Search Results (80 total)

I discuss some simple aspects of the low-energy physics of a nontrivial scale invariant sector of an effective field theory—physics that cannot be described in terms of particles. I argue that it is important to take seriously the possibility that the unparticle stuff described by such a theory might actually exist in our world. I suggest a scenario in which some details of the production of unparticle stuff can be calculated. I find that in the appropriate low-energy limit, unparticle stuff with scale dimension d_U looks like a nonintegral number d_U of invisible particles. Thus dramatic evidence for a nontrivial scale invariant sector could show up experimentally in missing energy distributions.

Motivated by recent works on “Higgsless theories,” I discuss an SU(2)₀×SU(2)^N×U(1) gauge theory with arbitrary bifundamental (or custodial SU(2) preserving) symmetry breaking between the gauge subgroups and with ordinary matter transforming only under the U(1) and SU(2)₀. When the couplings, g_j, of the other SU(2)s are very large, this reproduces the standard model at the tree level. I calculate the W and Z masses and other electroweak parameters in a perturbative expansion in 1/g_j², and give physical interpretations of the results in a mechanical analog built out of masses and springs. In the mechanical analog, it is clear that even for arbitrary patterns of symmetry breaking, it is not possible (in the perturbative regime) to raise the Higgs mass by a large factor while keeping the S parameter small.

We construct renormalizable, asymptotically free, four-dimensional gauge theories that dynamically generate a fifth dimension.

We note that orbifold boundary conditions that produce chiral fermion zero modes in compactified higher dimensional theories may distort scalar field vacuum expectation values, giving rise to nontrivial dependence on the extra dimensions. We illustrate this in a simple five-dimensional model, which has chiral fermion zero modes stuck to fat branes. The model could provide a simple and explicit realization of the separation of quarks and leptons in the fifth dimension. We discuss the Kaluza-Klein expansion in some detail. We find that there are, in general, non-zero-mode states stuck to the brane, just as the chiral zero mode is. We see explicitly the transition from the states dominated by the internal structure of the fat brane to those dominated by the compactification.

We discuss an alternative to the top-color seesaw mechanism. In our scheme, all the light quarks carry top-color, and there are many composite SU(2) doublets. This makes it possible to get the observed t quark mass and observed SU(2)×U(1) breaking in a way that is quite different from the classic seesaw mechanism. We discuss a model of this kind that arises naturally in the context of dynamically broken top-color. There are many composite scalars in a theory of this kind. This has important effects on the Pagels-Stokar relation and the Higgs boson mass. We find m_Higgs≲330 GeV, lighter than in typical top-color models. We also show that the electroweak singlet quarks in such a model can be lighter than the corresponding quarks in a seesaw model.

Recent data suggest a simple and intriguing form of the neutrino mass matrix. We show how the data may constrain solar neutrino oscillations to be nearly maximal [and rule out the small-angle Mikheyev-Smirnov-Wolfenstein (MSW) explanation of solar neutrino observations] if relic neutrinos comprise at least three percent of the critical mass density of the universe.

The top quark seesaw mechanism offers a method for constructing a composite Higgs field without the usual difficulties that accompany traditional technicolor or top-color theories. The focus of this article is to study the phenomenology of the new physics required by this mechanism. After establishing a set of criteria for a plausible top quark seesaw theory, we develop two models, the first of which has a heavy weak singlet fermion with hypercharge 4 / 3 while the second has, in addition, a heavy weak singlet hypercharge -2 / 3 fermion. At low energies, these theories contain one or two Higgs doublets, respectively. We then derive the low energy effective Higgs potential in detail for the two-doublet theory as well as study the likely experimental signatures for both theories. A strong constraint on the one-doublet model is the measured value of the ρ parameter which permits the new heavy fermion to have a mass of about 5–7 TeV, when the Higgs boson has a mass greater than 300 GeV. In the two-doublet model, mixing of the new heavy Y=-2 / 3 fermion and the b quark affects the prediction for R_b. In order to agree with the current limits on R_b, the mass of this fermion should be at least 12 TeV. The mass of the heavy Y=4 / 3 fermion in the two-doublet model is not as sharply constrained by experiments and can be as light as 2.5 TeV.

