Bogdan A. Dobrescu

Top Quark Seesaw Theory of Electroweak Symmetry Breaking

We study electroweak symmetry breaking involving the seesaw mechanism of quark condensation. These models produce a composite Higgs boson involving the left-handed top quark, yet the top mass arises naturally at the observed scale. We describe a schematic model which illustrates the general dynamical ideas. We also consider a generic low-energy effective theory which includes several composite scalars, and we use the effective potential formalism to compute their spectrum. We develop a more detailed model in which certain features of the schematic model are replaced by additional dynamics.

Electroweak Symmetry Breaking via a Top Condensation Seesaw Mechanism

We propose a new mechanism in which all of the electroweak symmetry breaking is driven by a natural top quark condensate, produced by {open_quotes}top-color{close_quotes} interactions at a multi-TeV scale. The scheme allows the observed top quark mass, and acceptable T and S parameters, by invoking a seesaw mechanism involving mixing of the top with an additional isosinglet quark. {copyright} {ital 1998} {ital The American Physical Society}

Light axion within the next-to-minimal supersymmetric standard model

We analyze the Higgs sector in the next-to-minimal Supersymmetric Standard Model, emphasizing the possibility of a light CP-odd scalar (axion) in the spectrum. We compute the coupling of the Standard-Model-like Higgs boson to a pair of axions, and show that it can be large enough to modify the Higgs branching fractions, with a significant impact on the Higgs searches. We delineate the range of parameters relevant for this scenario, and also derive analytic expressions for the scalar masses and couplings in two special cases - a decoupling limit where all scalars other than the axion are heavier than the Standard-Model-like Higgs boson, and the large tan β limit.

Higgs boson decays to CP-odd scalars at the Fermilab Tevatron and beyond

In extended Higgs models, the Higgs boson may decay into a pair of light CP-odd scalars, with distinctive collider signatures. We study the ensuing Higgs signals at the upgraded Fermilab Tevatron, considering the subsequent decays of the scalars into pairs of gluons or photons. For CP-odd scalars lighter than a few GeV, the Higgs boson manifests itself as a diphoton resonance and can be discovered up to masses of a few hundred GeV. For heavier CP-odd scalars the reach extends at most up to M{sub h}{approx}120GeV. We also discuss the capabilities of the CERN LHC and lepton colliders in these channels.

Resonances from two universal extra dimensions

Standard model gauge bosons propagating in two universal extra dimensions give rise to heavy spin-1 and spin-0 particles. The lightest of these, carrying Kaluza-Klein numbers (1,0), may be produced only in pairs at colliders, whereas the (1,1) modes, which are heavier by a factor of {radical}(2), may be singly produced. We show that the cascade decays of (1,1) particles generate a series of closely-spaced narrow resonances in the tt invariant mass distribution. At the Tevatron, s-channel production of (1,1) gluons and electroweak bosons will be sensitive to tt resonances up to masses in the 0.5-0.7 TeV range. Searches at the LHC for resonances originating from several higher-level modes will further test the existence of two universal extra dimensions.

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.

Chiral compactification on a square

We study quantum field theory in six dimensions with two of them compactified on a square. A simple boundary condition is the identification of two pairs of adjacent sides of the square such that the values of a field at two identified points differ by an arbitrary phase. This allows a chiral fermion content for the four-dimensional theory obtained after integrating over the square. We find that nontrivial solutions for the field equations exist only when the phase is a multiple of π/2, so that this compactification turns out to be equivalent to a T2/Z4 orbifold associated with toroidal boundary conditions that are either periodic or anti-periodic. The equality of the lagrangian densities at the identified points in conjunction with six-dimensional Lorentz invariance leads to an exact Z8 × Z2 symmetry, where the Z2 parity ensures the stability of the lightest Kaluza-Klein particle.

GENERIC AND CHIRAL EXTENSIONS OF THE SUPERSYMMETRIC STANDARD MODEL

We construct extensions of the Standard Model in which the gauge symmetries and supersymmetry prevent the dangerously large effects that may potentially be induced in a supersymmetric standard model by Planck scale physics. These include baryon number violation, flavor-changing neutral currents, the μ-term, and masses for singlet or vector-like fields under the Standard Model gauge group. For this purpose we introduce an extra non-anomalous U(1)μ gauge group. Dynamical supersymmetry breaking in a secluded sector triggers the breaking of the U(1)μ and generates soft masses for the superpartners via gauge mediation, with the scalars possibly receiving sizable contributions from the U(1)μ D-term. We find several classes of complete and calculable models, in which the messengers do not present cosmological problems and neutrino masses can also be accommodated. We derive the sparticle spectrum in these models and study the phenomenological consequences. We give an exhaustive list of the potential experimental signatures and discuss their observability in the upcoming Tevatron runs. One class of models exhibits interesting new discovery channels, namely WWT, WγT and WZT, which arise when the next-to-lightest supersymmetric particle is a short-lived SU(2)w neutralino.

Coupling-mass mapping of dijet peak searches

We study hypothetical gauge bosons that may produce dijet resonances at the LHC. Simple renormalizable models include leptophobic Zbosons or colorons that have flavor- independent couplings and decay into a color-singlet or -octet quark-antiquark pair, respec- tively. We present the experimental results on dijet resonances at hadron colliders as limits in the coupling-versus-mass plane of a gauge boson associated with baryon number. This theoretical framework facilitates a direct comparison of dijet resonance searches performed at different center-of-mass energies or at different colliders.

Six-dimensional gauge theory on the chiral square

We construct gauge theories in two extra dimensions compactified on the chiral square, which is a simple compactification that leads to chiral fermions in four dimensions. Stationarity of the action on the boundary specifies the boundary conditions for gauge fields. Any six-dimensional gauge field decomposed in Kaluza-Klein modes includes a tower of heavy spin-1 particles whose longitudinal polarizations are linear combinations of the extra-dimensional components, and a tower of heavy spin-0 particles corresponding to the orthogonal combinations. These linear combinations depend on the Kaluza-Klein numbers, and are independent of the gauge fixing. If the gauge symmetry is broken by the vacuum expectation value of a six-dimensional scalar, at each Kaluza-Klein level three spinless fields in the adjoint representation mix to provide the longitudinal polarization of the spin-1 mode, leaving the orthogonal states as two spin-0 particles. We derive the interactions of the Kaluza-Klein modes for generic gauge theories, laying the groundwork for the Standard Model in two universal extra dimensions, and more generally for future model building and phenomenological studies.