David Bailin

A three-generation orbifold-compactified superstring model

Abstract 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 SUC(3)×SUL(3)×U2(1). Yukawa couplings in this model are also discussed.

Orbifold compactified superstring models with tetrahedral and octahedral point groups

Abstract For orbifolds with tetrahedral or octahedral point group and embedding of the complete space group in the gauge group, we study simultaneously the conditions for modular invariance to be satisfied, for complete generations of quarks and leptons to survive, and for realistic gauge symmetry breaking to occur either by “Wilson lines” alone or with additional intermediate scale breaking by a 27 + 27 . Models with four complete generations and a realistic gauge group are obtained.

Intersecting D5-brane models with massive vector-like leptons

We construct eight-stack intersecting D5-brane models, with an orbifold transverse space, that yield the (non-supersymmetric) standard model up to vector-like leptons. The matter includes right-chiral neutrinos and the models have the renormalisable Yukawa couplings to tachyonic Higgs doublets needed to generate mass terms for all matter, including the vector-like leptons. The models are constrained by the requirement that twisted tadpoles cancel, that the gauge boson coupled to the weak hypercharge U(1)Y does not get a string-scale mass via a generalised Green-Schwarz mechanism, and that there are no surviving, unwanted gauged U(1) symmetries coupled to matter. Gauge coupling constant ratios close to those measured are easily obtained for reasonable values of the parameters, consistently with having the string scale close to the electroweak scale, as required to avoid the hierarchy problem. Unwanted (colour-triplet, charged-singlet, and neutral-singlet) scalar tachyons can be removed by a suitable choice of the parameters.

Flavour-dependent CP violation and natural suppression of the electric dipole moments

Abstract We revisit the supersymmetric CP problem and find that it can be naturally resolved if the origin of CP violation is closely related to the origin of flavour structures. In this case, the supersymmetry breaking dynamics do not bring in any new CP-violating phases. This mechanism requires hermitian Yukawa matrices which naturally arise in models with a U (3) flavour symmetry. The neutron electric dipole moment (NEDM) is predicted to be within one-two orders of magnitude below the current experimental limit. The model also predicts a strong correlation between A CP ( b → sγ ) and the NEDM. The strong CP problem is mitigated although not completely solved.

MODULAR SYMMETRIES, THRESHOLD CORRECTIONS AND MODULI FOR ℤ2 × ℤ2 ORBIFOLDS

ℤ2 × ℤ2 Coxeter orbifolds are constructed with the property that some twisted sectors have fixed planes for which the six-torus cannot be decomposed into a direct sum T2 ⊕ T4 with the fixed plane lying in T2. The string loop threshold corrections to the gauge coupling constants are derived, and display symmetry groups for the T and U moduli that are subgroups of the full modular group PSL(2, ℤ). The effective potential for duality invariant gaugino condensate in the presence of hidden sector matter is constructed and minimized for the values of the moduli. The effect of Wilson lines on the modular symmetries is also studied.

Constructing the supersymmetric Standard Model from intersecting D6-branes on the Z6′ orientifold

Abstract Intersecting stacks of supersymmetric fractional branes on the Z 6 ′ orientifold may be used to construct the supersymmetric Standard Model. If a , b are the stacks that generate the SU ( 3 ) colour and SU ( 2 ) L gauge particles, then, in order to obtain just the chiral spectrum of the (supersymmetric) Standard Model (with non-zero Yukawa couplings to the Higgs multiplets), it is necessary that the number of intersections a ∩ b of the stacks a and b, and the number of intersections a ∩ b ′ of a with the orientifold image b ′ of b satisfy ( a ∩ b , a ∩ b ′ ) = ( 2 , 1 ) or ( 1 , 2 ) . It is also necessary that there is no matter in symmetric representations of the gauge group, and not too much matter in antisymmetric representations, on either stack. Fractional branes having all of these properties may be constructed on the Z 6 ′ orientifold. We provide a number of new examples having these properties, some of which may be extended to give the Standard Model spectrum. Specifically, we construct four-stack models with two further stacks, each with just a single brane, which have the matter spectrum of the supersymmetric Standard Model, including a single pair of Higgs doublets, plus three right-chiral neutrino singlets. Ramond–Ramond tadpole cancellation is achieved by the introduction of background H ¯ 3 flux, the 3-form field strength associated with the Kalb–Ramond 2-form field B 2 . There remains a single unwanted gauged U ( 1 ) B − L .

String unification and extra matter multiplets below the string scale

Abstract The implications of the running of gauge coupling constants from low energies to the sring scale for the presence of matter multiplets in addition to those of the supersymmetric standard model are discussed for string theories with flipped SU(5)×U(1) and SO(6)×SO(4) grand unification.

String Loop Threshold Corrections For ZN Coxeter Orbifolds.

The moduli dependence of string loop threshold corrections to gauge coupling constants is investigated for those ZN Coxeter orbifolds with the property that some twisted sectors have fixed planes for which the six-torus T6 cannot be decomposed into a direct sum T4 ⊕ T2 with the fixed plane lying in T2.

String unification, grand unification and string loop threshold corrections☆

Abstract A study is made of all ZN and ZM×ZN orbifolds to discover for which orbifolds string loop threshold corrections to the renormalisation group equations for the gauge coupling constants enable string unification together with SO(6)×SO(4), [SU(3)]3 or flipped SU(5)×U(1) grand unification to occur at a scale less than 1018 GeV.

Duality Symmetries in Orbifold Models.

Abstract We derive the duality symmetries relevant to moduli dependent gauge coupling constant threshold corrections, in Coreter Z N orbifolds. We consider those orbifolds for which the point group leaves fixed a two-dimensional sublattice Λ 2 , of the six-dimensional torus lattice Λ 6 , where Λ 6 cannot be decomposed as Λ 2 ⊗ Λ 4 .