We consider the novel Kaluza-Klein (KK) scenario where gravity propagates in the 4 + n dimensional bulk of spacetime, while gauge and matter fields are confined to the 3 + 1 dimensional world-volume of a brane configuration. For simplicity we assume compactification of the extra n dimensions on a torus with a common scale R, and identify the massive KK states in the four-dimensional spacetime. For a given KK level n there are one spin-2 state, (n − 1) spin-1 states and n(n − 1)/2 spin-0 states, all mass-degenerate. We construct the effective interactions between these KK states and ordinary matter fields (fermions, gauge bosons and scalars). We find that the spin-1 states decouple and that the spin-0 states only couple through the dilaton mode. We then derive the interacting Lagrangian for the KK states and Standard Model fields, and present the complete Feynman rules. We discuss some low energy phenomenology for these new interactions for the case when 1/R is small compared to the electroweak scale, and the ultraviolet cutoff of the effective KK theory is on the order of 1 TeV. Phys. Rev. D59, 105006 (1999). * Here we choose the gauge condition for the sake of clarity; the definitions of physical fields in Eq. (17) do not depend on the gauge choice. Published in

In this paper we compute one-loop corrections to masses and couplings in the minimal supersymmetric standard model. We present explicit formulae for the complete corrections and a set of compact approximations which hold over the unified parameter space associated with radiative electroweak symmetry breaking. We illustrate the importance of the corrections and the accuracy of our approximations by scanning over the parameter space. We calculate the supersymmetric one-loop corrections to the W -boson mass, the effective weak mixing angle, and the quark and lepton masses, and discuss implications for gauge and Yukawa coupling unification. We also compute the one-loop corrections to the entire superpartner and Higgs-boson mass spectrum. We find significant corrections over much of the parameter space, and illustrate that our approximations are good to O(1%) for many of the superparticle masses.

We show how the motion through the extra dimensions of a gas of branes and antibranes can, under certain circumstances, produce an era of inflation as seen by observers trapped on a 3-brane, with the inflaton being the inter-brane separation. Although most of our discussion refers to arbitrary p-branes, when we need to be specific we assume that they are D-branes of Type II or Type I string theory. For realistic brane couplings, such as those arising in string theory, the inter-brane potentials are too steep to inflate the universe for acceptably long times. However, for special regions of the parameter space of brane-antibrane positions the brane motion is slow enough for there to be sufficient inflation. Inflation would be more generic in models where the inter-brane interactions are much weaker. The spectrum of primordial density fluctuations predicted has index n slightly less than 1, and an acceptable amplitude, provided that the extra dimensions have linear size 1/r ∼ 10 12 GeV. Reheating occurs as in hybrid inflation, with the tachyonic instability of the brane-antibrane system taking over for small separations. The tachyon field can induce a cascade mechanism within which higher-dimension branes annihilate into lower-dimension ones. We argue that such a cascade naturally stops with the production of 3-branes in 10-dimensional string theory.

Motivated by orbifold grand unified theories, we construct a class of three-family Pati-Salam models in a Z 6 abelian symmetric orbifold with two discrete Wilson lines. These models have marked differences from previously-constructed three-family models in prime-order orbifolds. In the limit where one of the six compactified dimensions (which lies in a Z 2 sub-orbifold) is large compared to the string length scale, our models reproduce the supersymmetry and gauge symmetry breaking pattern of 5d orbifold grand unified theories on an S 1 /Z 2 orbicircle. We find a horizontal 2 + 1 splitting in the chiral matter spectra -2 families of matter are localized on the Z 2 orbifold fixed points, and 1 family propagates in the 5d bulk -and identify them as the first-two and third families. Remarkably, the first two families enjoy a non-abelian dihedral D 4 family symmetry, due to the geometric setup of the compactified space. In all our models there are always some color triplets, i.e. (6, 1, 1) representations of the Pati-Salam group, survive orbifold projections. They could be utilized to spontaneously break the Pati-Salam symmetry to that of the Standard Model. One model, with a 5d E 6 symmetry, may give rise to interesting low energy phenomenology. We study gauge coupling unification, allowed Yukawa couplings and some of their phenomenological consequences. The E 6 model has a renormalizable Yukawa coupling only for the third family. It predicts a gauge-Yukawa unification relation at the 5d compactification scale, and is capable of generating reasonable quark/lepton masses and mixings. Potential problems are also addressed, they may point to the direction for refining our models.

A three-generation Pati-Salam model is constructed by compactifying the heterotic string on a particular T^6/Z_6 Abelian symmetric orbifold with two discrete Wilson lines. The compactified space is taken to be the Lie algebra lattice G_2+SU(3)+SO(4). When one dimension of the SO(4) lattice is large compared to the string scale, this model reproduces many features of a 5d SO(10) grand unified theory compactified on an S^1/Z_2 orbifold. (Of course, with two large extra dimensions we can obtain a 6d SO(10) grand unified theory.) We identify the orbifold parities and other ingredients of the orbifold grand unified theories in the string model. Our construction provides a UV completion of orbifold grand unified theories, and gives new insights into both field theoretical and string theoretical constructions.Comment: 15 pages, 3 figures, 1 table, revtex

We study the observability of a Standard Model-like Higgs boson at an upgraded Fermilab Tevatron via the mode h → W * W * . We concentrate on the main channel gg → h → W * W * → ℓνlν. We also find the modeWe perform detector level simulations by making use of a Monte Carlo program SHW. Optimized searching strategy and kinematical cuts are developed. We find that with a c. m. energy of 2 TeV and an integrated luminosity of 30 fb −1 the signal should be observable at a 3σ level or better for the mass range of 145 GeV < ∼ m h < ∼ 180 GeV. For 95% confidence level exclusion, the mass reach is 135 GeV < ∼ m h < ∼ 190 GeV. We also present results of studying these channels with a model-independent parameterization. Further improvement is possible by including other channels. We conclude that the upgraded Fermilab Tevatron will have the potential to significantly advance our knowledge of Higgs boson physics.

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We analyse the cosmological implications of brane-antibrane systems in string-theoretic orbifold and orientifold models. In a class of realistic models, consistency conditions require branes and antibranes to be stuck at different fixed points, and so their mutual attraction generates a potential for one of the radii of the underlying torus or the 4D string dilaton. Assuming that all other moduli have been fixed by string effects, we find that this potential leads naturally to a period of cosmic inflation with the radion or dilaton field as the inflaton. The slow-roll conditions are satisfied more generically than if the branes were free to move within the space. The appearance of tachyon fields at certain points in moduli space indicates the onset of phase transitions to different non-BPS brane systems, providing ways of ending inflation and reheating the corresponding observable brane universe. In each case we find relations between the inflationary parameters and the string scale to get the correct spectrum of density perturbations. In some examples the small numbers required as inputs are no smaller than 0.01, and are the same small quantities which are required to explain the gauge hierarchy.