String-inspired Models Research Articles

CONSTRAINING THE RUNAWAY DILATON AND QUINTESSENTIAL DARK ENERGY

Dark energy is some of the weirdest and most mysterious stuff in the universe that tends to increase the rate of expansion of the universe. Two commonly known forms of dark energy are the cosmological constant, a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli whose energy density can vary with time. We explore one particular model for dynamic dark energy: quintessence driven by a scalar dilaton field. We propose an ansatz for the form of the dilaton field, |ϕ(a)|mP ≡ α1 ln t + α2tn = α ln a + βa2ζ, where a is the scale factor and α and ζ are parameters of the model. This phenomenological ansatz for ϕ can be motivated by generic solutions of a scalar dilaton field in many effective string theory and string-inspired gravity models in four dimensions. Most of the earlier discussions in the literature correspond to the choice that ζ = 0 so that ϕ(t) ∝ ln t or ϕ(t) ∝ ln a(t). Using a compilation of current data including type Ia supernovae, we impose observational constraints on the slope parameters like α and ζ and then discuss the relation of our results to analytical constraints on various cosmological parameters, including the dark energy equation of state. Some useful constraints are imposed on model parameters like α and ζ as well as on the dark energy/dark matter couplings using results from structure formation. The constraints of this model are shown to encompass the cosmological constant limit within 1σ error bars.

CPT Violation and decoherence in quantum gravity

In this brief review I discuss ways and tests of CPT-Violation in the context of quantum gravity theories with space-time foam vacua, which entail quantum decoherence of matter propagating in such backgrounds. I cover a wide variety of sensitive probes, ranging from cosmic neutrinos to meson factories. I pay particular emphasis on associating the latter with specific, probably unique (‘smoking-gun’), effects of this type of CPT Violation, related to a modification of Einstein-Podolsky-Rosen (EPR) correlations in the entangled states of the relevant neutral mesons. I also present some semi-microscopic estimates of these latter effects, in the context of a specific string-inspired model of space-time foam (‘D-particle foam’).

Moduli stabilization in non-supersymmetric Minkowski vacua with anomalous U(1) symmetry

We study how two moduli can be stabilized in a Minkowski/de Sitter vacuum for a wide class of string-inspired Supergravity models with an effective Fayet-like Supersymmetry breaking. It is shown under which conditions this mechanism can be made natural and how it can give rise to an interesting spectrum of soft masses, with a relatively small mass difference between scalar and gaugino masses. In absence of a constant superpotential term, the above mechanism becomes completely natural and gives rise to a dynamical supersymmetry breaking mechanism. Some specific type IIB and heterotic string inspired models are considered in detail.

Generalization of Einstein-Lovelock theory to higher order dilaton gravity

A higher order theory of dilaton gravity is constructed as a generalization of the Einstein-Lovelock theory of pure gravity. Its Lagrangian contains terms with higher powers of the Riemann tensor and of the first two derivatives of the dilaton. Nevertheless, the resulting equations of motion are quasilinear in the second derivatives of the metric and of the dilaton. This property is crucial for the existence of brane solutions in the thin wall limit. At each order in the derivatives the contribution to the Lagrangian is unique up to an overall normalization. Relations between symmetries of this theory and the $O(d,d)$ symmetry of the string-inspired models are discussed.

Narrow trans-TeV Higgs bosons andH→hhdecays: two LHC search paths for a hidden sector Higgs boson

We consider the addition of a condensing singlet scalar field to the Standard Model. Such a scenario may be motivated by any number of theoretical ideas, including the common result in string-inspired model building of singlet scalar fields charged under some hidden sector gauge symmetry. For concreteness, we specify an example model of this type, and consider the relevant constraints on Higgs physics, such as triviality, perturbative unitarity and precision electroweak analysis. We then show that there are two unique features of the phenomenology that present opportunities for discovery at the Large Hadron Collider (LHC). First, it is possible to identify and discover a narrow trans-TeV Higgs boson in this scenario -- a mass scale that is well above the scale at which it is meaningful to discuss a SM Higgs boson. Second, the decays of the heavier scalar state into the lighter Higgs bosons can proceed at a high rate and may be the first discovery mode in the Higgs sector.

