radius $\ensuremath{\sim}{\mathrm{Dr}}_{0}^{\ensuremath{-}1}.$ The asymptotic formula for large multipole number l and arbitrary D is derived. In addition, the WKB formula for computing QN modes, developed to the third order beyond the eikonal approximation, is extended to the sixth order here. This gives us an accurate and economic way to compute quasinormal frequencies.

How long before the end of inflation were observable perturbations produced

Abstract: We reconsider the issue of the number of e-foldings before the end of inflation at which observable perturbations were generated. We determine a plausible upper limit on that number for the standard cosmology which is around 60, with the expectation that the actual value will be up to 10 below this. We also note a special property of the $\ensuremath{\lambda}{\ensuremath{\varphi}}^{4}$ model which reduces the uncertainties in that case and favors a higher value, giving a fairly definite prediction of 64 e-foldings for that model. We note an extreme (and highly implausible) situation where the number of e-foldings can be even higher, possibly up to 100, and discuss the shortcomings of quantifying inflation by e-foldings rather than by the change in $\mathrm{aH}.$ Finally, we discuss the impact of nonstandard evolution between the end of inflation and the present, showing that again the expected number of e-foldings can be modified, and in some cases significantly increased.

Interacting quintessence solution to the coincidence problem

Abstract: We show that a suitable interaction between a scalar field and a matter fluid in a spatially homogeneous and isotropic spacetime can drive the transition from a matter dominated era to an accelerated expansion phase and simultaneously solve the coincidence problem of our present Universe. For this purpose we study the evolution of the energy density ratio of these two components. We demonstrate that a stationary attractor solution is compatible with an accelerated expansion of the Universe. We extend this study to account for dissipation effects due to interactions in the dark matter fluid. Finally, type Ia supernovae and primordial nucleosynthesis data are used to constrain the parameters of the model.

Refinements in electroweak contributions to the muon anomalous magnetic moment

Abstract: Effects of strong interactions on the two loop electroweak radiative corrections to the muon anomalous magnetic moment, $a_\mu=(g_\mu-2)/2$, are examined. Short-distance logs are shown to be unaffected. Computation of long-distance contributions is improved by use of an effective field theory approach that preserves the chiral properties of QCD and accounts for constraints from the operator product expansion. Small, previously neglected, two loop contributions, suppressed by a $1-4\sin^2\theta_W$ factor, are computed and the complete three loop leading short-distance logs are reevaluated. These refinements lead to a reduction in uncertainties and a slight shift in the total electroweak contribution to $a_\mu^{\rm EW} = 154(1)(2)\times 10^{-11}$ where the first error corresponds to hadronic uncertainties and the second is primarily due to the allowed Higgs mass range.

Next-to-leading order calculation of three-jet observables in hadron-hadron collision

Abstract: The production of three jets in hadron-hadron collisions is the first complex process which allows us to define a branch of variables in order to make more precise measurements of the strong coupling and the parton distribution function of the proton. This process is also suitable for studying the geometrical properties of the hadronic final state at hadron colliders. This requires a next-to-leading order prediction of the three-jet observables. In this paper we describe the theoretical formalism of such a calculation with sufficient detail. We use the dipole method to construct a Monte Carlo program for calculating three-jet observables at next-to-leading order accuracy. We present a theoretical prediction for inclusive and exclusive cross sections and for some relevant event shape variables such as the transverse thrust, transverse jet broadening, and ${E}_{t3}$ variable.

Cosmological dynamics of a phantom field

Abstract: We study the general features of a dynamics of a phantom field in the cosmological context. In the case of an inverse coshyperbolic potential, we demonstrate that the phantom field can successfully drive the observed current accelerated expansion of the Universe with the equation of state parameter ${w}_{\ensuremath{\varphi}}l\ensuremath{-}1.$ The de Sitter universe turns out to be the late time attractor of the model. The main features of the dynamics are independent of the initial conditions and the parameters of the model. The model fits the supernova data very well, allowing for $\ensuremath{-}2.4l{w}_{\ensuremath{\varphi}}l\ensuremath{-}1$ at a 95% confidence level.

