We study the infrared behavior of the effective Coulomb potential in lattice SU(3) Yang-Mills theory in the Coulomb gauge. We use lattices up to a size of ${48}^4$ and three values of the inverse coupling, $\beta =5.8$, 6.0, and 6.2. While finite-volume effects are hardly visible in the effective Coulomb potential, scaling violations and a strong dependence on the choice of Gribov copy are observed. We obtain bounds for the Coulomb string tension that are in agreement with Zwanziger’s inequality relating the Coulomb string tension to the Wilson string tension.

We nonperturbatively investigate a fermion spectrum at finite temperature in a chiral invariant linear sigma model. Coupled Schwinger-Dyson equations for fermion and boson are developed in the real time formalism and solved numerically. From the coupling of a massless fermion with a massive boson, the fermion spectrum shows a three-peak structure at some temperatures even for the strong coupling region. This means that the three-peak structure which was originally found in the one-loop calculation is stable against higher order corrections even in the strong coupling region.

In the standard model, scalar contributions to leptonic and semileptonic decays are helicity suppressed. The hypothesis of additional physical neutral/charged Higgses can enhance such scalar contributions and give detectable effects especially in $B$ physics. For the charged Higgs, experimental information on both $\mathrm{Br}(B\to D\tau \nu )$ and $\mathrm{Br}(B\to \tau \nu )$ has already become available, and in particular, the $B\to D\tau \nu$ branching ratio measurements will be further improved in the coming years. Hadronic uncertainties of scalar contributions in semileptonic decays are already in much better shape than the ones plaguing the helicity-suppressed leptonic decays $B\to \tau \nu$. Combining existing experimental information from the $B$ factories, we explore which existing and future lattice estimates will be useful to directly address new physics effects from measurements of $\mathrm{Br}(B_{u,d,s}\to D_{u,d,s}\tau \nu )$, which can be performed also at hadron colliders.

We study the phase diagram and the pion modes in the electrically neutral two flavor Polyakov–Nambu–Jona-Lasinio model. One of the main results of this paper is that in the model with massive quarks, when electrical neutrality is required, pions do not condense in the ground state of the model: the isospin chemical potential ${\mu }_I=-{\mu }_e/2$ is always smaller than the value required for pion condensation to occur. Moreover we investigate on the pions and $\sigma$ mass spectra. We find that the qualitative behavior of the masses resembles that obtained in the Nambu–Jona-Lasinio model. We close this paper by studying the intriguing possibility that bound states with the quantum numbers of the pions can be formed above the chiral phase transition.

Several spin and flavor dependent parameters characterizing the strangeness content of the nucleon have been calculated in the chiral constituent quark model with configuration mixing ($\chi {\mathrm{CQM}}_{\mathrm{config}}$) which is known to provide a satisfactory explanation of the “proton spin crisis” and related issues. In particular, we have calculated the strange spin polarization $\Delta s$, the strangeness contribution to the weak axial vector couplings ${\Delta }_8$ etc., strangeness contribution to the magnetic moments $\mu (p{)}^s$ etc., the strange quark flavor fraction $f_s$, the strangeness dependent quark flavor ratios $\frac{2\overline{s}}{u+d}$ and $\frac{2\overline{s}}{\overline{u}+\overline{d}}$ etc. Our results are consistent with the recent experimental observations.

We consider a modification of the standard Einstein theory in four dimensions, alternative to R. Jackiw and S.-Y. Pi, Phys. Rev. D 68, 104012 (2003), since it is based on the first-order (Einstein-Cartan) approach to general relativity, whose gauge structure is manifest. This is done by introducing an additional topological term in the action which becomes a Lorentz-violating term by virtue of the dependence of the coupling on the space-time point. We obtain a condition on the solutions of the Einstein equations, such that they persist in the deformed theory, and show that the solutions remarkably correspond to the classical solutions of a collection of independent $2+1$-dimensional (topological) Chern-Simons gravities. Finally, we study the relation with the standard second-order approach and argue that they both coincide to leading order in the modulus of the Lorentz-violating vector field.

We show that any magnetostatic axially symmetric solution of the Einstein-Maxwell equations can be endowed with a specific charged fluid source of the Polanco et al. type via a simple procedure requiring the knowledge of exclusively the magnetostatic seed spacetime. Using this procedure we construct yet another exact solution for a massive magnetic dipole surrounded by a static charged fluid which is different from the Polanco et al. metric.

