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Field Theory On the World Sheet: Mean Field Expansion And Cutoff Dependence

Description: Continuing earlier work, we apply the mean field method to the world sheet representation of a simple field theory. In particular, we study the higher order terms in the mean field expansion, and show that their cutoff dependence can be absorbed into a running coupling constant. The coupling constant runs towards zero in the infrared, and the model tends towards a free string. One cannot fully reach this limit because of infrared problems, however, one can still apply the mean field method to the high energy limit (high mass states) of the string.
Date: January 10, 2007
Creator: Bardakci, Korkut
Partner: UNT Libraries Government Documents Department

Supergravity Analysis of Hybrid Inflation Model from D3--D7 System

Description: The slow-roll inflation is a beautiful paradigm, yet the inflaton potential can hardly be sufficiently flat when unknown gravitational effects are taken into account. However, the hybrid inflation models constructed in D = 4 N = 1 supergravity can be consistent with N = 2 supersymmetry, and can be naturally embedded into string theory. This article discusses the gravitational effects carefully in the string model, using D = 4 supergravity description. We adopt the D3--D7 system of Type IIB string theory compactified on K3 x T^2/Z_2 orientifold for definiteness. It turns out that the slow-roll parameter can be sufficiently small despite the non-minimal Kahler potential of the model. The conditions for this to happen are clarified in terms of string vacua. We also find that the geometry obtained by blowing up singularity, which is necessary for the positive vacuum energy, is stabilized by introducing certain 3-form fluxes.
Date: November 20, 2003
Creator: Koyama, Fumikazu; Tachikawa, Yuji & Watari, Taizan
Partner: UNT Libraries Government Documents Department

Towards a string formulation of vortex dynamics

Description: We derive an exact equation of motion for a non-relativistic vortex in two- and three-dimensional models with a complex field. The velocity is given in terms of gradients of the complex field at the vortex position. We discuss the problem of reducing the field dynamics to a closed dynamical system with non-locally interacting strings as the fundamental degrees of freedom.
Date: January 1, 1998
Creator: Inst.], Elsebeth Schroeder & Toernkvist, Ola
Partner: UNT Libraries Government Documents Department

Spectra in standard-like Z_3 orbifold models

Description: General features of the spectra of matter states in all 175 models found in a previous work by the author are discussed. Only twenty patterns of representations are found to occur. Accomodation of the Minimal Supersymmetric Standard Model (MSSM) spectrum is addressed. States beyond those contained in the MSSM and nonstandard hypercharge normalization are shown to be generic, though some models do allow for the usual hypercharge normalization found in SU(5) embeddings of the Standard Model gauge group. The minimum value of the hypercharge normalization consistent with accommodation of the MSSM is determined for each model. In some cases, the normalization can be smaller than that corresponding to an SU(5) embedding of the Standard Model gauge group, similar to what has been found in free fermionic models. Bizarre hypercharges typically occur for exotic states, allowing for matter which does not occur in the decomposition of SU(5) representations--a result which has been noted many times before in four-dimensiona lstring models. Only one of the twenty patterns of representations, comprising seven of the 175 models, is found to be without an anomalous U(1). The sizes of nonvanishing vacuum expectation values induced by the anomalous U(1) are studied. It is found that large radius moduli stabilization may lead to the breakdown of sigma-model perturbativity. Various quantities of interest in effective supergravity model building are tabulated for the set of 175 models. In particular, it is found that string moduli masses appear to be generically quite near the gravitino mass. String scale gauge coupling unification is shown to be possible, albeit contrived, in an example model. The intermediate scales of exotic particles are estimated and the degree of fine-tuning is studied.
Date: August 31, 2001
Creator: Giedt, Joel
Partner: UNT Libraries Government Documents Department

Axions from string decay

Description: We have studied numerically the evolution and decay of axion strings. These global defects decay mainly by axion emission and thus contribute to the cosmological axion energy density. The relative importance of this source relative to misalignment production of axions depends on the spectrum. Radiation spectra for various string loop configurations are presented. They support the contention that the string decay contribution is of the same order of magnitude as the contribution from misalignment.
Date: July 9, 1998
Creator: Hagmann, C., LLNL
Partner: UNT Libraries Government Documents Department

