Standard Model Gauge Symmetry Research Articles

Quasi-supersymmetric G3 unification from intersecting D6-branes on Type IIA orientifolds

We construct three quasi-supersymmetric G3 GUT models with S3 symmetry and gauge coupling unification from intersecting D6-branes on Type IIA orientifolds. The Standard Model fermions and Higgs doublets can be embedded into the bifundamental representations in these models, and there is no any other unnecessary massless representation. Especially in Model I with gauge group U(4)3, we just have three-family SM fermions and three pairs of Higgs particles. The G3 gauge symmetry in these models can be broken down to the Standard Model gauge symmetry by introducing light open string states. And 1 TeV scale supersymmetry breaking soft masses imply the reasonable intermediate string scale.

U(1) masses in intersecting D-brane SM-like models

For recently constructed classes of D6-brane models, yielding the Standard Model fermion spectrum and gauge symmetry, we compute lower bounds on the masses of new U(1) fields that such models predict in addition to the hypercharge U(1) Y . In models with extra dimensions, generic uncertainties due to unknown values of the compactification radii of the extra dimensions affect the value of the string scale and thus the predictive power of such models. Using ρ parameter and Z– U(1) mixing-angle constraints we show how to avoid such uncertainties, to provide lower mass bounds for the additional U(1) fields. These are in the region above 750 GeV for mixing angles less than 1.5×10 −3 (and as low as 550 GeV for mixing angles of 3×10 −3).

τ−μflavor violation as a probe of the scale of new physics

Motivated by the recent strong experimental evidence of large ${\ensuremath{\nu}}_{\ensuremath{\mu}}\ensuremath{-}{\ensuremath{\nu}}_{\ensuremath{\tau}}$ neutrino mixing, we explore current bounds on the analogous mixing in the charged lepton sector. We present a general formalism for dimension-6 fermionic effective operators involving $\ensuremath{\tau}\ensuremath{-}\ensuremath{\mu}$ mixing with a typical Lorentz structure $(\overline{\ensuremath{\mu}}\ensuremath{\Gamma}\ensuremath{\tau})({q}^{\ensuremath{\alpha}}\ensuremath{\Gamma}{q}^{\ensuremath{\beta}}),$ and discuss their relationship to the standard model gauge symmetry and the underlying flavor dynamics. We derive the low-energy constraints on the new physics scale associated with each operator, mostly from current experimental bounds on rare decay processes of $\ensuremath{\tau},$ hadrons or heavy quarks. For operators involving at least one light quark $(u,d,s),$ these constraints typically give a bound on the new physics scale of a few TeV or higher. Those operators with two heavy quarks turn out to be more weakly constrained at the present, giving bounds of a few hundred GeV. A few scalar and pseudoscalar operators are free from all current experimental constraints.

Family unification in five and six dimensions

In family unification models, all three families of quarks and leptons are grouped together into an irreducible representation of a simple gauge group, thus unifying the Standard Model gauge symmetries and a gauged family symmetry. Large orthogonal groups, and the exceptional groups $E_7$ and $E_8$ have been much studied for family unification. The main theoretical difficulty of family unification is the existence of mirror families at the weak scale. It is shown here that family unification without mirror families can be realized in simple five-dimensional and six-dimensional orbifold models similar to those recently proposed for SU(5) and SO(10) grand unification. It is noted that a family unification group that survived to near the weak scale and whose coupling extrapolated to high scales unified with those of the Standard model would be evidence accessible in principle at low energy of the existence of small (Planckian or GUT-scale) extra dimensions.

SO(10)grand unification in five dimensions: Proton decay and theμproblem

We construct a minimal supersymmetric SO(10) grand unified model in 5 dimensions. The extra dimension is compactified on an S^1/(Z_2 x Z_2^\prime) orbifold which has two in-equivalent fixed points. These are flat 4-dimensional Minkowski spaces: the visible and the hidden branes. By orbifolding, the gauge symmetry on the hidden brane is reduced down to the Pati--Salam gauge symmetry SU(4) x SU(2)_L x SU(2)_R. On the visible brane the SO(10) is broken by the ordinary Higgs mechanism down to SU(5). The resulting 4-dimensional theory has the standard model gauge symmetry (the intersection of SU(5) and SU(4) x SU(2)_L x SU(2)_R) and the massless spectrum consists of the MSSM gauge fields and two Higgs doublets. The matter fields are assumed to live on the visible brane. We discuss gauge coupling unification in our 5-dimensional model in terms of corrections to the conventional 4-dimensional unification. Supersymmetry is broken on the hidden brane (where mass terms for gauginos and a mu term are generated) and communicated to squarks and sleptons via gaugino mediation. We also discuss a possibility of linking the supersymmetry breaking on the hidden brane to the Higgs mechanism responsible for partial breaking of the gauge symmetry on the visible brane via the shining mechanism. Finally, there are no operators of dimension 5 leading to proton decay. Proton decay through dimension 6 operators is enhanced compared to conventional GUTs and can be seen in current or next generation proton decay experiments.