Energy in generic higher curvature gravity theories

Abstract: We define and compute the energy of higher curvature gravity theories in arbitrary dimensions. Generically, these theories admit constant curvature vacua (even in the absence of an explicit cosmological constant), and asymptotically constant curvature solutions with nontrivial energy properties. For concreteness, we study quadratic curvature models in detail. Among them, the one whose action is the square of the traceless Ricci tensor always has zero energy, unlike conformal (Weyl) gravity. We also study the string-inspired Einstein-Gauss-Bonnet model and show that both its flat and anti–de Sitter vacua are stable.

Cosmology with tachyon field as dark energy

Abstract: We present a detailed study of cosmological effects of homogeneous tachyon matter coexisting with nonrelativistic matter and radiation, concentrating on the inverse square potential and the exponential potential for the tachyonic scalar field. A distinguishing feature of these models (compared to other cosmological models) is that the matter density parameter and the density parameter for tachyons remain comparable even in the matter dominated phase. For the exponential potential, the solutions have an accelerating phase, followed by a phase with $a(t)\ensuremath{\propto}{t}^{2/3}$ as $\stackrel{\ensuremath{\rightarrow}}{t}\ensuremath{\infty}.$ This eliminates the future event horizon present in cold dark matter models with a cosmological constant $(\ensuremath{\Lambda}\mathrm{CDM})$ and is an attractive feature from the string theory perspective. A comparison with supernova type Ia data shows that for both the potentials there exists a range of models in which the universe undergoes an accelerated expansion at low redshifts which are also consistent with the requirements of structure formation. They do require fine-tuning of parameters but not any more than in the case of $\ensuremath{\Lambda}\mathrm{CDM}$ models or quintessence models.

Phenomenology of the little Higgs model

Abstract: We study the low-energy phenomenology of the little Higgs model. We first discuss the linearized effective theory of the ``littlest Higgs model'' and study the low-energy constraints on the model parameters. We identify sources of the corrections to low-energy observables, discuss model-dependent arbitrariness, and outline some possible directions of extensions of the model in order to evade the precision electroweak constraints. We then explore the characteristic signatures to test the model in the current and future collider experiments. We find that the CERN LHC has great potential to discover the new $\mathrm{SU}(2)$ gauge bosons and the possible new $U(1)$ gauge boson to the multi-TeV mass scale. Other states such as the colored vectorlike quark T and doubly charged Higgs boson ${\ensuremath{\Phi}}^{++}$ may also provide interesting signals. At a linear collider, precision measurements on the triple gauge boson couplings could be sensitive to the new physics scale of a few TeV. We provide a comprehensive list of the linearized interactions and vertices for the littlest Higgs model in the appendices.

Can the Chaplygin gas be a plausible model for dark energy

Abstract: In this paper two cosmological models representing the flat Friedmann universe filled with a Chaplygin fluid, with or without dust, are analyzed in terms of the recently proposed ``statefinder'' parameters. Trajectories of both models in the parameter plane are shown to be significantly different with respect to the ``quiessence'' and ``tracker'' models. The generalized Chaplygin gas model with an equation of state of the form $p=\ensuremath{-}A/{\ensuremath{\rho}}^{\ensuremath{\alpha}}$ is also analyzed in terms of the statefinder parameters.

The state of the dark energy equation of state

Abstract: By combining data from seven cosmic microwave background experiments (including the latest WMAP results) with the Hubble parameter measurement from the Hubble space telescope and luminosity measurements of type Ia supernovae, we demonstrate the bounds on the dark energy equation of state ${w}_{Q}$ to be $\ensuremath{-}1.45l{w}_{Q}l\ensuremath{-}0.74$ at the 95% confidence level. Although our limit on ${w}_{Q}$ is improved with respect to previous analyses, cosmological data do not rule out the possibility that the equation of state parameter ${w}_{Q}$ of the dark energy Q is less than $\ensuremath{-}1.$ We present a tracking model that ensures ${w}_{Q}l~\ensuremath{-}1$ at recent times and discuss the observational consequences.

