arXiv subjects

Bogdan A. Dobrescu

Publications and source records attributed to Bogdan A. Dobrescu.

87 records · Page 5Linked to original sources

arXiv1999

Electroweak symmetry breaking as a consequence of compact dimensions

It has been shown recently that the Higgs doublet may be composite, with the left-handed top-bottom doublet and a new right-handed anti-quark as constituents bound by some four-quark operators with nonperturbative coefficients. I show that these operators are naturally induced if there are extra space dimensions with a compactification scale in the multi-TeV range. The Higgs compositness is due mainly to the Kaluza-Klein modes of the gluons, while flavor symmetry breaking may be provided by various fields propagating in the compact dimensions. I comment briefly on the embedding of this scenario in string theory.

hep-ph↗

arXiv1999

Gauge Coupling Unification with Extra Dimensions and Gravitational Scale Effects

We study gauge coupling unification in the presence of extra dimensions compactified at a few TeV. Achieving unification requires a large number of gauge boson Kaluza-Klein excitations lighter than the string scale, such that the higher-dimensional gauge couplings are O(1) in string scale units. Corrections to the gauge couplings from two or more loops are about 10% or larger, hence string (or M) theory is generally expected to be strongly coupled in TeV-scale extra-dimensional scenarios. Higher-dimensional operators induced by quantum gravitational effects can shift the gauge couplings by a few percent. These effects are sufficiently large that even the minimal Standard Model, or the MSSM, allow unification at a scale in the TeV range. The strongly coupled unified theory may induce dynamical electroweak symmetry breaking.

hep-ph↗

arXiv1999

Higgs Compositeness from Top Dynamics and Extra Dimensions

The fundamental Higgs doublet may be replaced in the Standard Model by certain non-perturbative four-quark interactions, whose effect is to induce a composite Higgs sector responsible for electroweak symmetry breaking. A simple composite two-Higgs-doublet model is presented. The four-quark interactions arise naturally if there are either extra spatial dimensions or larger gauge symmetries at a multi-TeV scale. Some theoretical and phenomenological implications of these scenarios are discussed.

hep-ph↗

arXiv1998

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 accomodated. 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 $WW \not E_T$, $Wγ\not E_T$ and $WZ \not E_T$, which arise when the next-to-lightest supersymmetric particle is a short-lived $SU(2)_W$ neutralino.

hep-ph↗

arXiv1998

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.

hep-ph↗

arXiv1998

Electroweak Symmetry Breaking via Top Condensation Seesaw

We propose a new mechanism in which all of the electroweak symmetry breaking is driven by a natural top quark condensate, produced by topcolor 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.

hep-ph↗

arXiv1998

Top-Bottom Splitting in Technicolor with Composite Scalars

We present a model of dynamical electroweak symmetry breaking in which the splitting between the top and bottom quark masses arises naturally. The W and Z masses are produced by a minimal technicolor sector, the top quark mass is given by the exchange of a weak-doublet technicolored scalar, and the other quark and lepton masses are induced by the exchange of a weak-doublet technicolor-singlet scalar. We show that, in the presence of the latter scalar, the vacuum alignment is correct even in the case of SU(2) technicolor. The fit of this model to the electroweak data gives an acceptable agreement (chi-squared = 28 for 20 degrees of freedom). The mass hierarchy between the standard fermions other than top can also be explained in terms of the hierarchy of squared-masses of some additional scalars. We discuss various possibilities for the compositeness of the scalars introduced here.

hep-ph↗

arXiv1998

A Chiral Supersymmetric Standard Model

We propose a supersymmetric extension of the Standard Model with an extra U(1) gauge symmetry, so that all supersymmetric mass terms, including the $μ$-term, are forbidden by the gauge symmetries. Supersymmetry is broken dynamically which results in U(1) breaking and generation of realistic $μ$ term and soft breaking masses. The additional fields required to cancel the U(1) anomalies are identified with the messengers of supersymmetry breaking. The gaugino masses arise as in the usual gauge mediated scenario, while squarks and sleptons receive their masses from both the U(1) $D$-term and the two-loop gauge mediation contributions. The scale of supersymmetry breaking in this model can be below $10^6$ GeV, yielding collider signatures with decays to goldstinos inside the detector.