Yukawa textures in Horava–Witten M-theory

Recent advances in 11-dimensional Horava–Witten M-theory based on non-standard embeddings with torus fibered Calabi–Yau manifolds have allowed the construction of three generation models with Wilson line breaking to the Standard Model gauge symmetry. Central to these constructions is the existence of a set of 5-branes in the bulk. We examine within this framework the general structure of the matter Yukawa couplings and show that M-theory offers an alternate possible way of achieving the CKM and quark mass hierarchies without introducing undue fine tuning or (as in conventional analysis) small parameters raised to high powers. A phenomenological example is presented in accord with all CKM and quark mass data requiring mainly that the 5-branes cluster near the second orbifold plane, and that the instanton charges of the physical orbifold plane vanish. An explicit example of a three generation model with vanishing physical plane instanton charges based on a torus fibered Calabi–Yau three fold with a del Pezzo base dP 7 and Wilson line breaking is constructed.

Self-breaking of the standard model gauge symmetry

If the gauge fields of the Standard Model propagate in TeV-size extra dimensions, they rapidly become strongly coupled and can form scalar bound states of quarks and leptons. If the quarks and leptons of the third generation propagate in 6 or 8 dimensions, we argue that the most tightly bound scalar is a composite of top quarks, having the quantum numbers of the Higgs doublet and a large coupling to the top quark. In the case where the gauge bosons propagate in a bulk of a certain volume, this composite Higgs doublet can successfully trigger electroweak symmetry breaking. The mass of the top quark is correctly predicted to within 20%, without the need to add a fundamental Yukawa interaction, and the Higgs boson mass is predicted to lie in the range 165 - 230 GeV. In addition to the Higgs boson, there may be a few other scalar composites sufficiently light to be observed at upcoming collider experiments.

CP violation in the Cabbibo-suppressed decay τ→ Kπντ with polarized τ leptons

CP violation from physics beyond the Standard Model (SM) is investigated in the Cabbibo-suppressed decay τ→ Kπν τ with polarized τ leptons, to which both the J P=1 − resonance K ∗ and the J P=0 + resonance K ∗ 0 contribute. In addition to the CP-odd rate asymmetry, τ polarization enables us to construct three additional CP-odd polarization asymmetries that can be enhanced due to the interference between the K ∗ and K ∗ 0 , and whose magnitudes depend crucially on the K ∗ 0 decay constant, f K ∗ 0 . Taking a QCD sum rule estimate of f K ∗ 0 =45 MeV and the present experimental constraints on the CP-odd parameters into account, we estimate quantitatively the maximally-allowed values for the CP-odd rate and polarization asymmetries in the multi-Higgs-doublet (MHD) model and the scalar-leptoquark (SLQ) models consistent with the SM gauge symmetry where neutrinos are massless and left-handed as in the SM. We find that the CP-odd rate and polarization asymmetries are of a similar size for highly-polarized τ leptons and, for their maximally-allowed values, CP violation in the MHD model and two SLQ models may be detected with about 10 6 and 10 7 τ's at the 1 σ level.

Quantum restoration of the U(1)Y symmetry in dynamically broken SUSY-GUT's

We propose a supersymmetric hypercolor SU(3)_H gauge theory interacting strongly at the grand unification scale, in which the hyperquark condensation breaks SU(5)_GUT down to SU(3)_C x SU(2)_L without unbroken U(1)_Y at the classical level. However, we show that the broken U(1)_Y symmetry is restored by quantum mechanical effects and hence there remains the standard-model gauge symmetry at the electroweak scale. The dynamics of the strong interactions also produces naturally a pair of massless Higgs doublets. In addition to these Higgs doublets, we have a pair of massless singlets which contributes to the renormalization-group equations of gauge coupling constants and hence affects the GUT unification. We discuss a simple solution to this problem.

New low-energy leptoquark interactions

We discuss an extension of the standard model (SM) with vector and scalar leptoquarks. The renormalizable leptoquark Lagrangian consistent with the SM gauge symmetry is presented including the leptoquark-Higgs interactions previously not considered in the literature. We discuss the importance of these new interactions for leptoquark phenomenology. After the electro-weak symmetry breaking they generate non-trivial leptoquark mass matrices. These lead to mixing between different SU (2) l -multiplets of the leptoquarks and induce at low energies new effective 4-fermion lepton-quark vertices. The latter affect the standard leptoquark phenomenology. We discuss constraints on these interactions from the helicity-suppressed π → ν + e decay.