Impact parameter dipole saturation model

Abstract: We develop a dipole model for DESY HERA deep inelastic scattering data which incorporates the impact parameter distribution of the proton. The model describes the inclusive total ${\ensuremath{\gamma}}^{*}p$ cross sections as well as the diffractive $J/\ensuremath{\psi}$ differential cross sections. We compare the model with previous approaches and show that the t distributions are sensitive to saturation phenomena. We estimate the boundary of the saturation region and show that it dominates the data in the low-${Q}^{2}$ region where the total ${\ensuremath{\gamma}}^{*}p$ cross section exhibits the same universal rise as hadronic cross sections. The model is then extended to nuclei and shows good agreement with the nuclear shadowing data at small x. Finally, we estimate the saturation scale in nuclei.

The Pressure of hot QCD up to g6 ln(1/g)

Abstract: The free energy density, or pressure, of QCD has at high temperatures an expansion in the coupling constant g, known so far up to order g^5. We compute here the last contribution which can be determined perturbatively, g^6 ln(1/g), by summing together results for the 4-loop vacuum energy densities of two different three-dimensional effective field theories. We also demonstrate that the inclusion of the new perturbative g^6 ln(1/g) terms, once they are summed together with the so far unknown perturbative and non-perturbative g^6 terms, could potentially extend the applicability of the coupling constant series down to surprisingly low temperatures.

Finite density QCD via an imaginary chemical potential

Abstract: We study QCD at nonzero temperature and baryon density in the framework of the analytic continuation from an imaginary chemical potential. We carry out simulations of QCD with four flavors of staggered fermions, and reconstruct the phase diagram in the temperature-imaginary $\ensuremath{\mu}$ plane. We consider Ans\"atze for the analytic continuation of the critical line and other observables motivated both by theoretical considerations and mean field calculations in four fermion models and random matrix theory. We determine the critical line, and the analytic continuation of the chiral condensate, up to ${\ensuremath{\mu}}_{B}\ensuremath{\simeq}500\mathrm{MeV}.$ The results are in qualitative agreement with the predictions of model field theories, and consistent with a first order chiral transition. The correlation between the chiral transition and the deconfinement transition observed at $\ensuremath{\mu}=0$ persists at nonzero density.

Higgs boson production in bottom quark fusion at next-to-next-to-leading order

Abstract: The total cross section for Higgs boson production in bottom-quark annihilation is evaluated at next-to-next-to-leading order in QCD. This is the first time that all terms at order ${\ensuremath{\alpha}}_{s}^{2}$ are taken into account. We find a greatly reduced scale dependence with respect to lower order results, for both the factorization and the renormalization scales. The behavior of the result is consistent with earlier determinations of the appropriate factorization scale for this process of ${\ensuremath{\mu}}_{F}\ensuremath{\approx}{M}_{H}/4,$ and supports the validity of the bottom parton density approach for computing the total inclusive rate. We present precise predictions for the cross section at the Fermilab Tevatron and the CERN Large Hadron Collider.

Cosmic microwave background lensing reconstruction on the full sky

Abstract: Gravitational lensing of the microwave background by the intervening dark matter mainly arises from large-angle fluctuations in the projected gravitational potential and hence offers a unique opportunity to study the physics of the dark sector at large scales. Studies with surveys that cover greater than a percent of the sky will require techniques that incorporate the curvature of the sky. We lay the groundwork for these studies by deriving the full sky minimum variance quadratic estimators of the lensing potential from the CMB temperature and polarization fields. We also present a general technique for constructing these estimators, with harmonic space convolutions replaced by real space products, that is appropriate for both the full sky limit and the flat sky approximation. This also extends previous treatments to include estimators involving the temperature-polarization cross-correlation and should be useful for next generation experiments in which most of the additional information from polarization comes from this channel due to sensitivity limitations.

