A three-generation orbifold-compactified superstring model

A generalised Z-orbifold model is described in which, as a result of nonstandard embedding of the point group and nontrivial embedding of the discrete translations, the number of generations is reduced to three and the observable gauge group is broken to SU_C(3)×SU_L(3)×U²(1). Yukawa couplings in this model are also discussed.     

Phenomenology of superstring theories with SO(10) unification

The phenomenology of superstring theories in which the gauge symmetry after compactification is SO(10) × E′₈, instead of E₆ × E′₈, is discussed. In particular, the generation of an intermediate scale, the radiative breaking of electroweak symmetry, and the supersymmetric particle spectrum are investigated.     

A superstring model of four generations

We construct a possibly realistic four-generation Calabi-Yau manifold by dividing the algebraic variety in CP₄ × CP₄ with the Z₂×Z₂ symmetry. The nontrivial embedding of Z₂×Z₂ in E(6) allows physically intriguing intermediate symmetry based on the U(1)×SU(2)_L×SU(2)_R×SU(4)_C group. Also, the group of honest symmetries G_H of the manifold is identified.     

A new mechanism for baryogenesis

A new mechanism for baryogenesis is proposed. It is argued that, after inflation, the scalar quarks and leptons of a supersymmetric GUT may have large expectation values. The subsequent evolution of such a system is shown to generate a significant baryon density. For typical values of the parameters, n_B/n_γ may be as large as 10³.     

Perturbative and nonperturbative contributions to a simple model for baryogenesis

Single field baryogenesis, a scenario for Dirac leptogenesis sourced by a time-dependent scalar condensate, is studied on a toy model. We compare the creation of the charge asymmetry by the perturbative decay of the condensate with the nonperturbative decay, a process of particle production commonly known in the context of inflation as preheating. Neglecting backreaction effects, we find that over a wide parametric range perturbative decay and preheating contribute by the same order of magnitude to the baryon asymmetry.     

Mac hypothesis in a superstring inspired model

We study a superstring inspired E₆ model based on the MAC hypothesis. We find two kinds of model in which the proton decay through the colored Higgs is naturally suppressed and every field in 27 of E₆ carries a role; a pair of colored fields in 27 are necessary to cause the MAC condensation and two doublet fields in 27 cause the usual Higgs mechanism at the electroweak breaking. The fields left in 27 are identified with quarks and leptons adding to v_L^c.     

An extended technicolour model for grand unification

An extended technicolour grand unification model based on the gauge groupE ₆×SU(7) extended technicolour is presented. The symmetry-breaking based on extended technicolour theory is discussed. It is shown that the existing phenomenology is well explained by the model. The strangeness changing neutral currents may not be a problem with this model.     

On inflation in the heterotic superstring model

The possibility is considered of achieving inflation in the field-theory limit of the E₈ X E₈ superstring model, which is an N = 1 supergravity theory possessing a “no-scale” SU(n, 1)/SU(n) × U(1) structure. It is shown that neither type I inflation (due to higher-derivative terms O(R²)), nor inflation due to a SUSY-breaking gaugino-condensation potential, is possible, essentially because of the absence of free dimensionless parameters. Kaluza-Klein type inflation is ruled out because the internal space is Ricci flat. The occurence of type II inflation (due to some gauge singlet “inflation” field φ) depends upon the form of the superpotential F and of the Kähler potential G, but this also seems not to be possible, unless the >SU(n, 1) symmetry can be broken in a particular way. Hence, some new type of compactification scheme may be called for, or a different type of inflation.     

