We study an exact model of string theory propagating in a space-time containing regions with closed timelike curves (CTCs) separated from a finite cosmological region bounded by a big bang and a big crunch. The model is an nontrivial embedding of the Taub-NUT geometry into heterotic string theory with a full conformal field theory (CFT) definition, discovered over a decade ago as a heterotic coset model. Having a CFT definition makes this an excellent laboratory for the study of the stringy fate of CTCs, the Taub cosmology, and the Milne/Misner-type chronology horizon which separates them. In an effort to uncover the role of stringy corrections to such geometries, we calculate the complete set of ${\alpha }^{\prime }$ corrections to the geometry. We observe that the key features of Taub-NUT persist in the exact theory, together with the emergence of a region of space with Euclidean signature bounded by timelike curvature singularities. Although such remarks are premature, their persistence in the exact geometry is suggestive that string theory is able to make physical sense of the Milne/Misner singularities and the CTCs, despite their pathological character in general relativity. This may also support the possibility that CTCs may be viable in some physical situations, and may be a natural ingredient in pre-big bang cosmological scenarios.

I use the recently developed massive field approach to calculate the scalar self-force on a static particle in a Schwarzschild spacetime. In this approach the scalar self-force is obtained from the difference between the (massless) scalar field, and an auxiliary massive scalar field combined with a certain limiting process. By applying this approach to a static particle in Schwarzschild I show that the scalar self-force vanishes in this case. This result conforms with a previous analysis [A. G. Wiseman, Phys. Rev. D 61, 084014 (2000).].

Model-independent observables to pick out the Abelian $Z^{\prime }$ signal in the Bhabha process are introduced at energies $\sqrt{s}\ge 200\mathrm{G}\mathrm{e}\mathrm{V}$. They measure separately the $Z^{\prime }$-induced vector and axial-vector four-fermion contact couplings. The analysis of the LEP2 data constrains the value of the $Z^{\prime }$-induced vector four-fermion coupling at the $2\sigma$ confidence level that corresponds to the Abelian $Z^{\prime }$ boson with the mass of order 1 TeV.

We consider a model with a small explicit breaking of a global symmetry, as suggested by gravitational arguments. Our model has one scalar field transforming under a nonanomalous $\mathrm{U}(1)$ symmetry, and coupled to matter and to gauge bosons. The spontaneous breaking of the explicitly broken symmetry gives rise to a massive pseudo-Goldstone boson. We analyze thermal and nonthermal production of this particle in the early universe, and perform a systematic study of astrophysical and cosmological constraints on its properties. We find that for very suppressed explicit breaking the pseudo-Goldstone boson is a cold dark matter candidate.

In models where the gravitino is the lightest supersymmetric particle, the next-to-lightest supersymmetric particle (NLSP) is long-lived. We consider an important charged NLSP candidate, the scalar tau $\tilde{\tau }$. Slow charged NLSPs may be produced at future colliders and they may be stopped in a massive stopper which simultaneously serves as a detector for the NLSP and its decay products. We found the number of events at a 1 kton to $\mathrm{O}(10) \mathrm{k}\mathrm{t}\mathrm{o}\mathrm{n}$ detector could be significant enough to study the NLSP decays with lifetime shorter than ${10}^{10}\sec $ at the LHC. The performance of existing 1 kton detectors may be good enough to do such studies at the LHC, if they can be placed close to the ATLAS/CMS detectors. At a future $e^{-}{e}^{-}$ collider, scalar electrons ${\tilde{e}}^{-}$'s are copiously produced. Slow NLSPs may be produced from the ${\tilde{e}}^{-}$ decay. The number of stopped NLSPs at a future linear collider could be large enough to study rare decay modes of the NLSP.

Collective quantization of a $B=1$ gravitating skyrmion is described. The rotational and isorotational modes are quantized in the same manner as the skyrmion without gravity. It is shown in this paper how the static properties of nucleons such as masses, charge densities, magnetic moments are modified by the gravitational interaction.