Generalized Israel junction conditions for a Gauss-Bonnet brane world

Abstract: In spacetimes of dimension greater than four it is natural to consider higher order (in $R)$ corrections to the Einstein equations. In this paper generalized Israel junction conditions for a membrane in such a theory are derived. This is achieved by generalizing the Gibbons-Hawking boundary term. The junction conditions are applied to simple brane world models, and are compared to the many contradictory results in the literature.

Calculating quenching weights

Abstract: We calculate the probability (``quenching weight'') that a hard parton radiates an additional energy fraction $\ensuremath{\Delta}E$ due to scattering in spatially extended QCD matter. This study is based on an exact treatment of a finite in-medium path length; it includes the case of a dynamically expanding medium, and it extends to the angular dependence of the medium-induced gluon radiation pattern. All calculations are done in the multiple soft scattering approximation [Baier-Dokshitzer-Mueller-Peign\'e-Schiff-Zakharov (BDMPSZ) formalism] and in the single hard scattering approximation $[N=1$ opacity approximation]. By comparison, we establish a simple relation between the transport coefficient, Debye screening mass and opacity, for which both approximations lead to comparable results. Together with this paper, a CPU-inexpensive numerical subroutine for calculating quenching weights is provided electronically. To illustrate its applications, we discuss the suppression of hadronic transverse momentum spectra in nucleus-nucleus collisions. Remarkably, the kinematic constraint resulting from finite in-medium path lengths reduces significantly the ${p}_{\ensuremath{\perp}}$ dependence of the nuclear modification factor, thus leading to consistency with the data measured at the BNL Relativistic Heavy Ion Collider.

Quantum gravitational corrections to the nonrelativistic scattering potential of two masses

Abstract: We treat general relativity as an effective field theory, obtaining the full nonanalytic component of the scattering matrix potential to one-loop order. The lowest order vertex rules for the resulting effective field theory are presented and the one-loop diagrams which yield the leading nonrelativistic post-Newtonian and quantum corrections to the gravitational scattering amplitude to second order in G are calculated in detail. The Fourier transformed amplitudes yield a nonrelativistic potential and our result is discussed in relation to previous calculations. The definition of a potential is discussed as well, and we show that the ambiguity of the potential under coordinate changes is resolved.

Dynamical Horizons and their Properties

Abstract: A detailed description of how black holes grow in full, non-linear general relativity is presented. The starting point is the notion of dynamical horizons. Expressions of fluxes of energy and angular momentum carried by gravitational waves across these horizons are obtained. Fluxes are local and the energy flux is positive. Change in the horizon area is related to these fluxes. A notion of angular momentum and energy is associated with cross-sections of the horizon and balance equations, analogous to those obtained by Bondi and Sachs at null infinity, are derived. These in turn lead to generalizations of the first and second laws of black hole mechanics. The relation between dynamical horizons and their asymptotic states —the isolated horizons— is discussed briefly. The framework has potential applications to numerical, mathematical, astrophysical and quantum general relativity.

Gauge conditions for long-term numerical black hole evolutions without excision

Abstract: We extend previous work on 3D black hole excision to the case of distorted black holes, with a variety of dynamic gauge conditions that are able to respond naturally to the spacetime dynamics. We show that the combination of excision and gauge conditions we use is able to drive highly distorted, rotating black holes to an almost static state at late times, with well behaved metric functions, without the need for any special initial conditions or analytically prescribed gauge functions. Further, we show for the first time that one can extract accurate waveforms from these simulations, with the full machinery of excision or no excision and dynamic gauge conditions. The evolutions can be carried out for long times, far exceeding the longevity and accuracy of even better resolved 2D codes. While traditional 2D codes show errors in quantities such as apparent horizon mass of over 100% by t ≈ 100M, and crash by t ≈ 150M, with our new techniques the same systems can be evolved for more than hundreds of M’s in full 3D with errors of only a few percent.