Strings and supersymmetry as tools for perturbative QCD

Description: We review techniques simplifying the analytic calculation of one-loop QCD amplitudes with many external legs, for use in next-to-leading-order corrections to multi-jet processes. We explain how a supersymmetry-inspired organization works well in conjunction with other tools, namely the color and helicity decompositions of amplitudes, and the constraints imposed by perturbative unitarity and collinear singularities. String theory seems most useful as a heuristic guide. Using these techniques, the complete set of one-loop five-parton QCD amplitudes, as well as certain sequences of special helicity amplitudes with an arbitrary number of external gluons, have been obtained.
Date: July 1, 1995
Creator: Dixon, L.
Partner: UNT Libraries Government Documents Department

Flavor tests of quark-lepton unification

Description: We could become convinced that a particular theory of very-high-energy physics is correct if (1) it has a tightly constrained structure and is linked strongly enough with observed particle interactions, or (2) it predicts new physics beyond the standard model which is discovered. The author makes the case that experiments of this decade and the next allow the possibility that we might become convinced that grand unification, a candidate theory of the second type, is correct.
Date: March 21, 1995
Creator: Hall, L. J.
Partner: UNT Libraries Government Documents Department

M theory fivebrane and SQCD

Description: A low energy effective theory of parallel D(irichlet) branes is a gauge theory with sixteen supercharges, but one can consider a web of brane to realize situations with reduced number of supersymmetry. In this talk, the authors discusses four-dimensional theories with N = 1 and 2 supersymmetry (i.e. four and eight supercharges). In the case of theories with N = 2 supersymmetry, the exact description of the Coulomb branch is given by reinterpreting the web of branes as a configuration of a single fivebrane in the IIA theory. Recently the authors studied the case with N = 1 supersymmetry, and found that description in terms of the fivebrane in M Theory captures strong coupling dynamics of the N = 1 gauge theory in four dimensions. In particular, they found that the configuration of the fivebrane geometrically encodes information on the Affleck-Dine-Seiberg superpotential and the structure of the quantum moduli space of vacua.
Date: September 1, 1997
Creator: Ooguri, H.
Partner: UNT Libraries Government Documents Department

Mathematics and string theory

Description: The continuation of the collaboration with Liu and Lian on the calculation of the II A model opened up the possibility to understand calculations for higher genus curves also; many detailed calculations were carried out. They provided evidence that the method is powerful enough to calculate GW invariants in many cases. Local mirror symmetry was worked out with Chiang, Klemm, and Zaslow; it is consistent with physics intuition. Work was carried out to advance the ideas of Stroninger-Yau-Zaslow's geometric version of mirror symmetry in terms of special Lagragian torus fibration. Several papers were written on understanding such duality; it fits well with the predictions, and the ideas are still being studied.
Date: November 25, 2002
Creator: Yau, Shing-Tung
Partner: UNT Libraries Government Documents Department

Issues in IIA Uplifting

Description: Moduli stabilization in the type IIA massive string theory so far was achieved only in the AdS vacua. The uplifting to dS vacua has not been performed as yet: neither the analogs of type IIB anti-D3 brane at the tip of the conifold, nor the appropriate D-terms have been identified. The hope was recently expressed that the F-term uplifting may work. We investigate this possibility in the context of a simplified version of the type IIA model developed in hep-th/0505160 and find that the F-term does not uplift the AdS vacua to dS vacua with positive CC. Thus it remains a challenging task to find phenomenologically acceptable vacua in the type IIA string theory.
Date: December 12, 2006
Creator: Kallosh, Renata & Soroush, Masoud
Partner: UNT Libraries Government Documents Department

Simple Stringy Dynamical SUSY Breaking

Description: We present simple string models which dynamically break supersymmetry without non-Abelian gauge dynamics. The Fayet model, the Polonyi model, and the O'Raifeartaigh model each arise from D-branes at a specific type of singularity. D-brane instanton effects generate the requisite exponentially small scale of supersymmetry breaking.
Date: August 8, 2007
Creator: Aharony, Ofer; /Weizmann Inst. /Stanford U., Phys. Dept. /SLAC; Kachru, Shamit; Silverstein, Eva & /Stanford U., Phys. Dept. /SLAC
Partner: UNT Libraries Government Documents Department