INTRODUCTION TO MODIFIED GRAVITY AND GRAVITATIONAL ALTERNATIVE FOR DARK ENERGY

We review various modified gravities considered as gravitational alternative for dark energy. Specifically, we consider the versions of f(R), f(G) or f(R, G) gravity, model with non-linear gravitational coupling or string-inspired model with Gauss-Bonnet-dilaton coupling in the late universe where they lead to cosmic speed-up. It is shown that some of such theories may pass the Solar System tests. On the same time, it is demonstrated that they have quite rich cosmological structure: they may naturally describe the effective (cosmological constant, quintessence or phantom) late-time era with a possible transition from decceleration to acceleration thanks to gravitational terms which increase with scalar curvature decrease. The possibility to explain the coincidence problem as the manifestation of the universe expansion in such models is mentioned. The late (phantom or quintessence) universe filled with dark fluid with inhomogeneous equation of state (where inhomogeneous terms are originated from the modified gravity) is also described.

N-flationary magnetic fields

There is increasing interest in the role played by pseudo Nambu–Goldstone bosons(pNGBs) in the construction of string-inspired models of inflation. In these models theinflaton is expected to be coupled to gauge fields, and will lead to the generation ofmagnetic fields that can be of cosmological interest. We study the production of such fieldsmainly focusing on the model of N-flation, where the collective effect of several pNGBsdrives inflation. Because the fields produced are maximally helical, inverse cascadeprocesses in the primordial plasma significantly increase their coherence length. We discussunder what conditions inflation driven by pNGBs can account for the cosmologicalmagnetic fields observed. A constraint on the parameters of this class of inflationaryscenarios is also derived by requiring that the magnetic field does not backreact on theinflating background.

Dilaton and moduli fields in D-term inflation

We investigate the possibility of D-term inflation within the framework of type-I string-inspired models. Although the D-term inflation model has the excellent property that it is free of the so-called $\ensuremath{\eta}$ problem, two serious problems appear when we embed D-term inflation in string theory: the magnitude of the FI term and the rolling motion of the dilaton. In the present paper, we analyze the potential of D-term inflation in type-I-inspired models and study the behavior of dilaton and twisted moduli fields. Adopting the nonperturbative superpotential induced by gaugino condensation, the twisted moduli can be stabilized. If the dilaton is in a certain range, it evolves very slowly and does not run away to infinity. Thus D-term-dominated vacuum energy becomes available for driving inflation. By studying the density perturbation generated by the inflation model, we derive the constraints on model parameters and give some implications on D-term inflation in type-I-inspired models.

Particle Spectra and Gauge Unification in SU(6) xSU(2)R Model

We study the path from the string scale physics to the low-energy physics in the $SU(6) \times SU(2)_R$ string-inspired model with the flavor symmetry ${\bf Z}_M \times {\bf Z}_N \times \tilde{D}_4$. The flavor symmetry controls the mass spectra of heavy particles as well as those of quarks and leptons in the intermediate energy region ranging from the string scale ($\sim 10^{18} {\rm GeV}$) to the electroweak scale. In this paper we examine the mass spectra of heavy particles in detail in our model. The renormalization group evolution of the gauge couplings is studied up to two-loop order. A consistent solution of the gauge unification around the string scale is found by adjusting the spectra of the anti-generation matter fields.

Global analysis of a supersymmetric Pati-Salam model

We perform a complete global phenomenological analysis of a realistic string-inspired model based on the supersymmetric Pati-Salam $SU(4)\times SU(2)_L \times SU(2)_R$ gauge group supplemented by a U(1) family symmetry, and present predictions for all observables including muon $g-2$, $\tau \to \mu \gamma$, and the CHOOZ angle. Our analysis demonstrates the compatibility of such a model with all laboratory data including charged fermion masses and mixing angles, LMA MSW and atmospheric neutrino masses and mixing angles, and $b \to s \gamma$, allowing for small deviations from third family Yukawa unification. We show that in such models the squark and slepton masses may be rather light compared to similar models with exact Yukawa unification.

