This paper discusses global properties of exact (in ${\alpha }^{\prime }$) string theory solutions: A deformed black hole solution in two dimensions and a Taub-NUT type solution in four dimensions. These models are exact by virtue of having CFT descriptions in terms of heterotic coset models. The analysis includes analytic continuations of the metric, motion of test particles, and the $T$-duality which acts as a map between different regions of the extended solutions, rendering the physical spacetimes nonsingular.

Recently certain nonsupersymmetric solutions of type IIb supergravity were constructed [V. Jejjala, O. Madden, S. F. Ross, and G. Titchener, Phys. Rev. D 71, 124030 (2005).], which are everywhere smooth, have no horizons and are thought to describe certain non-Bogomolnyi-Prasad-Sommerfield microstates of the D1-D5 system. We demonstrate that these solutions are all classically unstable. The instability is a generic feature of horizonless geometries with an ergoregion. We consider the end point of this instability and argue that the solutions decay to supersymmetric configurations. We also comment on the implications of the ergoregion instability for Mathur’s fuzzball proposal.

Newly formed black holes are expected to emit characteristic radiation in the form of quasinormal modes, called ringdown waves, with discrete frequencies. LISA should be able to detect the ringdown waves emitted by oscillating supermassive black holes throughout the observable Universe. We develop a multimode formalism, applicable to any interferometric detectors, for detecting ringdown signals, for estimating black-hole parameters from those signals, and for testing the no-hair theorem of general relativity. Focusing on LISA, we use current models of its sensitivity to compute the expected signal-to-noise ratio for ringdown events, the relative parameter estimation accuracy, and the resolvability of different modes. We also discuss the extent to which uncertainties on physical parameters, such as the black-hole spin and the energy emitted in each mode, will affect our ability to do black-hole spectroscopy.

The “optical springs” regime of the signal-recycled configuration of laser interferometric gravitational-wave detectors is analyzed taking into account optical losses in the interferometer arm cavities. This regime allows one to obtain sensitivity better than the standard quantum limits both for a free test mass and for a conventional harmonic oscillator. The optical losses restrict the gain in sensitivity and achievable signal-to-noise ratio. Nevertheless, for parameter values planned for the Advanced LIGO gravitational-wave detector, this restriction is insignificant.

We study the possibility to explain the nonbaryonic dark matter abundance and improve the present fits on the muon anomalous magnetic moment through the same new physics. In this work we show that massive brane fluctuations (branons) in large extra-dimensions models can provide an economical way to deal with these two issues. This is so because the low-energy branon physics depends effectively on essentially only three parameters. Next collider experiments, such as LHC or ILC, will be sensitive to branon phenomenology in the natural parameter region where the theory is able to account for the two effects.

We conduct a general discussion of supersymmetric models with three families in the Higgs sector. We analyze the scalar potential and investigate the minima conditions, deriving the mass matrices for the scalar, pseudoscalar, and charged states. Depending on the Yukawa couplings and the Higgs spectrum, the model might allow the occurrence of potentially dangerous flavor-changing neutral currents at the tree level. We compute model-independent contributions for several observables, and as an example we apply this general analysis to a specific model of quark-Higgs interactions, discussing how compatibility with current experimental data constrains the Higgs sector.

The requirement of the $\mu -\tau$ symmetry in the neutrino sector that yields the maximal atmospheric neutrino mixing is shown to yield either $\sin {\theta }_{13}=0$ (referred to as C1)) or $\sin {\theta }_{12}=0$ (referred to as C2)), where ${\theta }_{12(13)}$ stands for the solar (reactor) neutrino mixing angle. We study general properties possessed by approximately $\mu -\tau$ symmetric textures. It is argued that the tiny $\mu -\tau$ symmetry breaking generally leads to $\cos 2{\theta }_{23}\sim \sin {\theta }_{13}$ for C1) and $\cos 2{\theta }_{23}\sim \Delta m_{\odot }^2/\Delta m_{\mathrm{atm}}^2(\equiv R)$ for C2), which indicates that the smallness of $\cos 2{\theta }_{23}$ is a good measure of the $\mu -\tau$ symmetry breaking, where $\Delta m_{\mathrm{atm}}^2$ ($\Delta m_{\odot }^2$) stands for the square mass difference of atmospheric (solar) neutrinos. We further find that the relation $R\sim {sin}^2{\theta }_{13}$ arises from contributions of $\mathcal{O}({sin}^2{\theta }_{13})$ in the estimation of the neutrino masses ($m_{1,2,3}$) for C1), and that possible forms of textures are strongly restricted to realize ${sin}^{2}2{\theta }_{12}=\mathcal{O}(1)$ for C2). To satisfy $R\sim {sin}^2{\theta }_{13}$ for C1), neutrinos exhibit the inverted mass hierarchy, or the quasi degenerate mass pattern with $|m_{1,2,3}|\sim \mathcal{O}(\sqrt{\Delta m_{\mathrm{atm}}^2})$, and, to realize ${sin}^{2}2{\theta }_{12}=\mathcal{O}(1)$ for C2), there should be an additional small parameter $\eta$ whose size is comparable to that of the $\mu -\tau$ symmetry breaking parameter $\epsilon$, giving $\tan 2{\theta }_{12}\sim \epsilon /\eta$ with $\eta \sim \epsilon$ to be compatible with the observed large mixing.