Heterotic orbifolds

Description: A review of orbifold geometry is given, followed by a review of the construction of four-dimensional heterotic string models by compactification on a six-dimensional Z{sub 3} orbifold. Particular attention is given to the details of the transition from a classical theory to a first-quantized theory. Subsequently, a discussion is given of the systematic enumeration of all standard-like three generation models subject to certain limiting conditions. it is found that the complete set is described by 192 models, with only five possibilities for the hidden sector gauge group. It is argued that only four of the hidden sector gauge groups are viable for dynamical supersymmetry breaking, leaving only 175 promising models in the class. General features of the spectra of matter states in all 175 models are discussed. Twenty patterns of representations are found to occur. Accommodation of the Minimal Supersymmetric Standard Model (MSSM) spectrum is addressed. States beyond those contains in the MSSM and nonstandard hypercharge normalization are shown to be generic, though some models do allow for the usual hypercharge normalization found in SU(5) embeddings of the Standard Model gauge group. Only one of the twenty patterns of representations, comprising seven of the 175 models, is found to be without an anomalous U(1). Various quantities of interest in effective supergravity model building are tabulated for the set of 175 models. String scale gauge coupling unification is shown to be possible, albeit contrived, in an example model.
Date: April 1, 2002
Creator: Giedt, Joel
Partner: UNT Libraries Government Documents Department

The size of a polymer of string-bits: a numerical investigation

Description: In string bit models, string is described as a polymer of point-like constituents. We attempt to use string-bit ideas to investigate how the size of string is affected by string interactions in a non-perturbative context. Lacking adequate methods to deal with the full complications of bit rearrangement interactions, we study instead a simplified analog model with only ``direct`` potential interactions among the bits. We use the variational principle in an approximate calculation of the mean-square size of a polymer as a function of the number of constituents/bits for various interaction strengths {ital g} in three specific models. 14 refs., 2 figs.
Date: April 1, 1997
Creator: Bergman, O. & Thorn, C.B.
Partner: UNT Libraries Government Documents Department

M-theory model-building and proton stability

Description: The authors study the problem of baryon stability in M theory, starting from realistic four-dimensional string models constructed using the free-fermion formulation of the weakly-coupled heterotic string. Suitable variants of these models manifest an enhanced custodial gauge symmetry that forbids to all orders the appearance of dangerous dimension-five baryon-decay operators. The authors exhibit the underlying geometric (bosonic) interpretation of these models, which have a Z{sub 2} x Z{sub 2} orbifold structure similar, but not identical, to the class of Calabi-Yau threefold compactifications of M and F theory investigated by Voisin and Borcea. A related generalization of their work may provide a solution to the problem of proton stability in M theory.
Date: September 1, 1997
Creator: Ellis, J.; Faraggi, A.E. & Nanopoulos, D.V.
Partner: UNT Libraries Government Documents Department

A family-universal anomalous U(1) in string models as the origin of supersymmetry breaking and squark degeneracy

Description: Recently a promising mechanism for supersymmetry breaking that utilizes both an anomalous U(1) gauge symmetry and an effective mass term m {approx} 1TeV of certain relevant fields has been proposed. In this paper we examine whether such a mechanism can emerge in superstring derived free fermionic models. We observe that certain three generation string solutions, though not all, lead to an anomalous U(1) which couples universally to all three families. The advantages of this three-family universality of U(1){sub A}, compared to the two-family case, proposed in earlier works, in yielding squark degeneracy, while avoiding radiative breaking of color and charge, are noted. The root cause of the flavor universality of U(1){sub A} is the cyclic permutation symmetry that characterizes the Z{sub 2} x Z{sub 2} orbifold compactification with standard embedding, realized in the free fermionic models by the NAHE set. It is shown that nonrenormalizable terms which contain hidden-sector condensates, generate the required suppression of the relevant mass term m, compared to the Planck scale. While the D-term of the family universal U(1){sub A} leads to squark degeneracy, those of the family dependent U(1)`s, remarkably enough, are found to vanish for the solutions considered, owing to minimization of the potential.
Date: December 1, 1997
Creator: Faraggi, A.E. & Pati, J.C.
Partner: UNT Libraries Government Documents Department

Dirichlet branes and nonperturbative aspects of supersymmetric string and gauge theories

Description: In chapter 1 the author reviews some elements of string theory relevant to the rest of this report. He touches on both the classical, i.e. perturbative, string physics before D-branes rise to prominence, and some of the progresses they brought forth. In chapter 2 he proceeds to give an exact algebraic formulation of D-branes in curved spaces. This allows one to classify them in backgrounds of interest and study their geometric properties. He applies this formalism to string theory on Calabi-Yau and other supersymmetry preserving manifolds. Then he studies the behavior of the D-branes under mirror symmetry in chapter 3. Mirror symmetry is known to be a symmetry of string theory perturbatively. He finds evidence for its nonperturbative validity when D-branes are also considered and compute some dynamical consequences. In chapter 4 he turns to examine the consistency of curved and/or intersecting D-brane configurations. They have been used recently to extract information about the field theories that arise in certain limits. It turns out that there are potential quantum mechanical inconsistencies associated with them. What saves the day are certain subtle topological properties of D-branes. This resolution has implications for the conserved charges carried by the D-branes, which he computes for the cases studied in chapter 2. In chapter 5 he uses intersecting brane configurations to study three dimensional supersymmetric gauge theories. There is also a mirror symmetry there that, among other things, exchanges classical and quantum mechanical quantities of a (mirror) pair of theories. It has an elegant realization in term of a symmetry of string theory involving D-branes. The author employs it to study a wide class of 3d models. He also predicts new mirror pairs and unconventional 3d field theories without Lagrangian descriptions.
Date: May 1, 1999
Creator: Yin, Zheng
Partner: UNT Libraries Government Documents Department