Within two-flavor approximation for the left right model, the equations connecting the parameters of the heavy neutrino sector and the charged Higgs sector (CHS) are found. For definition of the CHS parameters the impact of the Higgs bosons on lepton flavor violation decays, low-energy light neutrino scattering, and the muon anomalous magnetic moment are investigated. It is shown, that the heavy neutrino masses $m_{N_{1,2}}$ can be expressed in terms of the triplet Yukawa coupling constants and the mass of the $W_2$ gauge boson only. Using the data of inverse muon decays, and also the constraints on the masses of the ${\tilde{\delta }}^{(-)}$, ${\Delta }_{1,2}^{(-)}$ and $W_2$ bosons the limits on $m_{N_{1,2}}$ are established in the cases both of the absence and the presence of the heavy neutrino masses (quasi)degeneration. The data on explanation of the $(g-2{)}_{\mu }$ anomaly is used for determination of the constraints on $m_{N_{1,2}}$ in the case of the masses (quasi)degeneration only. The suggested scheme for estimations of the heavy neutrino masses and mixing, based on the Higgs sector investigation, can be used for any gauge electroweak theory with an extended Higgs sector and the “see-saw” mechanism.

I study the very recently discovered $S=-2$ resonance ${\Xi }^{--}(1886)$ by the NA49 Collaboration at the CERN SPS. I find that repulsion excludes the ${\Xi }^{--}$ as a $ddss\overline{u}$ $s$-wave pentaquark. I explore the ${\Xi }^{--}$ as a heptaquark, equivalent to a $N+K+K$ linear molecule, with positive parity and total isospin $I=3/2$. I find that the kaon-kaon repulsion is canceled by the attraction existing in the kaon-nucleon channels. In our framework this state is easier to bind than the ${\Theta }^{+}$ described by a $\pi +K+N$ Borromean bound state. The remaining $I=1/2$ doublet and $I=1$ triplet of the exotic antidecuplet are also studied, and the coupling to $p$-wave decay channels is briefly addressed.

We present a search for $B$ decays to charmless final states involving charged or neutral $a_0$ mesons. The data sample corresponds to $89\times {10}^6$ $B\overline{B}$ pairs collected with the BABAR detector operating at the PEP-II asymmetric-energy $B$ Factory at Stanford Linear Accelerator Center. We find no significant signals and determine the following 90% C.L. upper limits: $\mathcal{B}(B^0\to a_0^{-}{\pi }^{+})<5.1\times {10}^{-6}$, $\mathcal{B}(B^0\to a_0^{-}{K}^{+})<2.1\times {10}^{-6}$, $\mathcal{B}(B^{-}\to a_0^{-}{\overline{K}}^0)<3.9\times {10}^{-6}$, $\mathcal{B}(B^{+}\to a_0^0{\pi }^{+})<5.8\times {10}^{-6}$, $\mathcal{B}(B^{+}\to a_0^0{K}^{+})<2.5\times {10}^{-6}$, and $\mathcal{B}(B^0\to a_0^0{K}^0)<7.8\times {10}^{-6}$, where in all cases $\mathcal{B}$ indicates the product of branching fractions for $B\to a_{0}X$ and $a_0\to \eta \pi$, where $X$ indicates $K$ or $\pi$.