The scalar partner of the top quark is relatively light in many models of supersymmetry breaking. We study the production of top squarks (stops) at the Tevatron collider and their subsequent decay through baryon-number violating couplings such that the final state contains no leptons. Performing a detector-level analysis, we demonstrate that, even in the absence of leptons or missing energy, stop masses up to $210\mathrm{GeV}/c^2$ can be accessible at the Tevatron.

We carry out a detailed analysis of the nonminimal supersymmetric standard model with lepton number violation. The model contains a unique trilinear lepton number violating term in the superpotential which can give rise to neutrino masses at the tree level. We search for the gauged discrete symmetries realized by cyclic groups $Z_N$ which preserve the structure of the associated trilinear superpotential of this model, and which satisfy the constraints of the anomaly cancellation. The implications of this trilinear lepton number violating term in the superpotential and the associated soft supersymmetry breaking term on the phenomenology of the light neutrino masses and mixing is studied in detail. We evaluate the tree and one-loop contributions to the neutrino mass matrix in this model. We search for possible suppression mechanism which could explain large hierarchies and maximal mixing angles.

Sneutrino-antisneutrino mixing occurs in a supersymmetric model where neutrinos have nonzero Majorana masses. This can lead to the sneutrino decaying into a final state with a “wrong-sign charged lepton”. Hence, in an $e^{-}\gamma$ collider, the signal of the associated production of an electron-sneutrino and the lighter chargino and their subsequent decays can be $e^{-}\gamma \to e^{+}{\tilde{\tau }}_1^{-}{\tilde{\tau }}_1^{-}+{\mathrm{p̸}}_T$ where the ${\tilde{\tau }}_1$s are long-lived and can produce heavily ionizing charged tracks. This signal is free of any Standard Model background and the supersymmetric backgrounds are small. Such a signal can be experimentally observable under certain conditions which are possible to obtain in an anomaly-mediated supersymmetry breaking scenario. Information on a particular combination of the neutrino masses and mixing angles can also be extracted through the observation of this signal. Possible modifications in the signal event and the accompanying Standard Model background have been discussed when the ${\tilde{\tau }}_1$s decay promptly.

We calculate the perturbative value of the free energy in lattice QCD in three dimensions, up to three loops. Our calculation is performed using the Wilson formulation for gluons in $SU(N)$ gauge theories. The free energy is directly related to the average plaquette. To carry out the calculation, we compute the coefficients involved in the perturbative expansion of the Free Energy up to three loops, using an automated set of procedures developed by us in Mathematica. The dependence on $N$ is shown explicitly in our results. For purposes of comparison, we also present the individual contributions from every diagram. These have been obtained by means of two independent calculations, in order to cross check our results.

We find the contribution of constituent quark-loop mechanism to the branching ratio of ${\pi }^0\to 4\gamma$ process to be $B_{4\gamma }^{\mathrm{hadr}}={\Gamma }_{{\pi }^0\to 4\gamma }/{\Gamma }_{{\pi }^0\to 2\gamma }\approx 5.45·{10}^{-16}$ for the reasonable choice of constituent quark mass $m\approx 280\mathrm{MeV}$. This result is in agreement with the vector-dominance approach result obtained years ago. Thus the main contribution arises from the QED mechanism ${\pi }^0\to \gamma ({\gamma }^{*})\to \gamma (3\gamma )$ including light-light scattering block with electron loop. This contribution was investigated in the paper of one of us and gave $B_{4\gamma }^{\mathrm{QED}}\sim 2.6·{10}^{-11}$.