hep-ph↗

arXiv1998

Testing Technicolor with Scalars at the First Muon Collider

An interesting class of models of dynamical electroweak symmetry breaking allows only the third generation fermions to acquire dynamical masses, such that the masses of the first two generations should be given by coupling to a nonstandard ``Higgs'' doublet. The scalars in this case have large couplings to the second generation, so that they are copiously produced at a muon collider. We analyze the potential for discovery of the neutral scalars in the $s$-channel, and we show that at resonance there will be observed in excess of $10^5$ events per year.

hep-ph↗

arXiv1997

Negative contributions to S in an effective field theory

We show that an effective field theory that includes non-standard couplings between the electroweak gauge bosons and the top and bottom quarks may yield negative contributions to both the S and T oblique radiative electroweak parameters. We find that that such an effective field theory provides a better fit to data than the standard model (the chi-squared per degree of freedom is half as large). We examine in some detail an illustrative model where the exchange of heavy scalars produces the correct type of non-standard couplings.

hep-ph↗

arXiv1997

B - L Mediated Supersymmetry Breaking

I present a realistic model of dynamical supersymmetry breaking, in which a $U(1)_{B-L}$ gauge interaction communicates supersymmetry breaking to the standard fields. A distinctive superpartner spectrum is predicted in this model.

hep-ph↗

arXiv1996

The strong CP problem versus Planck scale physics

We discuss conditions that should be satisfied by axion models for solving the strong CP problem. It has been observed that Planck scale effects may render the axion models ineffective if there are gauge invariant operators of dimension less than 10 which break explicitly the Peccei-Quinn (PQ) symmetry. We argue that only those operators formed of fields which have vacuum expectation values are dangerous. Supersymmetric axion models fail to prevent even this restricted class of operators. Furthermore, the models that relate the PQ scale and the supersymmetry breaking scale are particularly sensitive to gauge invariant PQ-breaking operators. By contrast, in non-supersymmetric composite axion models the PQ scale arises naturally, and the dangerous operators can be avoided. However, the composite axion models contain heavy stable particles which are cosmologically ruled out. Another problem is a Landau pole for the QCD coupling constant. Both these problems may be solved if the unification of color with the gauge interactions which bind the axion could be achieved.

hep-ph↗

arXiv1996

Vacuum Instability in Low-Energy Supersymmetry Breaking Models

We show that for the simplest models of gauge mediated supersymmetry breaking, including all existing models, the true vacuum will not preserve QCD if it communicates supersymmetry breaking to the visible sector. We show that the desired supersymmetry breaking color preserving vacuum can nonetheless be stable cosmologically, but only if certain couplings are sufficiently small. We also present simple modifications to the sector which communicates supersymmetry breaking in which the true vacuum is acceptable, suggesting desirable properties to be sought in dynamical models of supersymmetry breaking.

hep-ph↗

arXiv1996

Gluino mass from dynamical supersymmetry breaking

We present a new mechanism for gluino mass generation in models of dynamical supersymmetry breaking. The mechanism requires two colored chiral superfields which feel a nonabelian gauge interaction such that a fermion condensate is formed at a scale of order 1 TeV. Renormalizable hidden sector models, which typically yield unacceptably light gauginos, become viable if a gauge singlet is coupled to these chiral superfields. Moreover, the interactions of the gauge singlet with the Higgs superfields substitute the $μ$-term. Visible sector models can also incorporate this mechanism; however, the models of dynamical supersymmetry breaking previously analyzed cannot be fitted in a simple visible sector model because they lead to vacuum expectation values for charged scalars.

hep-ph↗

arXiv1995

Fermion Masses without Higgs: A Supersymmetric Technicolor Model

We propose a supersymmetric technicolor model in which the electroweak symmetry breaking is communicated to the quarks and leptons by technicolored $SU(2)_W$-singlet scalars. When the technifermions condense, the quarks and leptons of the third generation acquire mass. The fermions of the other generations do not couple to the technicolored scalars but they receive masses from radiative corrections involving superpartners. As a result, the mass hierarchy between the fermion generations arises naturally. The model predicts the CP asymmetries in $B$ meson decays and in $ΔS = 1$ transitions to be smaller by two orders of magnitude than the ones predicted in the Standard Model.

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