In this paper, globally $\mathcal{N}=1$ supersymmetric configurations of intersecting D6-branes on the ${\mathbb{Z}}_6$ orientifold are discussed, involving also fractional branes. It turns out rather miraculously that one is led almost automatically to just one particular class of five stack models containing the standard model gauge group, which all have the same chiral spectrum. The further discussion shows that these models can be understood as exactly the supersymmetric standard model without any exotic chiral symmetric/antisymmetric matter. The superpartner of the Higgs finds a natural explanation and the hypercharge remains massless. However, the nonchiral spectrum within the model class is very different and does not in all cases allow for a $\mathcal{N}=2$ low energy field theoretical understanding of the necessary breaking $\mathrm{U}(1)\times \mathrm{U}(1)\to \mathrm{U}(1)$ along the Higgs branch, which is needed in order to get the standard Yukawa couplings. Also the left-right symmetric models belong to exactly one class of chiral spectra, where the two kinds of exotic chiral fields can have the interpretation of forming a composite Higgs. The aesthetical beauty of these models, involving only nonvanishing intersection numbers of an absolute value three, seems to be unescapable.

We calculate the amplitude of gravitational waves produced by inflation on a de Sitter brane embedded in five-dimensional anti-de Sitter bulk space-time, extending previous calculations in Randall-Sundrum type cosmology to include the effect of induced gravity corrections on the brane. These corrections arise via a term in the brane action that is proportional to the brane Ricci scalar. We find that, as in the Randall-Sundrum case, there is a mass-gap between the discrete zero-mode and a continuum of massive bulk modes, which are too heavy to be excited during inflation. We give the normalization of the zero-mode as a function of the Hubble rate on the brane and are thus able to calculate the high-energy correction to the spectrum of gravitational wave (tensor) modes excited on large scales during inflation from initial vacuum fluctuations on small scales. We also calculate the amplitude of density (scalar) perturbations expected due to inflaton fluctuations on the brane, and show that the usual four-dimensional consistency relation for the tensor/scalar ratio remains valid for brane inflation with induced gravity corrections.

Using the framework of the chromodielectric model we perform an analysis of color electric flux tubes in mesonlike $q\overline{q}$ and baryonlike $qqq$ quark configurations. We discuss the Abelian color structure of the model and point out a symmetry in color space as a remnant of the SU(3) symmetry of QCD. The generic features of the model are discussed by varying the model parameters. We fix these parameters by reproducing the string tension $\tau =980\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{f}\mathrm{m}$ and the transverse width $\rho =0.35\mathrm{f}\mathrm{m}$ of the $q\overline{q}$ flux tube obtained in lattice calculations. We use a bag constant $B^{1/4}=(240-260)\mathrm{M}\mathrm{e}\mathrm{V}$, a glueball mass $m_g=(1000-1700)\mathrm{M}\mathrm{e}\mathrm{V}$, and a strong coupling constant $C_F{\alpha }_s=0.2-0.3$. We show that the asymptotic string profile of an infinitely long flux tube is already reached for $q\overline{q}$ separations $R\ge 1.0\mathrm{f}\mathrm{m}$. A connection to the dual color superconductor is made by extracting a magnetic current from the model equations and a qualitative agreement between the two descriptions of confinement is shown. In the study of the $qqq$ system we observe a $\Delta$-like geometry for the color electric fields and a $\mathsf{Y}$-like geometry in the scalar fields both in the energy density distribution and in the corresponding potentials. The resulting total $qqq$ potential is described neither by the $\Delta$-picture nor by the $\mathsf{Y}$-picture alone.

The general issue of large $N_c$ QCD at nonzero chemical potential is considered with a focus on understanding the difference between large $N_c$ QCD with an isospin chemical potential and large $N_c$ QCD with a baryon chemical potential. A simple diagrammatic analysis analogous to 't Hooft’s analysis at $\mu =0$ implies that the free energy with a given baryon chemical potential is equal to the free energy with an isospin chemical potential of the same value plus $1/N_c$ corrections. Phenomenologically, these two systems behave quite differently. A scenario to explain this difference in light of the diagrammatic analysis is explored. This scenario is based on a phase transition associated with pion condensation when the isospin chemical potential exceeds $m_{\pi }/2$; associated with this transition there is breakdown of the $1/N_c$ expansion—in the pion condensed phase there is a distinct $1/N_c$ expansion including a larger set of diagrams. While this scenario is natural, there are a number of theoretical issues which at least superficially challenge it. Most of these can be accommodated. However, the behavior of quenched QCD which raises a number of apparently analogous issues cannot be easily understood completely in terms of an analogous scenario. Thus, the overall issue remains open.