We define a general scheme for the evolution of fragmentation functions which resums both soft gluon logarithms and mass singularities in a consistent manner and to any order, and requires no additional theoretical assumptions. Using the double logarithmic approximation and the known perturbative results for the splitting functions, we present our scheme with the complete contribution from the double logarithms, being the largest soft gluon logarithms. We show that the resulting approximation is more complete than the modified leading logarithm approximation even with the fixed order contribution calculated to leading order only, and find, after using it to fit quark and gluon fragmentation functions to experimental data, that this approximation in our scheme gives a good description of the data from the largest $x_p$ values to the peak region in $\xi =\ln (1/x_p)$, in contrast to other approximations. In addition, we develop a treatment of hadron mass effects which gives additional improvements at large $\xi$.

At NNLO it is particularly important to have a Variable-flavor Number Scheme (VFNS) to deal with heavy quarks because there are major problems with both the zero-mass variable-flavor number scheme and the fixed-flavor number scheme. I illustrate these problems and present a general formulation of a Variable-flavor Number Scheme (VFNS) for heavy quarks that is explicitly implemented up to NNLO in the strong coupling constant ${\alpha }_S$, and may be used in NNLO global fits for parton distributions. The procedure combines elements of the ACOT($\chi$) scheme and the Thorne-Roberts scheme. Despite the fact that at NNLO the parton distributions are discontinuous as one changes the number of active quark flavors, all physical quantities are continuous at flavor transitions and the comparison with data is successful.

The modification of the Paschos-Wolfenstein relation is investigated when the charge symmetry violations of valence and sea quark distributions in the nucleon are taken into account. We also study qualitatively the impact of charge symmetry violation (CSV) effect on the extraction of ${sin}^2{\theta }_w$ from deep-inelastic neutrino- and antineutrino-nuclei scattering within the light-cone meson-baryon fluctuation model. We find that the effect of CSV is too small to give a sizable contribution to the NuTeV result with various choices of mass difference inputs, which is consistence with the prediction that the strange-antistrange asymmetry can account for largely the NuTeV deviation in this model. It is noticeable that the effect of CSV might contribute to the NuTeV deviation when the larger difference between the internal momentum scales, ${\alpha }_p$ of the proton and ${\alpha }_n$ of the neutron, is considered.

In this paper we present a simple analytic expression for the (spin-averaged) neutrino-nucleon cross section for ultrahigh energies at twist-2, obtained as the asymptotic limit of our previous findings. This expression gives values for the cross section in remarkable numerical agreement with the previous numerical evaluation in the energy region relevant for forthcoming neutrino experiments. Moreover, we discuss the role and the relevance of saturation and recombination effects in our approach, in comparison with other recent suggestions.

We calculate the one-loop flavor changing neutral current top quark decay $t\to cgg$ in the standard model. We demonstrate that the rate for $t\to cgg$ exceeds the rate for a single gluon emission $t\to cg$ by about 2 orders of magnitude, while the rate for $t\to cq\overline{q}$ $q=u$ is slightly smaller than for $t\to cg$.

We study spontaneous gauge symmetry breaking in the framework of orbifold compactifications of heterotic string theory. In particular, we investigate the electroweak symmetry breakdown via the Higgs mechanism. Such a breakdown can be achieved by continuous Wilson lines. Exploiting the geometrical properties of this scheme, we develop a new technique which simplifies the analysis used in previous discussions.

The influence of electromagnetic vacuum fluctuations in the presence of the perfectly conducting plate on electrons is studied with an interference experiment. The evolution of the reduced density matrix of the electron is derived by the method of influence functional. We find that the plate boundary anisotropically modifies vacuum fluctuations that in turn affect the electron coherence. The path plane of the interference is chosen either parallel or normal to the plate. In the vicinity of the plate, we show that the coherence between electrons due to the boundary is enhanced in the parallel configuration, but reduced in the normal case. The presence of the second parallel plate is found to boost these effects. The potential relation between the amplitude change and phase shift of interference fringes is pointed out. The finite conductivity effect on electron coherence is discussed.

Computing the perturbation spectrum in the recently proposed Island Cosmology remains an open problem. In this paper we present a classical computation of the perturbations generated in this scenario by assuming that the NEC-violating field behaves as a classical phantom field. Using an exactly solvable potential, we show that the model generates a scale-invariant spectrum of scalar perturbations, as well as a scale-invariant spectrum of gravitational waves. The scalar perturbations can have sufficient amplitude to seed cosmological structure, while the gravitational waves have a vastly diminished amplitude.