We explore the formation of cosmic string Y-junctions when strings of two different types collide, which has recently become important since string theory can yield cosmic strings of distinct types. Using a model containing two types of local U(1) string and stable composites, we simulate the collision of two straight strings and investigate whether the dynamics matches that previously obtained using the Nambu-Goto action, which is not strictly valid close to the junction. We find that the Nambu-Goto action performs only moderately well at predicting when the collision results in the formation of a pair of Y-junctions (with a composite string connecting them). However, we find that when they do form, the late-time dynamics matches those of the Nambu-Goto approximation very closely. We also see little radiative emission from the Y-junction system, which suggests that radiative decay due to bridge formation does not appear to be a means via which a cosmological network of such string would rapidly lose energy.

I calculate a hybrid cross-power spectrum estimator from the WMAP 5-year CMB temperature maps, discuss the goodness of fit, and then constrain cosmological parameters. The spectrum and results are generally consistent with previous results, though the power spectrum error bars are slightly smaller and there are small shifts at high $l$. The small improvement in error bars is obtained at very low numerical cost but does not significantly improve parameter constraints. I discuss the accuracy of the likelihood model and how constraints on the optical depth translate into constraints on the reionization history allowing for helium reionization. In the appendices I propose a simple reionization parametrization that determines the history in terms of a midpoint reionization redshift, and suggest a new likelihood approximation for chi-squared-like distributions with varying skewness.

We describe higher-dimensional theories whose low-energy 4D descriptions contain larger gauge or global symmetry groups. As an example, we construct a Higgsless $\mathrm{SU}(2)\times \mathrm{U}(1{)}_{B-L}$ model of electroweak symmetry breaking. The 5D SU(2) gauge invariance contains both the weak SU(2) gauge group and a custodial symmetry that protects the rho parameter. Fermions obtain isospin-violating masses while maintaining universal gauge couplings among all three generations. As a further example, we construct a model of chiral color based on a single SU(3).

I derive and systematically analyze scalar glueball correlation functions in both the hard-wall and dilaton soft-wall approximations to holographic QCD. The dynamical content of the holographic correlators is uncovered by examining their spectral density and by relating them to the operator product expansion, a dilatational low-energy theorem and a recently suggested two-dimensional power correction associated with the short-distance behavior of the heavy-quark potential. This approach provides holographic estimates for the three lowest-dimensional gluon condensates or alternatively their Wilson coefficients, the two leading moments of the instanton size distribution in the QCD vacuum and an effective UV gluon mass. A remarkable complementarity between the nonperturbative physics of the hard- and soft-wall correlators emerges, and their ability to describe detailed QCD results can be assessed quantitatively. I further provide the first holographic estimates for the decay constants of the $0^{++}$ glueball and its excitations. The hard-wall background turns out to encode more of the relevant QCD physics, and its prediction $f_S\simeq 0.8–0.9\mathrm{GeV}$ for the phenomenologically important ground-state decay constant agrees inside errors with recent QCD sum rule and lattice results.

A number of theories, spanning a wide range of mass scales, predict dark matter candidates that have lifetimes much longer than the age of the Universe, yet may produce a significant flux of gamma rays in their decays today. We constrain such late-decaying dark matter scenarios model-independently by utilizing gamma-ray line emission limits from the Galactic Center region obtained with the SPI spectrometer on INTEGRAL, and the determination of the isotropic diffuse photon background by SPI, COMPTEL, and EGRET observations. We show that no more than $\sim 5\%$ of the unexplained MeV background can be produced by late dark matter decays either in the Galactic halo or cosmological sources.

We study the dynamics and evolution of Nambu-Goto strings in a warped spacetime, where the warp factor is a function of the internal coordinates giving rise to a “throat” region. The microscopic equations of motion for strings in this background include potential and friction terms, which attract the strings towards the bottom of the warping throat. However, by considering the resulting macroscopic equations for the velocities of strings in the vicinity of the throat, we note the absence of enough classical damping to guarantee that the strings actually reach the warped minimum and stabilize there. Instead, our classical analysis supports a picture in which the strings experience mere deflections and bounces around the tip, rather than strongly damped oscillations. Indeed, 4D Hubble friction is inefficient in the internal dimensions and there is no other classical mechanism known, which could provide efficient damping. These results have potentially important implications for the intercommuting probabilities of cosmic superstrings.