We investigate the interaction of fermions having both Dirac and left-handed and right-handed Majorana mass terms with vacuum domain walls. By solving the equations of motion in a thin-wall approximation, we calculate the reflection and transmission coefficients for the scattering of fermions off walls.

Heavy baryon chiral perturbation theory for pentaquarks is applied beyond leading order. The mass splitting in the pentaquark antidecuplet is calculated up to next-to-next-to-leading order in the absence of other exotic multiplets nearby in mass. An expansion in the coupling of the antidecuplet to nonexotic baryons simplifies calculations and makes the pentaquark masses insensitive to the pentaquark-nucleon mass difference. It is assumed that no other pentaquark multiplets are nearby in the mass spectrum. The possibility of determining coupling constants in the chiral Lagrangian on the lattice is discussed. Both positive and negative parities are considered.

We compare the prediction of Skyrme-like effective Lagrangians with data for electromagnetic form factors of nucleons and consider the possibility of fixing the parameters of these higher-order Lagrangians. Our results indicate that one- or two-parameter models can lead to better agreement with the data but more accurate determination of the effective Lagrangian faces theoretical uncertainties.

We perform various resummations of the hot QCD pressure based on the actual knowledge of the perturbation series which includes the $\sim g_s^6\ln (1/g_s)$ and part of the $\sim g_s^6$ terms. Resummations are performed separately for the short- and long-distance parts. The $\sim g_s^6$ term of the short-distance pressure is estimated on the basis of the known UV cutoff dependence of the long-distance part. The resummations are of the Padé and Borel-Padé type, using in addition the (Padé)resummed expression for the squared Debye screening mass $m_{\mathrm{E}}^2$ and, in some cases, even for the electrostatic QCD coupling parameter $g_{\mathrm{E}}^2$. The resummed results depend weakly on the yet unknown $\sim g_s^6$ terms and on the short-range renormalization scale, at all temperatures. The dependence on the long-range renormalization scale is appreciable at low temperatures $T\lesssim 1\mathrm{G}\mathrm{e}\mathrm{V}$. The resulting dependence of pressure on temperature $T$ is compatible with the results of the lattice calculations at low $T$.

This paper presents a first study of the pseudoscalar meson masses to two loops, or next-to-next-to-leading order, within the supersymmetric formulation of partially quenched chiral perturbation theory ($\mathrm{P}\mathrm{Q}\chi \mathrm{P}\mathrm{T}$). The expression for the pseudoscalar meson mass in the case of three valence and three sea quarks with equal masses, but different from each other, is given to $\mathcal{O}(p^6)$, along with a numerical analysis.

Neutrino oscillation experiments provide a unique tool for probing neutrino-matter interactions, especially those involving the tau neutrino. We describe the sensitivity of the present atmospheric neutrino data to these interactions in the framework of a three-flavor analysis. Compared to the two-flavor ${\nu }_{\mu }-{\nu }_{\tau }$ analyses, we find qualitatively new features, in particular, that large nonstandard interactions, comparable in strength to those in the Standard Model, can be consistent with the data. The existence of such interactions could imply a smaller value of the neutrino mixing angle and larger value of the mass-squared splitting than in the case of standard interactions only. This and other effects of nonstandard interactions may be tested in the next several years by MINOS, KamLAND, and solar neutrino experiments.

The energy spectrum of an isotropic harmonic oscillator as a function of the noncommutativity parameter $\Theta$ is studied. It is shown that for a dense set of values of $\Theta$ the spectrum is degenerated and the algebra responsible for degeneracy can always be chosen to be SU(2). The generators of the algebra are constructed explicitly.