Precocious Gauge Symmetry Breaking in SU(6) x SU(2)R Model

In the $SU(6) \times SU(2)_R$ string-inspired model, we evolve the couplings and the masses down from the string scale $M_S$ using the renormalization group equations and minimize the effective potential. This model has the flavor symmetry including the binary dihedral group $\tilde{D}_4$. We show that the scalar mass squared of the gauge non-singlet matter field possibly goes negative slightly below the string scale. As a consequence, the precocious radiative breaking of the gauge symmetry down to the standard model gauge group can occur. In the present model, the large Yukawa coupling which plays an important role in the symmetry breaking is identical with the colored Higgs coupling related to the longevity of the proton.

EXACT SOLUTIONS AND THE COSMOLOGICAL CONSTANT PROBLEM IN DILATONIC-DOMAIN-WALL HIGHER-CURVATURE STRING GRAVITY

In this paper, we extend the previous work by the authors, and elaborate further on the structure of the general solution to the graviton and dilaton equations of motion in brane world scenaria, in the context of five-dimensional effective actions with [Formula: see text] higher-curvature corrections, compatible with bulk string-amplitude calculations. We consider (multi)brane scenaria, dividing the bulk space into regions, in which one matches two classes of general solutions, a linear Randall–Sundrum solution and a (logarithmic) dilatonic domain wall (bulk naked singularity). We pay particular attention to examining the possibility of resolving the mass hierarchy problem together with the vanishing of the vacuum energy on the observable world, which is taken to be a positive tension brane. The appearance of naked dilatonic domain walls provides a dynamical restriction of the bulk space–time. Of particular interest is a dilatonic-wall solution, which after appropriate coordinate transformation, results in a linear dilaton conformal field theory. The latter may provide a holographic resolution of the naked singularity problem. All the string-inspired models involved have the generic feature that the brane tensions are proportional to the string coupling gs; it remains a challenge for string theory, therefore, to show whether microscopic models respecting this feature can be constructed.

Non-Anomalous Flavor Symmetries and SU(6) xSU(2)R Model

We introduce the flavor symmetry ZM ×ZN ×D4 into the SU(6) ×SU(2)R string-inspired model. The cyclic group ZM and the dihedral group D4 are R symmetries, while ZN is a non-R symmetry. By imposing the anomaly-free conditions on the model, we obtain a viable solution under many phenomenological constraints coming from the particle spectra. for the neutrino sector, we find a LMA-MSW solution but no SMA-MSW solution. The solution includes phenomenologically acceptable results concerning fermion masses and mixings and also concerning hierarchical energy scales including the GUT scale, the μ scale and the Majorana mass scale of R-handed neutrinos.

Density perturbations in the ekpyrotic Universe and string-inspired generalizations

We study density perturbations in several cosmological models motivated by string theory. The evolution and the spectra of curvature perturbations ${\cal R}$ are analyzed in the Ekpyrotic scenario and nonsingular string cosmologies. We find that these string-inspired models generally exhibit blue spectra in contrast to standard slow-roll inflationary scenarios. We also clarify the parameter range where ${\cal R}$ is enhanced on superhorizon scales.

On the dilaton and axion potentials

We extend the Vecchia–Veneziano–Witten model of quantum chromodynamics in the chiral limit and for large colour number Nc, by introducing an effective dilaton–gluon coupling from which we derive both the axion and dilaton potentials. Furthermore, using a string-inspired model, we determine a new interquark potential as a perturbative series in terms of the interquark distance r. Our potential goes beyond Dick's potential (Dick R 1999 Eur. Phys. J. C 6 701) and shares the same features as those of the Bian–Huang–Shen potential (VBHS) which depends only on odd powers of r (Bian J G, Huang T and Shen Q X 1994 Commun. Theor. Phys. 21 333).

