Meeting the constraint of neutrino—Higgsino mixing in gravity unified theories

In gravity unified theories all operators that are consistent with the local gauge and discrete symmetries are expected to arise in the effective low-energy theory. Given the absence of multiplets like 126 of SO(10) in string models, and assuming that B - L is violated spontaneously to generate light neutrino masses via a seesaw mechanism, it is observed that string theory solutions generically face the problem of producing an excessive mixing mass at the GUT scale, which is some nineteen orders of magnitude larger than the experimental bound of 1 MeV. The suppression of mixing, like proton longevity, thus provides one of the most severe constraints on the validity of any string theory solution. We examine this problem in a class of superstring derived models. We find a family of solutions within this class for which the symmetries of the models and an allowed pattern of VEVs, surprisingly, succeed in adequately suppressing the neutrino-Higgsino mixing terms. At the same time they produce the terms required to generate small neutrino masses via a seesaw mechanism.     

Yukawa couplings for a three-generation superstring compactification

We explicitly calculate the Yukawa couplings for a three-generation superstring compactification and show in detail how the calculation reduces to algebraic manipulations of certain polynomials. We also discuss the form of the low energy effective theory and find a basis for which these terms are diagonal.     

Soft supersymmetry breaking in superstring-based low-energy supergravity

We study four-dimensional supergravity theories consistent with those symmetries of the underlying ten-dimensionl supergravity (or superstring) theory, which are preserved by string loop effects. On very general grounds, we derive constraints that the Kähler potential of these theories must satisfy, in order that soft supersymmetry breaking terms appear in the low-energy observable sector at tree level.     

Superstring theory

Dual string theories, initially developed as phenomenological models of hadrons, now appear more promising as candidates for a unified theory of fundamental interactions. Type I superstring theory (SST I), is a ten-dimensional theory of interacting open and closed strings, with one supersymmetry, that is free from ghosts and tachyons. It requires that an SO(n) or Sp(2n) gauge group be used. A light-cone-gauge string action with space-time supersymmetry automatically incorporates the superstring restrictions and leads to the discovery of type II superstring theory (SST II). SST II is an interacting theory of closed strings only, with two D = 10 supersymmetries, that is also free from ghosts and tachyons. By taking six of the spatial dimensions to form a compact space, it becomes possible to reconcile the models with our four-dimensional perception of spacetime and to define low-energy limits in which SST I reduces to N = 4, D = 4 super Yang-Mills theory and SST II reduces to N = 8, D = 4 supergravity theory. The superstring theories can be described by a light-cone-gauge action principle based on fields that are functionals of string coordinates. With this formalism any physical quantity should be calculable. There is some evidence that, unlike any conventional field theory, the superstring theories provide perturbatively renormalizable (SST I) or finite (SST II) unifications of gravity with other interactions.     

Constraints from proton decay in the flipped SU(5) × U(1) superstring model

We discuss the constraints that emerge from the existence of dimension-5 baryon-violating operators in the flipped SU(5) × U(1) superstring model. These are constraints on matter field assignments and on singlet VEV values. Although baryon-violating dimension operators that appear as quintic non-renormalizable terms vanish as has been proven before and as we verify here, effective dimension-5 operators resultig from Higgs exchange put non-trivial but feasible constraints on the model. Constraints are also extracted from the presence of higher order non-renormalizable terms that generate such operators which do not a priori vanish.     

Nonmetricity and torsion induced by dilaton gravity in two dimension

We develop a theory in which there are couplings amongst Dirac spinor, dilaton and non-Riemannian gravity and explore the nature of connection-induced dilaton couplings to gravity and Dirac spinor when the theory is reformulated in terms of the Levi-Civita connection. After presenting some exact solutions without spinors, we investigate the minimal spinor couplings to the model and in conclusion we cannot find any nontrivial dilaton couplings to spinor.     

Evaluation of 273 Yukawa couplings in a three-generation superstring model

We compute the Yukawa couplings in the (1,1) sector for a phenomenologically promising three-generation superstring inspired model, and briefly discuss their consequences. Our results are valid to all finite orders in σ-model perturbation theory. The effects of world sheet instantons are considered, and we show that they do not significantly alter our results.     

The effective interactions of chiral families in four-dimensional superstrings

The low-energy effective interactions in four-dimensional superstrings with N = 2 and N = 1 space-time supersymmetry and massless twisted (family) sector are obtained. Our results rely on some general symmetry properties of superstring particle states and on tensor-calculus techniques for supergravity couplings. The novel feature is that the N = 2 quaternionic manifold and N = 1 Kähler space of the scalar superpartners of family multiplets are non-symmetric spaces whose structure can be obtained by “integrating out” the massive superstring modes.     

Infrared regularization of superstring theory and the one-loop calculation of coupling constants

Infrared regularized versions of 4-D N = 1 superstring ground states are constructed by curving the spacetime. A similar regularization can be performed in field theory. For the IR regularized string ground states we derive the exact one-loop effective action for non-zero U(1) or chromomagnetic fields as well as gravitational and axionic-dilatonic fields. This effective action is IR and UV finite. Thus, the one-loop corrections to all couplings (gravitational, gauge and Yukawas) are unambiguously computed. These corrections are necessary for quantitative string superunification predictions at low energies. The one-loop corrections to the couplings are also found to satisfy Infrared Flow Equations.     