The subject of cosmological hydrogen recombination has received much attention recently because of its importance to predictions for and cosmological constraints from cosmic microwave background observations. While the central role of the two-photon decay $2s\to 1s$ has been recognized for many decades, high-precision calculations require us to consider two-photon decays from the higher states $ns$, $nd\to 1s$ ($n\ge 3$). Simple attempts to include these processes in recombination calculations with an effective two-photon decay coefficient analogous to the $2s$ decay coefficient ${\Lambda }_{2s}=8.22{\mathrm{s}}^{-1}$ have suffered from physical problems associated with the existence of kinematically allowed sequences of one-photon decays, e.g. $3d\to 2p\to 1s$, that technically also produce two photons. These correspond to resonances in the two-photon spectrum that are optically thick to two-photon absorption, necessitating a radiative transfer calculation. We derive the appropriate equations, develop a numerical code to solve them, and verify the results by finding agreement with analytic approximations to the radiative transfer equation. The related processes of Raman scattering and two-photon recombination are included using similar machinery. Our results show that early in recombination the two-photon decays act to speed up recombination, reducing the free electron abundance by 1.3% relative to the standard calculation at $z=1300$. However, we find that some photons between $\mathrm{Ly}\alpha$ and $\mathrm{Ly}\beta$ are produced, mainly by $3d\to 1s$ two-photon decay and $2s\to 1s$ Raman scattering. At later times, these photons redshift down to $\mathrm{Ly}\alpha$, excite hydrogen atoms, and act to slow recombination. Thus, the free electron abundance is increased by 1.3% relative to the standard calculation at $z=900$. Our calculation involves a very different physical argument than the recent studies of Wong and Scott and Chluba and Sunyaev, and produces a much larger effect on the ionization history. The implied correction to the cosmic microwave background power spectrum is negligible for the recently released WMAP and ACBAR data, but at Fisher matrix level will be $7\sigma$ for Planck.

To enforce the conservation of baryon number $B$ and not lepton number $L$ in supersymmetry, a new $U(1{)}_X$ gauge symmetry is recommended. An example is offered with new particles interacting under $U(1{)}_X$ which are good candidates for the dark matter of the Universe.

We discuss how naturalness predicts the scale of new physics. Two conditions on the scale are considered. The first is the more conservative condition due to Veltman [Acta Phys. Pol. B 12, 437 (1981)]. It requires that radiative corrections to the electroweak mass scale be reasonably small. The second is the condition due to Barbieri and Giudice [Nucl. Phys. B306, 63 (1988)], which is more popular lately. It requires that physical mass scale not be oversensitive to the values of the input parameters. We show here that the above two conditions behave differently if higher-order corrections are taken into account. Veltman’s condition is robust (insensitive to higher-order corrections), while the Barbieri-Giudice condition changes qualitatively. We conclude that higher-order perturbative corrections take care of the fine-tuning problem, and, in this respect, scalar field is a natural system. We apply the Barbieri-Giudice condition with higher-order corrections taken into account to the standard model and obtain new restrictions on the Higgs boson mass.

We argue that a version of the four dimensional Brans-Dicke theory can be embedded in the standard flat spacetime electroweak theory. The embedding involves a change of variables that separates the isospin from the hypercharge in the electroweak theory.

We consider the generalized monopole in the five-dimensional Euclidean space. A numerical solution with the hedgehog ansatz is studied. The Bogomol’nyi equation becomes a second-order autonomous nonlinear differential equation. The equation can be translated into the Abel’s differential equation of the second kind and is an algebraic differential equation.

Recently the partial-wave cutoff method was developed as a new calculational scheme for a functional determinant of quantum field theory in radial backgrounds. For the contribution given by an infinite sum of large partial waves, we derive explicitly radial-WKB series in the angular momentum cutoff for $d=2$, 3, 4, and 5 ($d$ is the space-time dimension), which has uniform validity irrespectively of any specific values assumed for other parameters. Utilizing this series, precision evaluation of the renormalized functional determinant is possible with a relatively small number of low partial-wave contributions determined separately. We illustrate the power of this scheme in a numerically exact evaluation of the prefactor (expressed as a functional determinant) in the case of the false vacuum decay of 4D scalar field theory.

We present a new numerical scheme to solve the initial value problem for black hole–neutron star binaries. This method takes advantage of the flexibility and fast convergence of a multidomain spectral representation of the initial data to construct high-accuracy solutions at a relatively low computational cost. We provide convergence tests of the method for both isolated neutron stars and irrotational binaries. In the second case, we show that we can resolve the small inconsistencies that are part of the quasiequilibrium formulation, and that these inconsistencies are significantly smaller than observed in previous works. The possibility of generating a wide variety of initial data is also demonstrated through two new configurations inspired by results from binary black holes. First, we show that choosing a modified Kerr-Schild conformal metric instead of a flat conformal metric allows for the construction of quasiequilibrium binaries with a spinning black hole. Second, we construct binaries in low-eccentricity orbits, which are a better approximation to astrophysical binaries than quasiequilibrium systems.