Muon anomalous magnetic moment and τ→μγ in a realistic string-inspired model of neutrino masses

We discuss the lepton sector of a realistic string-inspired model based on the Pati-Salam $SU(4)\times SU(2)_L \times SU(2)_R$ gauge group supplemented by a U(1) family symmetry. The model involves third family Yukawa unification, predicts large $\tan \beta\sim 50$, and describes all fermion masses and mixing angles, including approximate bi-maximal mixing in the neutrino sector. Atmospheric neutrino mixing is achieved via a large 23 entry in the neutrino Yukawa matrix which can have important phenomenological effects. We find that the recent BNL result on the muon ($g-2$) can be easily accommodated in a large portion of the SUSY parameter space of this model. Over this region of parameter space the model predicts a CP-even Higgs mass near 115 GeV, and a rate for $\tau\to \mu\gamma$ which is close to its current experimental limit.

Direct detection of neutralino dark matter and the anomalous dipole moment of the muon

We compare predictions for the spin-independent contribution to the neutralino-proton scattering cross section ${\ensuremath{\sigma}}_{\ensuremath{\chi}p}$ and for the anomalous magnetic dipole moment of the muon, ${a}_{\ensuremath{\mu}}{=(g}_{\ensuremath{\mu}}\ensuremath{-}2)/2,$ in models with gravity-mediated supersymmetry breaking. We nearly always find a positive correlation between these two measurables; i.e., scenarios with larger ${a}_{\ensuremath{\mu}}$ also tend to have larger ${\ensuremath{\sigma}}_{\ensuremath{\chi}p},$ but the detailed prediction differs greatly between models. In particular, we find that for the popular MSUGRA scenario with universal soft breaking masses at the scale of grand unification, measurements of ${a}_{\ensuremath{\mu}}$ currently seem more promising. On the other hand, if scalar soft breaking masses at the GUT scale receive sizable contributions from SO(10) D terms, one often finds scenarios with large ${\ensuremath{\sigma}}_{\ensuremath{\chi}p}$ but ${a}_{\ensuremath{\mu}}$ below the currently foreseen sensitivity. A string-inspired model with a nonuniversal scalar spectrum at the GUT scale falls between these two cases.

Neutrino masses and mixing angles in a realistic string-inspired model

We analyze a supersymmetric string-inspired model of all fermion masses and mixing angles based on the Pati-Salam $\mathrm{SU}(4)\ifmmode\times\else\texttimes\fi{}{\mathrm{SU}(2)}_{L}\ifmmode\times\else\texttimes\fi{}{\mathrm{SU}(2)}_{R}$ gauge group supplemented by a ${U(1)}_{X}$ flavor symmetry. The model involves third family Yukawa unification and predicts the top quark mass and the ratio of the vacuum expectation values $\mathrm{tan}\ensuremath{\beta}.$ The model also provides a successful description of the CKM matrix and predicts the masses of the down and strange quarks. However, our main focus is on the neutrino masses and MNS mixing angles, and we show how the recent atmospheric neutrino mixing observed by Super-Kamiokande and the MSW solution to the solar neutrino problem lead to important information about the flavor structure of the model near the string scale. We show how single right-handed neutrino dominance may be implemented by the use of ``Clebsch zeros,'' leading to the LMA MSW solution, corresponding to bimaximal mixing. The LOW MSW and SMA MSW solutions are also discussed.

Neutrino masses in SUSY models

In this talk we assume the conventional see-saw mechanism, and contruct a hierarchical pattern of three active neutrinos with bi-maximal mixing. In order to enforce the hierarchy we use the single right handed neutrino dominance mechanism which is a very nice way to ensure a neutrino mass hierarchy in the presence of large mixing angles. We show how this mechanism can be organised in the framework of a U(1) family symmetry, then discuss a realistic string-inspired model which includes such a symmetry.

Supersymmetric CP-violating phases and the LSP relic density and detection rates

For varying values of tan β, we study the effect of CP-violating phases from the soft supersymmetry breaking terms in string-inspired models on the relic abundance and detection rates of the lightest neutralino (LSP). We find that the phases have no significant effect on the LSP relic abundance but can have a substantial impact on its detection rates.