New higher-derivative R4 theorems for graviton scattering

The nonminimal pure spinor formalism for the superstring is used to prove two new multiloop theorems which are related to recent higher-derivative R4 conjectures of Green, Russo, and Vanhove. The first theorem states that when 0相似文献   

R-parity violating contribution to the neutron electric dipole moment at one-loop order

We present the full result for the down squark mass-squared matrix in the complete theory of supersymmetry without R parity where all kinds of R-parity violating terms are admitted without bias using an optimal parametrization. The major result is a new contribution to LR squark mixing, involving both bilinear and trilinear R-parity violating couplings. Among other things, the latter leads to neutron electric dipole moment at one-loop level. Similar mechanisms lead to electron electric dipole moment at the same level. We present here a short report on major features of neutron electric dipole moment from supersymmetry without R parity and give the interesting constraints obtained.     

Compactification of ten-dimensional superstring theories over Ricci-flat coset spaces

In this paper the compactification of ten-dimensional superstring theories over homogeneous coset spaces S/R is analyzed. We construct explicitly the Ricci-flattening spin connection with torsion for the non-symmetric spaces G₂/SU(3), SU(3)/U(1) × U(1) and Sp(4)/SU(2) × U(1). These spaces provide a solution to the classical field equations with vanishing energy-momentum tensor and therefore one might expect that the conformal invariance of the string theory is preserved. We discuss the two-dimensional non-linear σ-model with Wess-Zumino term corresponding to the homogeneous spaces S/R. In addition, we investigate the constraints for the compactification coming from the requirement of unbroken supersymmetry and anomaly cancellation and give explicit examples of consistent compactification with S and R being embedded into the ten-dimensional gauge group E₈.     

Axions,dilatons and Wess-Zumino terms in superstring theories

The nature of the gauge singlet pseudoscalar and scalar particles in the four-dimensional low energy effective theory derived from the superstring is clarified. Including the Wess-Zumino terms of the intermediate ten-dimensional field theory does not affect the result of naive dimensional reduction that one pseudoscalar does not couple to $\text{F}\text{F}\text{̃}$ and is a true Goldstone boson, while the other pseudoscalar couples identically to both the hidden and observable gauge sectors, and is a true axion. The absence of any new physics in the Wess-Zumino terms of the ten-dimensional anomaly-free supergravity theory is emphasized.     

Dirac neutrino masses from generalized supersymmetry breaking

We demonstrate that Dirac neutrino masses in the experimentally preferred range are generated within supersymmetric gauge extensions of the standard model with a generalized supersymmetry breaking sector. If the superpotential neutrino Yukawa terms are forbidden by the gauge symmetry [such as a U(1)'], sub-eV scale effective Dirac mass terms can arise at tree level from hard supersymmetry breaking Yukawa couplings, or at one loop due to nonanalytic soft supersymmetry breaking trilinear scalar couplings. The radiative neutrino magnetic and electric dipole moments vanish at one-loop order.     

New manifolds for superstring compactification

We construct new manifolds with SU(3) holonomy that are candidate vacua for superstring theory and give a detailed explanation of the techniques involved. Some of these manifolds have a non-abelian fundamental group and thus can lead to a rank five low energy gauge group. Models with an odd number of generations are obtained.CP violation is discussed in this context and shown to occur generically. Topological features of the Yukawa couplings are also discussed.     

T-dualization of type II superstring theory in double space

In this article we offer a new interpretation of the T-dualization procedure of type II superstring theory in the double space framework. We use the ghost free action of type II superstring in pure spinor formulation in approximation of constant background fields up to the quadratic terms. T-dualization along any subset of the initial coordinates, \(x^a\), is equivalent to the permutation of this subset with subset of the corresponding T-dual coordinates, \(y_a\), in double space coordinate \(Z^M=(x^\mu ,y_\mu )\). Requiring that the T-dual transformation law after the exchange \(x^a\leftrightarrow y_a\) has the same form as the initial one, we obtain the T-dual NS–NS and NS–R background fields. The T-dual R–R field strength is determined up to one arbitrary constant under some assumptions. The compatibility between supersymmetry and T-duality produces a change of bar spinors and R–R field strength. If we dualize an odd number of dimensions \(x^a\), such a change flips type IIA/B to type II B/A. If we T-dualize the time-like direction, one imaginary unit i maps type II superstring theories to type \(\hbox {II}^\star \) ones.     

Lepton phenomenology and neutrinoless double β-decay in superstring inspired theories

The properties of neutrinos,s-leptons,s-quarks as well as the coloured isosinglet \(\tilde D\) and \(\tilde D^c \) are studied in superstring inspired models. Furthermore the neutrinoless double β-decay process is investigated, including light and heavy majorana neutrinos, as well as the contribution of the exotic Higgs scalars of the theory. From the current experimental limits onov-ββ-decay, useful constraints are imposed on the masses and Yukawa couplings of the theory.     

Non-commutative superstring worldsheet

In this paper we consider the worldsheet of the superstring as a non-commutative space. Some additional terms can be added to the superstring action, such that for an ordinary worldsheet they are zero. The expansion of this extended action up to the first order of the non-commutativity parameter leads to the new supersymmetric action for the string. For the closed superstring, we obtain the boundary state that describes a brane. From the open string point of view, the new boundary conditions on the worldsheet bosons generalize the non-commutativity of spacetime. Finally, we suggest some definitions for the non-commutativity parameter of the superstring worldsheet. Received: 19 April 2002 / Published online: 18 October 2002 RID="a" ID="a" e-mail: kamani@theory.ipm.ac.ir