Topics in conformal invariance and generalized sigma models

Description: This thesis consists of two different parts, having in common the fact that in both, conformal invariance plays a central role. In the first part, the author derives conditions for conformal invariance, in the large N limit, and for the existence of an infinite number of commuting classical conserved quantities, in the Generalized Thirring Model. The treatment uses the bosonized version of the model. Two different approaches are used to derive conditions for conformal invariance: the background field method and the Hamiltonian method based on an operator algebra, and the agreement between them is established. The author constructs two infinite sets of non-local conserved charges, by specifying either periodic or open boundary conditions, and he finds the Poisson Bracket algebra satisfied by them. A free field representation of the algebra satisfied by the relevant dynamical variables of the model is also presented, and the structure of the stress tensor in terms of free fields (and free currents) is studied in detail. In the second part, the author proposes a new approach for deriving the string field equations from a general sigma model on the world sheet. This approach leads to an equation which combines some of the attractive features of both the renormalization group method and the covariant beta function treatment of the massless excitations. It has the advantage of being covariant under a very general set of both local and non-local transformations in the field space. The author applies it to the tachyon, massless and first massive level, and shows that the resulting field equations reproduce the correct spectrum of a left-right symmetric closed bosonic string.
Date: May 1, 1997
Creator: Bernardo, L.M.
Partner: UNT Libraries Government Documents Department

Where's the glue: A strong interaction puzzle

Description: After recalling why most strong interaction phenomena cannot be studied with quark-gluon perturbation theory, the author suggests that a phenomenological form of the old dual string theory may be a good model for the nonperturbative aspects of QCD. He begins by describing some recent progress within this framework in understanding the status of the quark model within QCD. The resulting picture of the ordinary hadrons, hybrids, and glueballs provides a simple framework for studying these elements of hadron spectroscopy.
Date: December 1, 1996
Creator: Isgur, Nathan
Partner: UNT Libraries Government Documents Department

Some remarks on discrete physics as an ultimate dynamical theory

Description: The standard model of quarks and leptons currently fails to meet `t Hooft`s criterion for an ``Ultimate Dynamical Theory`` in that it contains 19 parameters which must be taken from experiment. Evaluating discrete physics in the same way we find that it requires 17 parameters and has already succeeded in computing 7 of them. While we are confident that the rest can also be computed, the very concept of an ultimate theory is incompatible with our attitude toward physics.
Date: October 1, 1995
Creator: Noyes, H.P.
Partner: UNT Libraries Government Documents Department

Evidence for string substructure

Description: The author argues that the behavior of string theory at high temperature and high longitudinal boosts, combined with the emergence of p-branes as necessary ingredients in various string dualities, point to a possible reformulation of strings, as well as p-branes, as composites of bits. He reviews the string-bit models, and suggests generalizations to incorporate p-branes.
Date: June 1, 1996
Creator: Bergman, O.
Partner: UNT Libraries Government Documents Department

Logarithmic unification from symmetries enhanced in the sub-millimeter infrared

Description: In theories with TeV string scale and sub-millimeter extra dimensions the attractive picture of logarithmic gauge coupling unification at 10{sup 16} GeV is seemingly destroyed. In this paper we argue to the contrary that logarithmic unification can occur in such theories. The rationale for unification is no longer that a gauge symmetry is restored at short distances, but rather that a geometric symmetry is restored at large distances in the bulk away from our 3-brane. The apparent ''running'' of the gauge couplings to energies far above the string scale actually arises from the logarithmic variation of classical fields in (sets of) two large transverse dimensions. We present a number of N = 2 and N = 1 supersymmetric D-brane constructions illustrating this picture for unification.
Date: August 21, 1999
Creator: Arkani-Hamed, Nima; Dimopoulos, Savas & March-Russell, John
Partner: UNT Libraries Government Documents Department