Pohlmeyer reduction of superstring sigma model

Motivated by a desire to find a useful 2d Lorentz-invariant reformulation of the AdS₅×S⁵ superstring world-sheet theory in terms of physical degrees of freedom we construct the “Pohlmeyer-reduced” version of the corresponding sigma model. The Pohlmeyer reduction procedure involves several steps. Starting with a coset space string sigma model in the conformal gauge and writing the classical equations in terms of currents one can fix the residual conformal diffeomorphism symmetry and kappa-symmetry and introduce a new set of variables (related locally to currents but non-locally to the original string coordinate fields) so that the Virasoro constraints are automatically satisfied. The resulting equations can be obtained from a Lagrangian of a non-Abelian Toda type: a gauged WZW model with an integrable potential coupled also to a set of 2d fermionic fields. A gauge-fixed form of the Pohlmeyer-reduced theory can be found by integrating out the 2d gauge field of the gauged WZW model. The small-fluctuation spectrum near the trivial vacuum contains 8 bosonic and 8 fermionic degrees of freedom with equal mass. We conjecture that the reduced model has world-sheet supersymmetry and is ultraviolet-finite. We show that in the special case of the AdS₂×S² superstring model the reduced theory is indeed supersymmetric: it is equivalent to the N=2 supersymmetric extension of the sine-Gordon model.     

On superdense superstring gases: A heretic string model approach

A simple model is proposed to describe a gas of free strings at very high energy density, particularly in connection with a recently discovered phase dominated by macroscopic strings. This phase appears at high energy density if open strings are forbidden. For oriented strings the probability density for strings in the energy interval (E, E + dE) is dE / E in this phase. If open strings are allowed no such phase appears. The model is compared with superstring calculations, and relevant quantities are shown to agree. The model gives a simple and intuitively appealing picture of dense, free string gases. The effective string tension is also discussed and shown to vanish at the critical temperature.     

Baryogenesis and entropy generation in a superstring model

We work out details of a previously proposed senario for baryogenesis in a superstring model, based on the Affleck-Dine mechanism exploiting the decays of large expectation values for squark and slepton fields after inflation. We show how the coefficients of the relevant quartic scalar operators can aquire the necessary imaginary parts. We examine possible sources of entropy among the gauge-singlet scalar and fermion fields including the gravitino, as well as the low-energy SU(2)_L × U(1)² → SU(2)_L × U(1)_Y → U(1)_em phase transitions. We argue that none of these dilute substantailly the baryon-to-entropy ratio, which can easily be in the range preferred by cosmology. This means, in particular, that the Polonyi and gravitino problems are solved.     

A grand unification preon model with E6 metacolor

We have constructed a version of the chiral three-preon model E₆ × SO(10) that satisfies the condition of asymptotic freedom in the metacolor and composite color-flavor sectors. The construction is based on the global color-flavor symmetry group SU(18). By applying the 't Hooft anomaly matching condition to the subgroup SU(16) × SU(2) of SU(18), together with a few physical constraints, we obtain a unique solution that gives rise to precisely three generations of the spinorial representation 16 of SO(10) without exotics. Except for N = 18, no solution exists for the global color-flavor group satisfying metacolor asymptotic freedom (N < 22) when SU(N) breaks to SU(16) × SU(N − 16).     

Brief comment on electro-weak breaking in superstring inspired model

We reconsider Kalyniak and Sundaresan's (KS's) criticism about the models obtained by Breit et al. totally from a more phenomenological point of view. We study which fields inN _f (27) must acquire the vacuum expectation value (vev) rather than ones in \(\delta (27 + \overline {27} )\) in order to break down toSU(3) _c ×U(1)_em symmetry without any extraU(1) at weak scale and endow quarks and leptons with masses. General sets of fields to acquire the vev are obtained. Further utilizing maximally the light components of \(\delta (27 + \overline {27} )\) , minimal sets of fields inN _f (27) to acquire the vev is listed up in each case of Breit et al. Also applying the conditions given there to more general cases, we find that there is only model free from KS's criticism.     

Phenomenology of a four generation superstring model

We discuss the quark-lepton phenomenology of a superstring model compactified on aCP ⁴-based Calabi-Yau manifold defined by? _i Z _i ⁵ = 5 _c Z ₁ Z ₂ Z ₃ Z ₄ Z ₅ leading to four fermion generations using the Yukawa couplings calculated by us in a previous paper. We note that maintaining supersymmetry implies that forc?1, one of theU(1)-subgroup ofE ₆ must break at the Planck scale; on the other hand forc?10⁺⁴, an intermediate scale of about 10¹¹ GeV or 10¹⁴ GeV may be generated without destabilizing the gauge hierarchy. The model may have problems with fast proton decay for some choice ofc; however, understanding small neutrino masses remains a problem.