The S₃ symmetry corresponding to permuting the positions of the quarks within a baryon allows us to study the 70-plet of L=1 baryons without an explicit choice for the spatial part of the quark wave functions: given a set of operators with definite transformation properties under the spin-flavor group SU(3)×SU(2) and under this S₃, the masses of the baryons can be expressed in terms of a small number of unknown parameters which are fit to the observed L=1 baryon mass spectrum. This approach is applied to study both the quark model and chiral constituent quark model. The latter theory leads to a set of mass perturbations which more satisfactorily fits the observed L=1 baryon mass spectrum (though we can say nothing, within our approach, about the physical reasonableness of the parameters in the fit). Predictions for the mixing angles and the unobserved baryon masses are given for both models as well as a discussion of specific baryons.

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 quark 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.

Simple models of top-color and top-color-assisted technicolor rely on a relatively strong U(1) gauge interaction to “tilt” the vacuum. This tilting is necessary to produce a top condensate, thereby naturally obtaining a heavy top quark, and to avoid producing a bottom condensate. We identify some peculiarities of the Nambu–Jona-Lasinio approximation often used to analyze the top-color dynamics. We resolve these puzzles by constructing the low-energy effective field theory appropriate to a mass-independent renormalization scheme. We construct the power-counting rules for such an effective theory. By requiring that the Landau pole associated with the U(1) gauge theory be sufficiently above the top-color gauge boson scale, we derive an upper bound on the strength of the U(1) gauge coupling evaluated at the top-color scale. The upper bound on the U(1) coupling implies that these interactions can shift the composite Higgs boson mass-squared by only a few percent and, therefore, that the top-color coupling must be adjusted to equal the critical value for chiral symmetry breaking to within a few percent.

Top-color and top-color-assisted technicolor provide examples of dynamical electroweak symmetry breaking which include top-quark condensation, thereby naturally incorporating a heavy top quark. In this paper we discuss the roles of the Nambu–Jona-Lasinio (NJL) and large-N approximations often used in phenomenological analyses of these models. We show that, in order to provide for top-quark condensation but not bottom-quark condensation, the top-color coupling must be adjusted to equal the critical value for chiral symmetry breaking up to O(1/N) in any theory in which the isospin-violating “tilting” interaction is a U(1) gauge interaction. A consequence of these considerations is that the potentially dangerous “bottom pions” are naturally light. We also show that the contributions to ρ-1 previously estimated are of leading order in N, are not included in the usual NJL analysis, and are the result of “vacuum alignment.”

Reparametrization invariance, a symmetry of heavy quark effective theory, appears in different forms in the literature. The most commonly cited forms of the reparametrization transformation are shown to induce the same constraints on operators that do not vanish under the equation of motion to order 1/m², and to be related by a redefinition of the heavy quark field. We give a new, very straightforward proof that the reparametrization invariance constraints apply to all orders in α_s under matching to full QCD and renormalization-group running, at least up to and including O(1/m²).

We investigate the effects of top quark compositeness on various physical parameters, and obtain lower limits on the compositeness scale from electroweak precision data. We consider corrections to top quark decay rates and other physical processes. Our results depend sensitively on whether the left-handed top quark is composite. A considerable enhancement of tt¯ production is possible if only the right-handed top quark is composite.

We study hadronic two-body decays of the orbitally excited, SU(6) 70-plet baryons in order to test the hypothesis that the successes of the nonrelativistic quark model have a natural explanation in the large-N_c limit of QCD. By working in a Hartree approximation, we isolate a specific set of operators that contribute to the observed s- and d-wave decays in leading order in 1/N_c. We fit our results to the current experimental decay data, and make predictions for a number of allowed but unobserved modes. Our tentative conclusion is that there is more to the nonrelativistic quark model of baryons than large N_c.

We examine a paradox, suggested by Banks and Dabholkar, concerning nonperturbative effects in an effective field theory which is obtained by integrating out a generation of heavy fermions, where the heavy fermion masses arise from Yukawa couplings. They argue that light fermions in the effective theory appear to decay via instanton processes, whereas their decay is forbidden in the full theory. We resolve this paradox by showing that such processes in fact do not occur in the effective theory, due to matching corrections which cause the relevant light field configurations to have infinite action.

If the number of colors is large the ratio m_η / m_η^′ is bounded from above. The bound is not satisfied by the observed η and η^′ masses.

We consider a minimal technicolor model in which the ordinary and technicolor sectors are coupled by a massless scalar doublet. When technicolor interactions become strong, the resulting technicolor condensate not only breaks the electroweak symmetry, but also causes the scalar to develop a vacuum expectation value. With the appropriate choice of the scalar's Yukawa couplings, fermion masses are generated, giving us the conventional pattern of flavor symmetry breaking. Although no explicit scalar mass term appears in the full Lagrangian of the model, the pseudoscalar states that remain in the low-energy effective theory gain sufficient mass through technicolor interactions to evade detection. We show that this model does not generate unacceptably large flavor-changing neutral currents, and is consistent with the experimental constraints on oblique electroweak radiative corrections. We determine the experimentally allowed region of the model's parameter space, and discuss the significance of a phenomenologically viable model that has no arbitrary dimensionful parameters. In terms of parameter counting, our model is the simplest possible extension of the standard model.

I identify a new realization of chiral SU(3)×SU(3) symmetry which, in a sense, encompasses both the Wigner-Weyl mode and the Nambu-Goldstone mode. The chiral symmetry is unbroken, but there are Goldstone bosons. Furthermore, the light vector mesons, as well as the pseudoscalar mesons, are intimately involved because the chiral partners of the pseudoscalars are the longitudinal components of the vectors. I discuss the connection of this realization with vector dominance, the Kawarabayashi-Suzuki-Riazuddin-Fayyazuddin relation, and the large-N approximation.

We present an electroweak theory in which left-handed quarks and leptons transform as doublets under separate SU(2) gauge groups. Spontaneous symmetry breakdown results in two charged and two neutral massive vector bosons. The lightest charged and neutral gauge bosons behave like the W and Z of the standard SU(2)×U(1) electroweak model. The heavier W and Z, which can be as light as several hundred GeV, couple primarily to quarks. Our theory predicts small deviations in the properties of the Z which will be visible at the SLAC Linear Collider and the CERN collider LEP.

We show that Suzuki’s model of composite vector bosons is consistent at the level of the effective low-energy theory. We generalize it to include couplings to quarks and leptons and discuss its phenomenology. We find observable corrections to the W mass.

I discuss a model of jet energy flow in which the jet is a continuous distribution of massless particles, uniform in the jet-jet center of mass. In the laboratory frame, all quark jets look the same near the jet direction. The transverse energy of a jet has a 1/r³ dependence on distance from the jet direction (r²=Δy²+Δφ²), with a small section in the center that depends on the shape of the jet in the center of mass. Only the height of the center depends on the jet energy.

We consider the phenomenology of composite technicolor standard models (CTSM’s). We estimate the CTSM contributions to weak neutral currents and CTSM corrections to the ρ parameter and Cabibbo universality. We show that current experimental limits on these CTSM corrections to the standard model constrain the compositeness scale and the flavor-symmetry structure of CTSM. We show that B_d⁰-B¯ _d⁰ mixing and the rate of the decay K→πνν¯ can differ significantly from the standard model and that CTSM effects in e⁺e^-→μ⁺μ^-,τ⁺τ^- should be observable in the nea future.

Low-energy theorems are proved for the scattering of longitudinally polarized W and Z bosons that hold at a scale intermediate between M_W and the characteristic mass scale of the symmetry-breaking sector. The theorems are proved without assuming a custodial SU(2) symmetry. Three methods are used: a perturbative power-counting analysis in the unitary gauge, a current-algebra derivation in renormalizable gauges, and a nonlinear chiral Lagrangian that is relevant in unitary or renormalizable gauges.

The experimentally verified relationship M_W=M_Zcosθ_W implies universal low-energy theorems for scattering of longitudinally polarized W and Z bosons provided that the symmetry-breaking sector contains no particles much lighter than 1 TeV.